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Endangered and Threatened Species of the Platte River (2005)

Chapter: 6 Piping Plover and Interior Least Tern

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Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

6
PIPING PLOVER AND INTERIOR LEAST TERN

Antithetical though it may seem from their vernacular name, shorebirds are common in North America’s vast interior. Sandpipers, willets, curlews, and their many relatives are found across the Great Plains and central Canada north to the high Arctic. Why piping plovers and interior least terns, now recognized as endangered and threatened species, reside in Nebraska, so far from ocean and sea, is immediately apparent to the astute observer. Great Plains rivers—with their braided channels, gravel bars, and sandy shores—offer conditions for rearing young that are remarkably similar to the conditions that support plovers and terns on distant coastal shores. But those conditions, which have been sustained for millennia by dynamic fluvial geomorphic processes, are changing rapidly. Impoundment and exports of Platte River Basin waters have altered the river’s hydrograph and caused the disappearance of landscape features that sustain the two species. As their habitat has been lost, populations of piping plovers and interior least terns have declined dramatically along the central Platte River.

The following chapter explores three specific questions posed to the committee: Do the present habitat conditions on the Platte River affect the likelihood of survival and recovery of the piping plover? Do they affect the likelihood of survival and recovery of the interior least tern? Is the currently designated critical habitat on the central Platte River for the piping plover supported by existing science? Observations in this chapter also support conclusions related to a more general question concerning gaps in knowledge regarding the two species.

Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

PIPING PLOVER

The piping plover (Charadrius melodus) is a small shorebird that has threatened or endangered status throughout its range (USFWS 1988; Haig 1992; Thompson et al. 1997). The species is distinguished from other, smaller plovers by a single black neck band that is present during the breeding season and a short, stout bill (orange during breeding), pale gray back and wings, white belly, and orange legs (Figure 1-4). The name “piping” refers to the bird’s distinctive flute-like vocalizations.

Distribution

The U.S. Fish and Wildlife Service (USFWS) and Nebraska Game and Parks Commission (NGPC) collect and maintain data on plover nesting. Methods used to monitor plovers in the Platte River are the same as or similar to those used by biologists elsewhere in the range of this species in North America. Protocols for monitoring breeding pairs, estimating productivity, and reporting results have been formalized.1 Since the Northern Great Plains (NGP) population was listed, Platte River nesting records have been consistently collected and maintained.

The general method used to obtain estimates of breeding pairs throughout the piping plover range is to walk toward potential nest habitat or approach it by boat (Plissner and Haig 2000). Potential habitat is identified on the basis of historical records (plovers are very site-faithful if habitat is suitable) and knowledge of habitat characteristics. In the central Platte, USFWS and NGPC biologists obtain estimates by using an airboat (Erika Wilson, USFWS, pers. comm., July-September 2003; John Dinan, NGPC, pers. comm., May-September 2003); they do not go on the land, because some of it is private property. At sandpits and Lake McConaughy, monitors approach plover habitat on foot (Mark Peyton, CNPPIR, pers. comm., September 2003; Jim Jenniges, Nebraska Public Power District, pers. comm., May-August 2003). USFWS’s surveys by river use two observers and are conducted in May and early June, depending on river conditions. At potential habitat sites, the monitors stop and check the landscape with a telescope and record the presence or absence of plovers. Monitors also record the Global Positioning System (GPS) location of the site. If plovers are present, the number of individual birds is counted, and their behavior in riverine habitat is recorded. Plovers typically vocalize and are usually visible to monitors because of their flight or rapid movements on the ground. If monitors are close to a nest, plovers may perform “broken-wing

1  

Governance Committee, Executive Director’s Office, Tern and Plover Monitoring Protocol Implementation Report, unpublished material, Feb.12, 2001.

Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

behavior” to distract the observer. Monitors record the presence or absence of birds at a particular site and note behavior that indicates nesting. If there is evidence of nesting or if the team believes that the birds will nest, they return daily or several times per week to determine whether chicks have hatched. Visits are made until adults leave or it is decided that no chicks are present, typically June-July. At the end of each season, data on pair estimates and nesting success are sent to NGPC, where the Nebraska piping plover database is maintained. Since records were first recorded for piping plovers, the methods have become more formalized, particularly after the methods section of the Cooperative Agreement Tern and Plover Monitoring Protocol was prepared.

The piping plover breeds only in North America. Its total population was estimated at nearly 6,000 adults in 2001 (Susan Haig, USGS, pers. comm., July-August 2003). The Great Lakes population is recognized as endangered under the Endangered Species Act (ESA), and the NGP and Atlantic Coast populations are recognized as threatened. Birds nesting on the Platte River are part of the NGP population. Piping plovers breed on open beaches along the Atlantic Ocean and Great Lakes, on alkali flats, on islands in broad prairie rivers, and along reservoir shorelines in the NGP. They winter along the Atlantic Coast from Virginia to southern Florida, in the Caribbean, and along the Gulf Coast from Texas and Mexico.

Historical distribution and nesting records exist for piping plovers in Nebraska back at least to the 1800s (EA Engineering, Science and Technology, Inc. 1988; Lutey 2002). Before listing, the species was reported from 32 of 97 counties in the state. Information on population status is more recent: statewide estimates were not conducted until after the “population” was listed in 1986. The presettlement plover breeding-population size in Nebraska is unknown.

The breeding range of the NGP population includes southern Alberta, southern Saskatchewan, and southern Manitoba; extends south to eastern Montana, North Dakota, South Dakota, southeastern Colorado, Iowa, and Nebraska; and extends east to Lake of the Woods in north central Minnesota. Most of the breeding pairs in the U.S. portion of the population’s range are in North Dakota, South Dakota, Montana, and Nebraska. USFWS conducted international winter and breeding censuses for piping plovers in 1991, 1996, and 2001. Trend data indicate that the NGP piping plover population declined by 15% from 1991 to 2001.

Plovers breed in Nebraska on sandbars, along reservoir shorelines, in commercial sand mines, and at other artificially created sites along three major rivers (Figure 6-1, 6-2, and 6-3). In the northeastern corner of the state, along the border with South Dakota, nesting occurs along about 64 km of the Upper Missouri River and 153 km of the lower Niobrara River

Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

FIGURE 6-1 Sandy low bars along central Platte River serve as nesting areas for piping plovers and interior least terns. Source: Photograph by W.L. Graf, May 2003.

(Figure 6-4). Farther south, plovers are found along about 390 km of the central and lower Platte River, from the Missouri River west to Lexington. Breeding also occurs at Lake McConaughy in western Nebraska and on the Middle Loup and Loup rivers in central and eastern Nebraska (L.C. Wemmer, USFWS, unpublished material, February 11, 2001). Census efforts during the breeding season estimated 398, 366, and 300 pairs of piping plovers in Nebraska in 1991, 1996, and 2001, respectively (Figure 6-5) (Susan Haig, USGS, pers. comm., July-August, 2003). Those estimates do not include plovers on the portion of the Missouri River that is shared with South Dakota. Current estimates indicate that nesting pairs of piping plovers in Nebraska make up 10-12% of the NGP population, and about 9% of Nebraska’s breeding piping plovers nest on the central Platte River (Dinan, NGPC, pers. comm., May-September 2003); about 1% of the NGP population of piping plovers nest on the central Platte River.

Lutey (2002) reported that the numbers of piping plovers observed on the Platte River declined from 1987 to 1998, a period during which the number of breeding pairs averaged just one for the South Platte, 35 for the North Platte (at Lake McConaughy), four on the upper Platte, 29 on the

Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

FIGURE 6-2 Sand mines along margin of central Platte River serve as nesting areas for piping plovers and interior least terns, but are not as suitable as sand masses in river. Source: Photograph by W.L. Graf, May 2003.

central Platte, and 50 on the lower Platte. John Dinan (NGPC, pers. comm., May-September 2003) reported a 61% decrease in piping plovers nesting on the central Platte River from 1991 to 2001, so piping plovers appear to have declined over the last decade at regional, state, and local levels. Declines at all levels are attributed primarily to human activities, including direct and inadvertent harassment of birds and nests by people, domesticated animals, and vehicles; destruction of shoreline habitat as a conse-quence of development projects; increased predation due to human presence in less-visited beach areas; and water-level regulation policies that result in changes in nesting habitat (Haig and Elliott-Smith, in press; Haig 1992). Causes of the declines recorded in the NGP population include predation of eggs and chicks, habitat destruction and degradation that result from channelization of rivers and modification of river flows, disturbance by humans and pets, contaminants, and inadequate regulatory mechanisms (L.C. Wemmer, USFWS, unpublished material, February 11, 2001).

To understand habitat use by plovers in the Platte River, it is important to establish whether birds are sedentary—that is, use one or a few sites during their lifetime—or disperse extensively. That is important because it

Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

FIGURE 6-3 Sandy shore of Lake McConaughy provides nesting areas for piping plovers and interior least terns when the reservoir is low enough to expose beaches. Source: Photograph by W.L. Graf, May 2003.

is a key to understanding local population dynamics, population persistence, and the spatial scale on which management of the Platte River will affect regional populations. Movements of plovers that hatched along the Platte River or were first banded as adults nesting along the Platte have been studied by Lingle (1993c). He reports resightings of 329 plovers banded from 1985 to 1989 between Lexington and Grand Island and observes that in the year after banding, 43% of the plovers banded as adults returned to the same site, but only 18% of the birds banded as chicks returned to the site where they hatched. Those observations suggest that missing birds either died or moved to other locations. An unknown number of birds were probably present in the study area but not detected. Without human observers present to look for banded birds, it is not possible to infer where the surviving banded birds eventually nested. Lingle did, however, obtain several dispersal records worth noting: seven birds were observed at other nest locations in the Platte River Valley, including an adult banded near Kearney that nested 155 mi downstream 2 years after banding and a banded bird captured at Lake McConaughy, 300 mi from its banding site on the lower Platte River. Lingle’s observations indicate that plovers from the Platte River are capable of moving far between breeding seasons and show the difficulty in making inferences

Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

FIGURE 6-4 Piping plovers and interior least terns distribution in Niobrara, Loup, and Platte Rivers, Nebraska. Source: Dinan 2003.

about local populations, movements of individuals, and patterns of habitat use without information on the behavior of marked birds. They also suggest strongly that piping plovers in Nebraska are part of a single demographic unit of interacting individuals.

Records on timing of breeding-season activities are maintained by the Wildlife Division of the NGPC (Dinan, NGPC, pers. comm., May-September 2003). Piping plovers typically arrive at the Platte River in middle to late April and depart by late August. During that interval, they locate a mate,

FIGURE 6-5 Estimated piping plover (PP) and interior least tern (LT) population numbers represented in pairs during 1991, 1996, and 2001 breeding season for Nebraska, excluding Missouri River. Source: Dinan 2003.

Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

construct a shallow nest on sand or gravel substrate, and, between early May and early July, lay eggs and incubate them for about 28 days. First hatching typically occurs in late May or early June but, depending on spring water flow, may extend into late June and July. Renesting can occur if the entire clutch is lost. Chicks require more than 21 days of growth and development after hatching before they are able to fly. After young birds fledge, they continue to feed and mature along the Platte River until they begin autumn migration in August.

Influence of Current Central Platte Conditions on Survival of Piping Plovers on Platte River

Plovers, like all species, have specific requirements for survival. USFWS identifies among the requirements space for population growth and normal behavior; nutritional or physiological requirements (such as food, water, air, light, and minerals); cover or shelter; sites for breeding, reproduction, and rearing of offspring which are protected from disturbance; and habitats representative of the historical distribution of the species (Fed. Regist. 67 (176): 57638 [2002]). Those survival requirements have been used by USFWS to identify the primary constituent elements (PCEs) of critical habitat in accordance with Section 3(5)(A)(i) of the ESA. PCEs for the NGP population of piping plovers are habitat components (physical and biological) essential for the biological needs of courtship, nesting, sheltering, broodrearing, foraging, roosting, intraspecific communication, and migration (Fed. Regist. 67 (176): 57638 [2002]).

According to the NGP population recovery plan (USFWS 1988) and critical habitat final rule (Fed. Regist. 67 (176): 57638 [2002]), PCEs on the Platte River include sparsely vegetated channel sandbars, sand and gravel beaches on islands, temporary pools on sandbars and islands, and interface zones with the Platte River (Figure 6-6). Those habitat features are directly influenced by dynamic precipitation cycles and longer-term climate patterns. Habitat area, abundance and availability of insect foods, brood and nesting cover, and lack of vegetation are all linked to weather and climate. Variability in flow can cause high rates of turnover of naturally variable sandbars. Flowing water creates diverse habitats for feeding, nesting, and brooding. Habitat variables were quantified by Ziewitz et al. (1992) during the 1988 breeding season and used to prepare USFWS’s target species suitable habitat document (USFWS, unpublished material, June 2000). Table 6-1 summarizes habitat characteristics important to the species.

Studies on the habitat requirements of the piping plover across its breeding range have been few. However, the study by Ziewitz et al. (1992) quantified a number of variables (such as channel width, sandbar area, mean nest elevation, and maximal nest elevation) present at the piping

Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

FIGURE 6-6 Area of central Platte River channel near Shelton with many primary constituent elements for the piping plover. Channel sandbars lack woodland cover, temporary pools on bare sand are abundant, and interface zones are plentiful. Source: Photograph by W.L. Graf, May 2003.

plover nesting sites along the Platte River. The effort was an important contribution to understanding of the piping plover habitat requirements along the central Platte River. Using an airboat, Ziewitz’s research team traveled the reach of the Platte River from Lexington to its confluence with the Missouri River in the middle of June. They located as many nest sites as possible and (to minimize disturbance to breeding birds) marked the nests for future study. Dates of hatching and egg laying were estimated by floating eggs in a container of water. Age is determined by the position of the egg as it floats in the water; older eggs float higher in the water column (Westerskov 1950). Ziewitz et al. (1992) used aerial videography to record the Platte River over the summer. They analyzed scenes at the 26 sites occupied by plovers and univariately compared physical characteristics of occupied and unoccupied sites. The study showed that more plovers nested on the lower Platte River than the central Platte, birds nested on river segments that were wider and had greater areas of sparsely vegetated sandbars, channels were wider and sandbars higher along the lower Platte River than on the central Platte, nests on the lower Platte had greater clearance

Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

above the river’s water line when initiated than did nests on the central Platte, and the average elevation of nests was greater on the lower Platte, which suggested that nest habitat was limited on the central Platte. Several other studies (e.g., Ducey 1988; Faanes 1983) attempted to quantify vegetation at piping plover nest sites, documenting that occupied sites tended to be sparsely vegetated or to lack vegetation early in the nesting season. No studies are available that quantify plover habitat characteristics during periods of especially low water flow.

Current information suggests that no suitable piping plover habitat now exists along the central Platte River (Erika Wilson, USFWS, pers. comm., July-September 2003; John Dinan, NGPC, pers. comm., May-September 2003). Flow has been so low that no new sandbars are being created. Sandbars that once were suitable are now unsuitable because no recent scouring has occurred to prevent establishment of vegetation, and many river reaches have been nearly dry in recent years. In 1988, about 25 pairs nested on the central Platte River and in sandpits; Ziewitz et al. (1992) concluded that reproduction by piping plovers was being compromised by low flows that were punctuated by sudden water peaks in July, which led to nest flooding. Lingle (1993a) attributed the July peaks to a combination of heavy local precipitation and releases of water to the river via the Johnson-2 return during local thundershowers. He reported that nest success was higher on the lower Platte River, where sandbar heights are greater than on the central Platte. Plover nesting in 1988-1996 varied from 20 to 42 pairs on the central Platte (1-15 pairs on the river, 13-30 pairs in sandpits). However, since 1996, fewer than five pairs per year have nested on the central Platte. Chick productivity at river sites along the central Platte declined to about 1.3 chicks per pair in 1997 and 1998; since 1999, there has been no successful reproduction.

In addition to riverine habitat, piping plovers nest in commercial sandpits on the central Platte River. Another important adjacent source of alternative habitat is Lake McConaughy to the west. Large numbers of plovers nest along the shoreline of Lake McConaughy when water levels are low. For example, high water elevation, 3,262 ft above mean sea level (msl), along the lake in 1986 resulted in narrow beaches (<25 m) and restricted habitat availability. In May 1991, in contrast, lake elevation was extremely low, 3,245 ft msl, and resulted in beach widths of 400-800 m and abundant nesting habitat (Wingfield 1993). It is important to consider potential habitat at Lake McConaughy because plovers produced at this reservoir may nest along the central Platte River, central Platte plovers may nest at Lake McConaughy (Wingfield 1993), and relationships between nesting along the river and the lake are probably related to water levels and habitat conditions at both locations. Finally, it is important to emphasize that lack of knowledge about population fidelity (tendency to return to same place to nest) and movement on all spatial scales (including both the

Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

TABLE 6-1 Habitat Characteristics Important to the Piping Plover

Piping Plover Habitat

Observed Measurements of Habitat Parameters (OMHPs)

Preliminary Goals for Habitat Management

References

Riverine habitat

Channel or sandbar characteristics

 

 

 

Channel width

975-1,554 ft

≥ 900 ft, initially

Ziewitz et al. 1992

Kirsch 1996

Sandbar area (early June, at nest initiation)

0.03-3.58 acres

Variable

Ziewitz et al. 1992

Kirsch 1996

Mean elevation above 400-cfs stage (early June, at nest initiation)a

0.2-2.0 ft (mean, 0.4 ft)

Low, ephemeral sandbars; high enough to provide dry, bare sand during nesting season

Ziewitz et al. 1992

Maximal Elevation above 400-cfs stage (early June, at nest initiation)a

0.4-4.4 ft (mean, 2.7 ft)

Low, ephemeral sandbars; high enough to provide dry, bare sand during nesting season

Ziewitz et al. 1992

Vegetation

 

 

 

Cover: at nest site

<5-20% (early in nesting season)

Sparsely vegetated or unvegetated

Schwalbach 1988

Ducey 1983

On sandbar

<10-25%

 

Faanes 1983

Height (at nest site)

<2 ft (early in nesting season)

Same as OMHP

Schwalbach 1988

Prindiville-Gaines and Ryan 1988

Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

Forage

 

 

 

Expanse of wet or moist substrate

 

Corn and Armbruster 1993

Sandpit habitat

Sand and gravel operation characteristics

 

 

 

Availability of forage

Within 1 mi

Corn and Armbruster 1993

Nordstrom and Ryan 1996

Total surface area

1.5-197 acres

(mean, 18.3 acres)

Sidle and Kirsch 1993

Water surface area

1-150 acres

(mean, 27.4 acres)

Sidle and Kirsch 1993

aTo compare water-surface-elevation datum from different nest sites to a common elevation datum, original data were adjusted to yield elevation measurements as if all central Platte River data were collected at a discharge of 11.3 cms (400 cfs).

Source: USFWS, unpublished material, June 16, 2000.

Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

interior NGP and the Platte River) compromises the ability to determine habitat use by central Platte River plovers.

Several studies have concluded that artificial habitats cannot provide the full complement of essential habitat requirements for piping plovers over the long term and therefore cannot substitute for riverine habitat (Corn and Armbruster 1993; Lingle 1993b; Sidle 1993). For example, prey-capture rates for plovers and densities of invertebrates were higher at river-channel sites than at sandpit sites (Corn and Armbruster 1993). (A similar conclusion was drawn for interior least terns [Sidle 1993; Wilson et al. 1993].) USFWS believes that sandpits do not meet the requirements of critical habitat (Fed. Regist. 67 (176): 57638 [2002]). Although plovers will nest in active sandpits, use of this habitat is temporary and terminates when vegetation becomes too dense. Moreover, water is often distant from sandpits, and adults must travel farther than a mile to forage. Finally, because sandpit sites are not isolated on islands, nests there are more vulnerable to predation. A number of investigators (including Erika Wilson, USFWS, pers. comm., July-September 2003; John Dinan, NGPC, pers. comm., May-September 2003; Lackey 1997 and Plettner 1997 in abstracts from 1997 Platte River Basin Ecosystem Symposium, Kearney, NE, February 18-19, 1997) have reported that despite some limitations, productivity in the sandpits can be high if the pits are intensively managed to exclude predators and human disturbances. No studies have examined whether survival from fledging to first breeding is higher in natural or in artificial habitats.

The contribution of alternative habitat to the survival and recovery of piping plovers can be summarized as follows: sandpits provide refuge and nesting substrate when water is high on the river, but they do not appear to provide the complete array of essential habitat elements required by piping plovers.

The presence of the habitat components required for survival influences whether piping plovers are able to nest, hatch eggs, and raise their offspring to fledging. According to USFWS and NGPC biologists (Erika Wilson and John Dinan), only a few plovers have attempted to nest on the river (as opposed to sandpits and other artificially created habitat) in the central Platte over the last 5 years. All nests apparently have failed to produce young; the failure is attributed to low water flow and lack of dynamic ecosystem processes appropriately timed to create new islands or sandbar habitat. It is believed that if water is too high or sandbars are too low during the breeding season, nest habitat (potentially including eggs and chicks) will be inundated and plovers will be unable to find appropriate locations to breed. If water is too low, because of water management or climatic conditions, scouring does not occur and vegetation develops on sandbars and small islands in the Platte River; in addition, reduced water flow fails to create the new sandbars that are necessary for nesting (Figure 6-7).

Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

FIGURE 6-7 Portion of central Platte River channel near Shelton. Typical low water condition fails to create new sandbars, and existing sandbars are becoming stabilized with vegetation. Source: Photograph by W.L. Graf, August 2003.

The primary causes of breeding failure in the central Platte River for the last decade appear to be nest flooding and reductions in habitat after periods of very low stream flow. Lingle (1993a,b) studied causes of nest failure and mortality of plovers along the central Platte from 1985 to 1990 and reported that the greatest causes of plover nest failure on the river were flooding (61%) and predation (19%). In contrast, predation and abandonment accounted for most (42% and 21%, respectively) failures at sandpits.

In summary, current conditions in the central Platte River appear to be compromising the continued existence—that is, the survival—of the NGP population of the piping plover. Supporting that conclusion are observations that few plovers have attempted to nest on the central Platte River in the last 5 years, no eggs have hatched or chicks fledged in the river during the last 5 years, hydrological conditions necessary for development and maintenance of nesting habitat have not occurred in the central Platte in the last 5 years, and loss of habitat along the river appears to be forcing birds to use alternative sites that are less secure from predators and other sources of disturbance and death and do not provide suitable habitat for reproduction. The loss of central Platte River habitat has reduced breeding options for the already-vulnerable regional population.

Recovery Goals for the Northern Great Plains Piping Plover Population

According to USFWS’s (1988) recovery plan for the Great Lakes and NGP piping plover populations, the NGP population can be considered for delisting when four recovery criteria are met: the number of birds in the

Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

population reaches 1,300 pairs, essential breeding and winter habitats are protected, the Canadian recovery objective of 2,500 birds for its prairie region is reached, and the 1,300 pairs in the United States are maintained in a specific distribution for 15 years (assuming at least three major censuses during this period). For Nebraska, the recovery goal is 465 pairs; the goal for the Platte River is 140 of those pairs. The NGP recovery plan has been revised twice since 1986, but neither revision has been formally approved by USFWS. The most recent draft revision proposes increasing the recovery goal for NGP from 1,300 to 2,400 breeding pairs (L.C. Wemmer, USFWS, unpublished material, February 11, 2001).

Lutey (2002) reported a 1987-1998 average of 84 pairs of piping plovers breeding on the Platte River including birds nesting in sandpits along the river. If Lake McConaughy birds are included, the average is 119 pairs. Current numbers therefore are about half those needed to meet the recovery goal for the Platte. It is important to note that the Lake McConaughy plovers usually are not included in the recovery goal for the Platte River, because the lake is not considered part of the Platte River (Erika Wilson, USFWS, pers. comm., July-September 2003; John Dinan, NGPC, pers. comm., May-September 2003). The Lake McConaughy birds, however, are included in Nebraska’s total number of breeding pairs and can contribute to Nebraska’s goal of 465 pairs as part of NGP population recovery. It should be noted here that re-establishment of a more natural Platte River hydrograph alone will not ensure recovery of the piping plover. The continued presence of reservoir and sand pit breeding sites may provide sink habitat situations that draw plover pairs into circumstances where survival of offspring is low, and this in turn could lead to reduced fitness and regional declines in population numbers. Ameliorating direct losses and harassment of birds and nests caused by recreation and other human activities on the river will be a necessary component of recovery. Losses of adults, juveniles, and eggs to predation are likely to increase and lead inevitably to increases in introduced species associated with human activities on the river. And, although the contribution of contaminants to current mortality is unknown, population losses from that source will probably need to be better understood before downlisting or delisting of the species.

Population Viability Analysis for Piping Plovers

Assertions regarding population sizes necessary for recovery can be tested with viability modeling. Population viability analysis (PVA) uses demographic models to predict the probability that a population will go extinct within a specific period under specific circumstances. PVAs are often used to derive estimates of the minimal number of individuals needed

Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

to ensure long-term population persistence. They also are used to simulate effects of various environmental phenomena and population characteristics on trends in population size. Three PVAs have been published for the NGP population of piping plovers (Ryan et al. 1993; Plissner and Haig 2000; Larson et al. 2000, 2002). The PVA by Plissner and Haig (2000) found that a 36% increase in productivity (offspring fledged per pair per year) or an 8-10% increase in adult and immature survival rates, was needed to ensure a 95% probability that the population will persist for 100 years. That level of reproduction would require a mean reproductive success of 1.7 chicks/pair per year, a scenario in which the population would decline but would not go extinct. To ensure population stability, Plissner and Haig reported that an annual mean productivity of 2.0 fledglings/pair was required. That assessment has been considered conservative by other biologists because of the low reproduction rate used in the model; see Larson et al. (2000, 2002). However, Plissner and Haig and Larson et al. agree that the NPG metapopulation (a set of local populations connected by migrating individuals) will not persist unless reproductive success is increased substantially.

Conclusions about effects on recovery of piping plovers are essentially the same as reported above for current conditions on survival. Plissner and Haig (2000) suggested that at least 1.7 chicks (preferably 2.0 to provide a buffer against stochastic events) must fledge per pair per year to meet recovery goals for the NGP population; the estimate may be reduced somewhat if mortality decreases. Ryan et al. (1993) suggested 1.13 chicks as sufficient for recovery, but they assumed an unreasonably high rate of juvenile survival; Larson et al. (2000, 2002) report a populationwide fledging rate of 1.1 fledglings/pair per year to stabilize simulated populations. Reproductive success for the central Platte River has been zero for the last 5 years, and it is unlikely that this is being offset by groups of birds elsewhere in the population’s range. On a more local scale, relatively high productivity was reported at Lake McConaughy, especially in 2002 and 2003 (1.7 chicks/pair) (Mark Peyton, CNPPID, pers. comm., August 2003). Although Lake McConaughy is not considered part of the Platte River—and, just as important, does not provide secure habitat, because it depends on Kingsley Dam operations—Lake McConaughy plovers currently contribute substantially to the Platte River population. However, without the ability to differentiate individual birds from different source populations, the relationship between birds that nest at Lake McConaughy and birds that use the Platte remains unknown.

The committee performed an elementary PVA on the NGP and Platte River piping plover populations and found, not surprisingly, that losing Platte and Loup Rivers habitat has little effect on the persistence of the

Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

NGP metapopulation. This finding is, however, a result of current low productivity by plovers on the Platte River. That is, when a declining population is removed from a group of stable or declining populations, it has little effect on overall persistence of the group. Indeed if the areas of the Platte and Loup Rivers, in their current conditions, are acting as sink habitats, removing them completely as potential breeding sites could actually increase the persistence probability of the NGP population. However, if habitat management succeeds in increasing habitat value to a point where it allows the proportion of nests that produce offspring to double, the Platte and Loup River population would have a moderate probability of persisting, and in turn again contribute to the regional metapopulation persistence. Current conditions can therefore be viewed as adversely affecting recovery. The analysis was based on a number of assumptions about the spatial structure of piping plovers in the NGP population; before the contribution of birds from the central Platte or the Platte River system can be accurately evaluated, movements of birds among habitat areas and their uses of those areas must be better understood.

Although insufficient data are available to construct a PVA for piping plovers on the Platte River that has a high level of resolution and predictive power, the committee used the VORTEX model 9.3 (Lacy et al. 2003) created by Plissner and Haig (2000) for the NGP to address several circumstances. First, to assess the effect of removing the Platte and Loup River birds from the model, resident birds were removed from the larger NGP population, and then the apparent carrying capacity (number of individuals that can be supported permanently in a given area) of the Platte River was decreased. Second, Platte and Loup River birds were separated from the Nebraska population to examine the effect of reducing plover productivity in the Platte River to zero. Finally, the percentage of nests producing fledglings in the Platte and Loup system was doubled to assess the effect of reducing nest failures.

No information exists regarding the population structure of piping plovers in Nebraska. Consequently, birds were modeled as two populations: one on the Missouri and Niobrara Rivers, and one on the Platte and Loup Rivers. Population recovery goals (USFWS 1988) were used to estimate carrying capacity. Carrying capacity for the Niobrara portion of the Missouri and Niobrara population is 100 birds, and carrying capacity for the Platte and Loup is 610 (Platte, 280; Loup, 330). Counts from 2001 were used to estimate current population sizes as 84 birds on the Niobrara and 216 on the Platte and Loup. Carrying capacity of those rivers is substantially higher than current populations; recovery goals are based on many sources of information (e.g., knowledge of distribution and abundance, survey data, historical population data, and loss of viable habitat) and include assessment of the potential to increase the number of breeding

Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

pairs at occupied sites and to establish pairs at unoccupied sites (L.C. Wemmer, USFWS, unpublished material, February 11, 2001).

Although breeding sites are dynamic—they are lost and created—it was assumed that on the average the number of sites and dispersal among sites was consistent across years. As data become available to evaluate that assumption, a more sophisticated PVA can be developed.

Tables 6-2 and 6-3 summarize the input values used for the initial model. Movement among other populations and the Platte and Loup

TABLE 6-2 Summary Input for the Piping Plover PVA Initial Model

 

Great Lakes

Manitoba and Lake of the Woods

Northern Prairie

Platte and Loup

Niobrara

Colorado

Carrying capacity

300

250

4,700

610

100

40

Initial population size

48

73

2,786

216

84

13

1st year mortality

56.8

56.8

56.8

56.8

56.8

56.8

Mortality (SD)

11.36

11.36

11.36

11.36

11.36

11.36

Adult mortality

34

34

34

34

34

34

Mortality (SD)

6.8

6.8

6.8

6.8

6.8

6.8

% nests failing

49.68

79.9

51.75

58

58

72.4

 

Source: Reed 2003.

TABLE 6-3 Migration Rates Among Piping Plover Populations

 

Great Lakes

Manitoba and Lake of the Woods

Northern Prairie

Platte and Loup

Niobrara

Colorado

Great Lakes

0.001

0

0

0

0

Manitoba and Lake of the Woods

0.001

0.01

0

0

0

Northern Prairie

0

0.01

0.01

0.01

0

Platte and Loup

0

0

0.01

0.02

0.005

Niobrara

0

0

0.01

0.02

0.005

Colorado

0

0

0

0.005

0.005

 

Source: Reed 2003.

Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

population and Niobrara population is the same as Plissner and Haig used for the Nebraska populations (half going to each population); migrations between the Platte and Loup population and the Niobrara population used the same rule as Plissner and Haig. Also assumed were no inbreeding depression, no correlation in environmental variance, no catastrophes, no additional mortality associated with dispersal, no density dependence, mo-nogamous mating, males and females breeding at the age of 1 year with a maximum of 10 years, an equal sex ratio, maximal clutch of four eggs, and a stable age distribution. Other model assumptions followed those of Plissner and Haig.

The first model run reduced carrying capacity for the Platte and Loup population (by 280 birds) without altering initial population size. Birds that did not emigrate died. In the second model, carrying capacity was restored, but reproductive success was reduced to zero. The last model also restored carrying capacity, and nest failures were halved (the percentage of nests failing was reduced by half), with proportions of nests producing one, two, three, and four eggs kept the same.

Three key results should be noted: removing the plover carrying capacity of the Platte and Loup Rivers slightly increased the rate at which the NPG metapopulation went extinct (Table 6-4), reducing Platte and Loup reproductive success to zero increased the time to metapopulation extinction and reduced population persistence time in the Platte and Loup Rivers system by two-thirds (from 28 years to 10) (Table 6-4), and reducing nest failure rate by half (increasing the 42% of nests producing at least one fledgling to 84%) had a large effect on population persistence. For the NGP metapopulation, the probability of persisting 200 years increased from 0% to 16.3%. For the Platte and Loup population, persistence probability went from 0% to 93.7%, and the mean time to extinction (when populations went extinct) increased from 28 years to 107 years. When populations persisted, their mean population size at 200 years was 456 (Table 6-4).

The analyses indicate that losing Platte and Loup River habitats has a measurable effect on regional metapopulation persistence. However, because all populations are declining, the effect is slight. If management actions could produce as much as a doubling of the proportion of nests that produce offspring (and the proportions of nests producing one, two, three, and four fledglings remain the same), the Platte and Loup population of plovers has a high probability of persisting. When the proportion of nests producing chicks, for example, is low, it is possible to double the proportion of nests that produce chicks by intensive nest-site management (e.g., nest exclosures, beach closure, and volunteer plover patrols) (Wemmer 2000), as recommended (Larson et al. 2002) for the Great Lakes and NGP populations of piping plovers.

Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

TABLE 6-4 Summary Statistics from PVA Similar to that of Plissner and Haig (2000), with Nebraska Birds Separated into Platte and Loup and Niobrara Populations

 

Default

Scenario

 

 

 

 

Platte-Loup Habitat Removed

Zero Reproductive Success in Platte-Loup

Doubled Nests Producing Fledglings in Platte-Loup

Northern Great Plains metapopulation

 

Probability of surviving 200 years

0.00

0.00

0.00

0.163

Mean final population size if persisting

116

Mean years to extinction if it goes extinct

27

22

24

40

Population growth rate (r)

–0.167

–0.168

–0.214

–0.123

Niobara population

 

Probability of surviving 200 years

0.00

0.00

0.00

0.025

Mean final population size if persisting

3

Mean years to extinction if it goes extinct

22

22

22

23

Population growth rate (r)

–0.144

–0.145

–0.148

–0.114

Platte and Loup Rivers population

 

Probability of surviving 200 years

0.00

0.00

0.937

Mean final population size if persisting

456

Mean years to extinction if it goes extinct

28

10

107

Population growth rate (r)

–0.145

–0.232

0.078

 

Source: Reed 2003.

Evaluation of Science Supporting Piping Plover Critical Habitat Designation

Critical habitat for the NGP breeding population of the piping plover was designated in 2002, including 19 critical habitat units totaling 183,422 acres (74,228 ha) and portions of four rivers totaling approximately 1,208 river miles (1,944 km) (Fed. Regist. 67 (176): 57638 [2002]). One unit (Unit NE-1) was designated that encompasses about 440 mi (707.9 km) of river habitat on the Platte, Loup, and Niobrara Rivers.

Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

The critical habitat designation for the NGP breeding population of piping plovers identified 405.5 km (252 mi) of the Platte River from the Lexington bridge east to the Platte’s confluence with the Missouri River. In designating critical habitat, USFWS is required to use the best scientific and commercial data available.

The critical habitat final rule for the NGP piping plover population (Fed. Regist. 67 (176): 57638 [2002]) reports that:

  • The best scientific and commercial data available were used.

  • Conservation of the NGP breeding population of piping plovers undertaken by local, state, tribal, and federal agencies operating within the range of the species since listing in 1986 was reviewed.

  • Steps necessary for recovery of the NGP piping plover population were identified in the NGP plover recovery plan (USFWS 1988).

  • Available information pertaining to the habitat requirements of the species (including new material since preparation of the recovery plan in 1988) was reviewed. Specific sources of information included data in reports submitted during Section 7 consultations and by biologists holding Section 10(a)(1)(A) recovery permits, the 1994 technical agency review draft revised recovery plan for piping plovers breeding on the Great Lakes and NGP (USFWS, unpublished material, Dec. 23, 1994), research published in peer-reviewed articles and presented in academic theses and agency reports, annual survey reports, regional geographic information system coverages, and personal communications from knowledgeable biologists.

The committee found that scientific knowledge supported designation of critical habitat for the piping plover on the Platte River by DOI agencies in 2002. USFWS used methods that are comparable with those successfully used for designation of critical breeding habitat for the Great Lakes population of piping plovers. The only major difference between the two efforts was that substantial data on habitat use and breeding biology of nesting piping plovers were not available in the Great Lakes case, but were available in the case of the central and lower Platte River. USFWS’s use of the data in designating the Platte River critical habitat resulted in greater confidence in the designation than would be possible without such data (Figure 6-8). Use of the central and lower Platte River by plovers is better known than use of most of the designated critical habitat elsewhere. For the NGP population of piping plovers, the PCEs of critical habitat are the essential habitat characteristics for the biological needs of courtship, nesting, sheltering, brood-rearing, foraging, roosting, intraspecific communication, and migration (Fed. Regist. 67 (176): 57638 [2002]). In the case of the central and lower Platte River, these characteristics are sparsely vegetated channel sandbars, sand and gravel beaches on islands, temporary pools on sandbars and islands, and interface zones with the river (USFWS 1988; Fed. Regist. 67 (176): 57638 [2002]).

Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

FIGURE 6-8 Piping plover nesting locations along critical habitat section of central Platte, 1987-2003. Source: J. Runge, USFWS, unpublished material, 2004.

The hydrology and geomorphology of the Platte River, driven by drought and flood cycles, create and maintain these habitat components. The water flow, sediment, vegetation, and channel morphology influence the abundance and availability of insect foods and brood and nesting cover. The variability of flow in the channel causes recycling of sediment in ever-changing forms, including sandbars and beaches that form, are destroyed, and reform in new configurations. The dynamic, sandy surfaces of these forms remain little vegetated and provide diverse habitats for feeding, nesting, and brooding. USFWS specified the “presence of dynamic ecological processes” (produced by the river processes) as the overarching PCE—an accurate, reasonable decision supported by published studies and agency reports (e.g., Faanes 1983; Haig 1992; Ziewitz et al. 1992; Lingle 1993a; Fed. Regist. 67 (176): 57638 [2002]).

In addition to the general systematic properties of the central and lower Platte River, USFWS investigated four more-specific characteristics required for designation of critical habitat. The designated habitat must be currently or recently used for breeding, or have a documented history of occupation by breeders, be deemed potential breeding habitat and fall

Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

within geographic boundaries of distribution, and be included in habitat complexes essential to the conservation of the species. The committee concluded that USFWS followed a specific process in addressing those points, as reviewed in the following paragraphs, and the committee found that the procedure resulted in scientifically valid designation of critical habitat for the piping plover on the central and lower Platte River.

USFWS began its designation process by using recovery plans (USFWS 1988; USFWS unpublished material, December 23, 1994) to identify the specific recovery needs of NGP piping plovers (including birds nesting on the central Platte River). The plans provided records of geographic nesting locations at the time the documents were prepared and included descriptions with quantitative data on habitat variables at some breeding locations. USFWS obtained and used more recent breeding-survey data to create a complete and current (as of 2001) database that reflected the best available knowledge of historical and current breeding by piping plovers. The database included inputs from census and survey efforts coordinated with USFWS, plover nest records maintained by the Nebraska Wildlife Division every year since 1984, and prelisting records. The result is a picture of plover distributions along the Platte River that constitutes the best available information on the subject. Recent methods for assessing plover use have increased in sophistication, particularly by including GPS, so the later records may be more accurate than the earlier ones. An important decision by USFWS was to designate the entire reach between Lexington and the Missouri and Platte confluence as critical habitat. The alternative was to attempt to identify individual parcels along the reach that had been used in the past by nesting plovers. For some plover nest habitat in North America, that parcel-by-parcel approach is possible, and it has been done to some extent in the Great Lakes region. However, given the ephemeral nature of plover habitat along the major prairie rivers including the Platte, sites are not predictable or stable. Extensive literature on plover habitat justifies the decision to designate the entire reach, but there will be some parcels in the designated area of critical habitat that do not meet all the requirements every year. USFWS (Fed. Regist. 67 (176): 57638 [2002]) correctly pointed out that because of the nature of the northern Great Plains, some of the designated habitats will not have the PCE every year but must have them over time to be considered critical habitat.

The decision to designate the entire reach was a practical solution. USFWS initially declined to designate critical habitat on the central and lower Platte River because it deemed the task too difficult and essentially infeasible. The agency did not designate critical habitat for the interior least tern for the same reasons. In response to legal action, the agency undertook the task at a relatively coarse scale for the Lexington to Missouri confluence segment of the river, but could not resolve a finer resolution down to parcel

Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

scale. The committee recognizes that the extensive investigation required to designate critical habitat at the parcel scale was not possible within a reasonable period because recent land-use changes—including the conversion of abandoned sand mines to urban housing, shoreline development around Lake McConaughy, and expansion of the use of the river bed by off-road vehicles—are proceeding too quickly.

INTERIOR LEAST TERN

The least tern (Sterna antillarum) is the smallest of North American terns. Adults weigh about 40-45 g and are distinguished from other small terns during the breeding season by a black cap and eye stripe, prominent white forehead, gray back and wings, and white underside. The least tern’s bill is orange or yellow and has a dark tip (Figure 1-5). The rarity of the least tern and loss of its nesting habitat have led the USFWS to designate as endangered, threatened, or a species of concern across most of its range.

Distribution

Least terns breed along coastal beaches and major interior rivers of North America. They winter on marine coastlines of Central America and South America (Thompson et al. 1997). The size of least tern populations are difficult to estimate accurately because of the wide geographic distribution of the species and a weak understanding of population limits. Thompson et al. (1997) estimated that at least 55,000 individuals could be found across all breeding sites in the United States during the 1980s and 1990s. Birds nesting on the Platte River are part of an “interior population” that was listed as endangered nearly 2 decades ago (Fed. Regist. 50:21784 [1985]).

Historical distribution and nesting records exist for interior least terns in Nebraska since the 1800s (EA Engineering, Science and Technology, Inc. 1988; Lutey 2002). Before listing, nesting and migration records were reported for 56% of the counties in Nebraska, but estimates for the size of the statewide population are not available until USFWS listed the population as endangered in 1985.

Historically, the interior least tern bred along the Colorado, Red, Rio Grande, Arkansas, Missouri, Ohio, and Mississippi river systems in a vast region covering much of the mid-continent region of the United States (Thompson et al. 1997). In this region, interior least terns often nest near piping plovers and for this reason are thought to have similar habitat requirements. Although the boundaries of the breeding range of least terns in the 1990s were similar to its boundaries a century earlier, the distribution of least terns in that area is now much more fragmented than previously, especially

Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

distribution of the interior population (Thompson et al. 1997). Kirsch and Sidle (1999) examined the status of the interior population after listing and reported that in 1995 the rangewide population exceeded the recovery goal of 7,000 pairs, primarily because of a tripling of tern numbers along the lower Mississippi River. They concluded that the increased population size was not a result of increased reproduction but probably reflected both more-thorough surveys and possible immigration of terns from the Gulf Coast. Kirsch and Sidle (1999) noted that the number of breeding pairs in most interior locations, including the Platte River, had not reached recovery goals.

Breeding terns in Nebraska typically co-occur with piping plovers on sandbars, on reservoir shorelines, in commercial sand mines, and at other artificially created sites along three major rivers (Figures 6-1, 6-2, and 6-3). In the northeastern portion of the state, nesting occurs along about 64 km (40 mi) of the upper Missouri River shared with South Dakota and along 153 km (95 mi) of the lower Niobrara River. Farther south, terns are found along 386 km (239 mi) of the central and lower Platte River from Lexington downstream to the confluence with the Missouri River. Breeding also occurs at Lake McConaughy in western Nebraska and on the Middle Loup and Loup Rivers in central and eastern Nebraska (Figure 6-4). Missouri River habitat is shared with South Dakota and population estimates for Nebraska typically do not include the Missouri River (John Dinan, NGPC, pers. comm., May-September 2003).

The structure of the tern population using the Platte River habitat is partly determined by the movement and distribution of birds during their lifetimes. Lingle (Lingle 1993c) examined the movements of interior least terns that had hatched along the Platte or that were first encountered as adults nesting along the river (1993c). From 1984 to 1989, Lingle banded 704 birds at locations between Lexington and Grand Island. He found that 29% of the terns banded as adults returned to their banding site, 26% returned to the site where they had hatched, and the rest died or moved to other locations. Emigrating Platte River terns were observed as far away as Stafford, Kansas: a tern chick banded in 1987 was found nesting at Quivira National Wildlife Refuge in 1990. An adult banded at Quivara was recorded on a nest near Kearney (dispersal distance, 170 mi or 274 km). Birds banded on the central Platte were found on the lower Platte and vice versa. The study strongly indicates that the interior population is open on spatial scales beyond that of the central Platte River, and that terns from the Platte River are capable of moving substantial distances between breeding seasons. Tern use of the Platte River is seasonal. Interior least terns arrive on the Platte River in early to middle May. After arrival, they locate mates, construct shallow nests on sand or gravel substrate, and lay eggs that are incubated for about 20 days. Egg-laying occurs from late May to middle

Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

July, depending on river levels and the ability of birds to locate suitable habitat. Birds renest if an entire clutch is destroyed. First hatching typically occurs in June but can extend to late July. Chicks are able to fly after about 20 days. After fledging, young birds feed and mature in the Platte River ecosystem until autumn migration in late July to early September. A mated pair of interior least terns, along with their offspring, remain on the Platte for about 4 months.

Influence of Current Central Platte Conditions on Survival of Interior Least Terns on Platte River

All species, including interior least terns, have specific requirements for survival, including space for population growth and normal behavior; nutritional or physiological requirements (such as food, water, air, light, and minerals); cover or shelter; sites for breeding, reproduction, and rearing of offspring; and habitats that are protected from disturbance or representative of the historical geographic and ecological distributions of the species (Fed. Regist. 67 (176): 57638 [2002]). Ensuring that those requirements are met is a key focus of conservation efforts. In addition to habitat for nesting, interior least terns require adjacent habitat for foraging. Unlike plovers, interior least terns do not obtain food in their nesting territories.

To fledge young along the Platte River, interior least terns require bare or nearly bare alluvial islands or sandbars, favorable water levels during the nesting season, and food. Island habitat is critical because nests that are on islands have lower rates of predation than nests on the mainland. Interior least terns forage in open river channels with pooled or slow-flowing water less than 15 cm deep. In general, terns forage within 0.8 km (0.5 mi) of riverside nest sites, but they may forage up to 2.4 km (1.5 mi) from sandpit colonies. Habitat variables were quantified by Ziewitz et al. (1992) and used to prepare a description of suitable habitat (USFWS, unpublished material, June 16, 2000) (Table 6-5).

A number of independent investigators have studied breeding habitat of least terns across their broad geographic range in North America (Thompson et al. 1997). Several studies have focused on interior least terns in Nebraska (e.g., Sidle and Kirsch 1993; Ziewitz et al. 1992; Kirsch 1996; Ducey 1988; Faanes 1983; Corn and Armbruster 1993). One study quantified environmental variables at interior least tern nest sites along the Platte River (Ziewitz et al. 1992). That effort focused on both terns and plovers and is widely cited in documents from which water-management recommendations were established for the central Platte River. Study methods for interior least terns were similar to those described above for piping plovers.

Using aerial videography, Ziewitz et al. (1992) obtained records of the river during the summer of 1988. They analyzed scenes at 26 sites occupied

Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

TABLE 6-5 Habitat Characteristics Important to the Interior Least Tern

Interior Least Tern Habitat

Observed Measurements of Habitat Parameters (OMHPs)

Preliminary Goals for Habitat Management

References

Riverine habitat

 

Channel and sandbar characteristics

 

 

 

Channel width

975-1,554 ft

≥900 ft, initially

Ziewitz et al. 1992

Kirsch 1996

Sandbar area (early June, at nest initiation)

0.3-3.58 acres

Variable

Ziewitz et al. 1992

Kirsch 1996

Mean elevation above 400-cfs stage (early June, at nest initiation)a

0.2-2.0 ft (mean, 0.4 ft) above stage at 400 cfs

Low, ephemeral sandbars; high enough to provide dry, bare sand during nesting season

Ziewitz et al. 1992

Maximal Elevation above 400-cfs stage (early June, at nest initiation)a

0.4-4.4 ft (mean 2.7 ft) above stage at 400 cfs

Low, ephemeral sandbars; high enough to provide dry, bare sand during nesting season

Ziewitz et al. 1992

Vegetation

 

 

 

Cover: at nest site

On sandbar

<5-20% (June and July)

28% (May and June)

Sparsely vegetated or unvegetated

Schwalbach 1988

Ducey 1988

Faanes 1983

Height (at nest site)

<2 ft (May and June)

Same as OMHP

Schwalbach 1988

Ducey 1988

Forage

 

 

 

Proximity to forage fish

0-2 mi

Typically within 1 mi

Within 1 mi

Smith and Renken 1990

Faanes 1983

Size of forage fish

2-10 cm

Same as OMHP

Atwood and Kelly 1984

Dugger 1997

Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

Sandpit habitat

 

Sand and gravel operation characteristics

 

 

 

Proximity to river and forage

0-2 mi

Smith and Renken 1990

Sidle and Kirsch 1993

Wilson 1991

Total surface area

1.5-197 acres (mean, 18.3 acres)

Sidle and Kirsch 1993

Water surface area

1-150 acres (mean, 27.4 acres)

Sidle and Kirsch 1993

aTo compare water-surface-elevation datum from different nest sites to a common elevation datum, original data were adjusted to yield elevation measurements as if all central Platte River data were collected at a discharge of 11.3 cms (400 cfs).

Source: USFWS, unpublished material, June 16, 2000.

Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

by plovers and terns and compared physical characteristics of occupied and vacant sites (every 3 mi along a river transect). The study showed that more terns nested on the lower Platte River than on the central Platte and that birds nested on river segments that were wider and had larger areas of unvegetated or sparsely vegetated sandbars than on vacant sites. Channels were wider and larger and sandbars higher on the lower Platte River than on the central Platte, and nests on the lower Platte had greater clearance above the river water line when initiated than did those on the central Platte. The average elevation of nests was higher than that of vacant sites available on the lower Platte, but lower than that of vacant sites available on the central Platte River. On the basis of those results, Ziewitz and colleagues concluded that habitat availability may be limiting populations on the central Platte River. Current reports describe riverine habitat as nonexistent along the central Platte (Erika Wilson, USFWS, pers. comm., July-September 2003; John Dinan, NGPC, pers. comm., May-September 2003). Recent flows are so low that new sandbars are not being created and old sandbars have become unsuitable because no scouring has occurred to prevent vegetation from colonizing. In 1988, about 60-65 pairs of interior least terns nested on the central Platte River; Ziewitz et al. (1992) contended that later reproductive success and nesting activity were compromised by low flows, which were punctuated by sudden water peaks in July that flooded nests. Tern nesting efforts between 1988 and 1996 varied between about 60 and 90 pairs per year on the central Platte River. From 1997 to 2001, the number of pairs attempting to nest ranged downward from 45 to 15. From 2001 to 2003, the number of pairs on the river was below 12. Since 1991, interior least tern productivity on the river has been no greater than two chicks fledged per pair; no chicks fledged in six of those years, and none during the last five.

Reproductive success of interior least terns appears to be influenced primarily by water-flow regimes. The birds require sandbars or small islands that are relatively free of vegetation and are at sufficient elevation for eggs and chicks not to be flooded. Sandbars and islands also provide isolation from mammalian predators. The relationship between water levels and site occupancy is well documented by numerous authors throughout the breeding range of least terns in North America. In a study by Lingle (1993b), predation and flooding accounted for 74% of nest failures in Platte River habitat. Predation was the greatest source of nest failures in sandpits, followed by human disturbance and weather.

USFWS and NGPC collect and maintain data on interior least tern nesting. Methods used to monitor terns in the river are the same as or similar to those used by biologists elsewhere in the range of the species in North America. Protocols for monitoring breeding pairs, estimating productivity, and reporting results have been formalized. Since the interior

Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

population of the least tern was listed, nesting records for the Platte River have been among the most consistently collected and maintained. Kirsch and Sidle (1999) reviewed the status of the interior population and stated that “methods, level of effort, and degree of coordination that biologists use on the upper Missouri, the Platte, and lower Mississippi rivers are needed throughout the interior breeding range”; that supports the idea that data used to recommend conservation and management strategies for interior least terns on the Platte River have been drawn from appropriate scientific approaches. Because piping plovers and interior least terns typically co-occur on the Platte River, the species are surveyed at the same time, and similar methods are used, as described earlier in this chapter.

Our findings are essentially the same as those reached for the piping plover. According to discussions with USFWS and NGPC biologists (Erika Wilson and John Dinan, respectively), who manage the interior least tern database and survey for nesting birds, only small numbers of terns have attempted to nest along the central Platte River over the last 5 years. All have failed to produce young. Failure is attributed to low water flow and lack of dynamic ecosystem processes appropriately timed to create new island and sandbar habitat without flooding nesting pairs. Therefore, current conditions on the central Platte River appear to be compromising the continued existence (survival) of the interior population of least terns. That conclusion was based on the following observations: few terns have attempted to nest on the central Platte River in the last 5 years, no eggs have hatched or chicks fledged along the river during the last 5 years, water levels deemed necessary to produce nesting habitat have not been reached on the central Platte in the last 5 years, and loss of habitat along the river is forcing birds to use alternative sites that tend not to offer adequate protection from predation and flooding and do not provide habitat conditions needed for survival of adults and chicks. In addition, loss of central Platte River habitats from the geographic range of the interior population of least terns reduces breeding options for an already vulnerable regional population.

Influence of Current Central Platte Conditions on Recovery of Interior Population of Least Terns

For many threatened and endangered species for which a recovery plan has been prepared, a population goal is set that must be reached before the species or population can be removed from the federal list. Additional criteria are usually stipulated for recovery. For the interior population of the least tern, the recovery plan (USFWS 1990) states that “in order to be considered for removal from the endangered species list, 1) interior least tern essential habitat will be properly protected and managed and

Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

2) populations will have increased to 7,000 birds.” The population goal is broken down into five river systems—Missouri River, Mississippi and Ohio Rivers, Arkansas River, Red River, and Rio Grande. The Platte River is included in the Missouri River system, for which specific recovery guidelines have been promulgated: the number of birds in the Missouri River system must increase to 2,100 adults; essential breeding habitat will be protected, enhanced, or restored; and breeding pairs will be maintained in a specific distribution (listed by state and river) for 10 years (assuming that a minimum of four censuses have been conducted). The recovery goal is 1,520 adults in Nebraska, including 750 adults along the Platte River. Lutey (2002) reported the average numbers of interior least tern pairs at sites along the Plate River from 1987 to 1998—South Platte River (one), North Platte (Lake McConaughy) (six), upper Platte (13), central Platte (74), and lower Platte (187)—for a total of 550 individuals, not counting six pairs at Lake McConaughy. From 1991 to 2001, interior least tern nesting on the central Platte River decreased by 47% (John Dinan, NGPC, pers. comm., May-September 2003).

Like recovery of the piping plover, least tern recovery will require more than the return of a more favorable Platte River hydrograph. Human activities not directly associated with habitat loss, including harassment of adults and young, and destruction of nests also contributed to reducing local populations. Losses of least terns, their offspring, and nests to predation from natural and exotic sources will need to be evaluated and most likely mitigated in some form. Also, as in the case of plovers on the Platte River, the role of contaminants in compromising breeding success needs study to assess its effects on least tern populations.

Population Viability Analysis for Least Terns

Two PVAs have been completed for least terns: one for birds in the Platte River (Boyce et al. 2002), and one for the California least tern (Akçakaya et al. 2003). The models are completely different in structure and sensitivity to altered vital rates. The model by Boyce et al. (2002) determines extinction probability from time-series data on population sizes, so the only parameters for the model are variance in population growth rate (as determined by changes in population size across years), initial population size, and carrying capacity. In contrast, the model by Akçakaya et al. (2003) is a matrix-based, stochastic simulation model of a metapopulation that uses RAMAS and includes age-specific vital rates.

The Boyce et al. model predicted a high probability of persistence of least terns on the central and lower Platte River—an almost 100% probability of persisting in at least one location for 100 years. In addition, it found that increased connectivity between the populations increased persis-

Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

tence probability. At least two problems, however, plague this type of PVA approach. First, sensitivity to altered vital rates cannot be evaluated, because data entering the model come solely from population size. How much immigration contributes to persistence, for example, cannot be assessed, even though it could be essential to population persistence. Second, there is some concern that this type of model is not able to predict persistence very far into the future. Work by Fieberg and Ellner (2000) and Ludwig (1999) suggests that even with perfect knowledge of population size, estimates of extinction probabilities based on this approach are accurate only to 20% of the length of the time series. That means that the 11 years of least tern data used by Boyce et al. might validly predict only 2 years into the future. The authors’ recognition that the central Platte River birds contribute substantially to the joint persistence of the populations is correct in the context of the model, but the model makes implicit assumptions about each of the local populations, for example, that Platte River birds are self-replacing (the population is not maintained in part by immigration), which probably is not valid. Consequently, it is difficult to determine what this model offers for predicting the contribution of Platte River birds to the persistence of their own population, or that of the regional population.

The model by Akçakaya et al. (2003) is designed for terns in California but is more useful in a Platte River application. The model treats clusters of colonies or individuals within 5 km (3.1 mi) of one another as elements of the same population. It includes age structure, year-to-year changes in survival and fecundity, regional catastrophes (such as strong El Niño-Southern Oscillation events), and local catastrophes (reproductive failure due to predation). The period used to assess risk of extinction and substantial population declines is 50 years. Lacking local data, they used data on dispersal from a Massachusetts population (Atwood 1999) and included spatial and temporal autocorrelation. Although Akçakaya et al. had no evidence of density dependence in reproductive success, they assumed that reproductive success declined when populations were at high and low population densities. The high-density effect is programmed to occur near carrying capacity, and an Allee effect occurs when a colony has five or fewer females. Sensitivity analysis was carried out by rerunning the model using low, medium, and high values for parameters and quantifying the effects on population viability. The model predicted that during normal (noncatastrophe) years, the finite rate of increase (λ) is ~0.994, which converts to a slow annual decline (-0.6%). In a revised model, λ = 1.07, so slow average population growth was predicted. Using medium values of all parameters, the model predicts zero risk of extinction or substantial decline in the next 50 years. Using lower vital rates, the model predicts a higher risk of decline but a low risk of least tern extinction in California over the next 50 years. The model was sensitive to survival and fecundity rates and moderately sensitive to carrying

Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

capacity. It appears that reproductive success, philopatry (tendency to return to hatching site to nest), and site fidelity are all greater for California birds than for Platte River birds.

A PVA commissioned by the committee sheds some light on several points that are important for the recovery of Platte River interior least terns. Analyses suggest that the least tern interior population is likely to persist for 200 years and that the Platte and Loup River birds contribute minimally to its persistence. Reproductive success is zero or very low on the Platte, so birds breeding on the Platte are probably coming from outside Nebraska. Therefore, Platte River habitat is serving as a “sink”; the population there is not replacing itself with local reproduction and could be drawing breeding birds into habitat not suitable for reproduction. Those conclusions, however, are based on a hypothetical spatial structure of least tern populations; before population persistence and the importance of the Platte River birds to the larger interior population can be accurately evaluated, the movements and habitat use of terns in the interior population must be understood.

The committee’s conclusions about effects on survival of interior least terns and piping plovers were similar. As in the case of the PVA for the piping plover, the committee used available data to ask several basic questions about the persistence of the Platte River interior least tern population. Again, it is important to note that data are not sufficient to describe in terms of parameters for a PVA with robust predictive capacity. Nonetheless, the committee built a simple three-population model for the interior United States: Niobrara River (eventually adding the Missouri River birds to this population would be reasonable), Platte and Loup Rivers, and all birds outside Nebraska. The model allowed the committee to consider the fate of the interior population after excluding Platte River birds, assuming a completely isolated population of 920 adult birds. The committee also considered the fate of Platte River terns with and without immigration and assuming increased reproductive success.

For the least tern PVA, the committee again used VORTEX (9.3). There is no information about the population structure of interior least terns in Nebraska (or, for that matter, the interior United States). Although breeding sites are dynamic—they are lost and created—it had to be assumed that on the average the number of sites and dispersal among sites were consistent. As data become available to evaluate that assumption, a more sophisticated PVA can be developed. Consequently, the PVA is exploratory, so the model and its results should be viewed as hypotheses to be tested.

The model used initial population sizes (N) and carrying capacity (K) from the recovery plan (Table 6-6). Survival data came from Thompson et al. (1997) or from California populations (Table 6-7). All populations are

Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

TABLE 6-6 Summary Input for Interior Least Tern PVA Initial Model

 

Interior United States Excluding Platte and Loup Rivers and Niobrara River Populations

Platte and Loup Rivers

Niobrara River

Initial population size (K)

7,000

920

200

Initial carrying capacity (N)

5,900

700

200

 

Source: Reed 2003.

assumed to have the same life-history parameters: age at first breeding, 2 years; longevity, 20 years; maximal clutch size, three eggs; adult survival, 85%; survival from fledging to age 1 year, and from age 1 year to age 2 years, 56% (Akçakaya et al. 2003); environmental variance for reproduction and survival, 20% of the parameter value; no inbreeding depression; no catastrophic environmental events; no change in carrying capacity over time; and no cost to dispersal. It appears that the percentage of nests that fail each year on the Platte River is increasing, so the base model was run with all populations experiencing a 20% failure rate, then with the Platte

TABLE 6-7 Some Parameter Values for California Least Tern PVA (Akçakaya et al. 2003) and Parameter Values from Platte River Data

Model Parameter

California PVA

Platte River Data

Reproduction (fledglings/female per year)

0.6964

0.47 (Kirsch 1996 as cited in Thompson et al. 1997)

Survival, hatching to fledging (normal years)

0.6237

0.504 (Smith and Renken 1993)

Survival, hatching to first breeding (normal years)

0.16

Unknown

Survival, from 1 to 2 years

0.5627

Unknown

Survival, from 2 to 3 years, and 3 to 4 years

0.81

Unknown

Survival after age 4 years

0.92

Unknown

Philopatry

>50%

26% (Lingle 1993c, in Thompson et al. 1997)

Site fidelity

70%

28% (Lingle 1993c, in Thompson et al. 1997)

 

Source: Reed 2003

Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

and Loup River population experiencing increasing failure rates. Fledglings per female surviving per year is lower than in California (Table 6-7) at 0.47 (presumably, this includes nest failures) (Kirsch 1996 as cited in Thompson et al. 1997). The fledgling survival per female as reported by Kirsch is very low; to duplicate this low estimate in the model output required use of high nest failure rates in the model. It was assumed that of the birds whose reproductive efforts did not fail, 70% had one offspring, 28% had two, and 2% had three. Those assumptions generated predicted rates of reproductive success higher than observed (just over one fledgling per nest), so the model was run with increased failure rates. In addition, the base model was run with the Platte and Loup population and carrying capacity removed from the model.

Dispersal rates are unknown, but philopatry and site fidelity appear to be lower in Nebraska than in Massachusetts or California (Akçakaya et al. 2003), so it was assumed that dispersal in Nebraska is higher than in Massachusetts or California. Lower site fidelity is expected in Nebraska because riverine sandbars are less stable than coastal beaches. High immigration from the greater interior metapopulation would inundate model results, so it was held to 1%. Migration between Nebraska populations was set at 30%; it was set at 10% between Nebraska and other interior sites. The model was run with a variety of dispersal values and with the nest failure rate set at 20%.

The current recovery plan has a goal of 920 adults in the Platte River and Loup River combined. A PVA assuming a completely isolated population of this size was run with the base-model parameters, first in a version with immigration and then in a version of the model that assumed increased reproductive success. To model immigration, the population was supplemented with 10 birds (five adult males and five adult females) each year. In a nonimmigration model, nest failure rate was decreased to 10% (from 20%); in another scenario, failure rate was sustained at 20%, but it was assumed that of the birds that did not fail, 50% had one offspring, 42% had two, and 8% had three. Finally, a last scenario used variances in reproductive success and survival that decreased to 10% (from 20%).

Most scenarios run with the model showed poor least tern population persistence probability (Table 6-8); all were characterized by multiple extinction and recolonization events among smaller populations.

The Platte and Loup River population contributed substantially to the persistence of the greater interior metapopulation as it was modeled. That was demonstrated by the increased extinction probability associated with increasing nest failure in the Platte and Loup population and after removing the population and its carrying capacity entirely (Table 6-8). Model results indicate that the interior metapopulation is more adversely affected by

Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

TABLE 6-8 Results of the Platte and Loup Habitat Removed Scenario PVAs, Varying Nest Failure Rate on Platte and Loup Rivers, and Removing Birds of Platte and Loup Rivers and Carrying Capacity (Fixed Dispersal Rate of Interior United States into Nebraska of 0.0005 and Between Nebraska Populations of 0.01)

Scenario

% Nest Failure in Platte and Loup Rivers

Metapopulation

Platte and Loup Rivers

Niobrara River

P(ext)a

Sizeb

Mean Timec

P(ext)a

Sizeb

Mean Timec

P(ext)a

Sizeb

Mean Timec

(1) Base

20

0.729

216

114

0.757

57

108

0.745

51

107

40

0.788

232

110

0.808

56

103

0.811

52

103

60

0.840

184

104

0.861

45

95

0.855

41

95

(2) Platte and Loup Rivers gone

20 (overall failure)

0.761

280

112

0.803

52

100

(3) Base, 20% failure

Dispersald

1

0.729

216

114

0.757

57

108

0.745

51

107

3

0.658

111

127

0.665

59

123

0.669

55

123

5

0.342

96

125

0.287

68

125

0.414

64

128

aProbability of extinction.

bMean N at 200 years for those which lasted.

cMean extinction time (years) for those which went extinct.

dImmigration rate from interior United States into Nebraska; fixed rates out of Nebraska (10%), and between Nebraska populations (30%).

Source: Reed 2003.

Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

Platte and Loup habitat serving as a sink than by an outright loss of Platte and Loup River habitat. That is reinforced when immigration from outside Nebraska is increased, causing a drain from the larger regional population to the smaller, sink population. The same pattern was found by Plissner and Haig (2000) for piping plovers.

Under current conditions, an isolated population of 920 adults in the Platte River and Loup River combined would have a poor probability of persisting (Table 6-9). Increasing reproductive success substantially did not have much effect on persistence, but decreasing variance in reproductive success and survival improved persistence (Table 6-9). Under the last scenario, populations almost always persisted, and population sizes tended toward 80% of carrying capacity, on the average, after 200 years. Adding immigrants equivalent to an influx of 10 adults, or 1% of the local carrying capacity, improved persistence probabilities almost to zero probability of extinction (Table 6-9).

Under the simulated model structure and conditions, the least tern metapopulation is unlikely to persist for 200 years. The Platte and Loup River population contributes to the persistence of the metapopulation but is a negative factor if reproductive success is very low and immigration into Nebraska is high. Clearly the model is sensitive to variance in reproductive success and survival, so it is important to determine the best estimates for those parameters.

Immigration into the population also had a considerable effect in increasing population persistence. That supports the need to understand the spatial structure of the tern populations in this area.

TABLE 6-9 PVA Results for Single Platte and Loup River Population with Different Amounts of Immigration

Scenario

P(ext)a

Sizeb

Mean timec

Isolated, base model

0.973

107

78

10 adult immigrants/year

0.000

781

Decrease nest failure to 10%

0.921

193

84

Reduced partial brood lossd

0.854

235

88

Last two combined

0.795

363

91

Same, but variance in reproduction and survival decreased to 10%

0.003

786

123

aProbability of extinction.

bMean N at 200 years for those which lasted.

cMean extinction time (years) for those which went extinct.

d50% had one offspring, 42% had two, and 8% had three.

Source: Reed 2003.

Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

SUMMARY AND CONCLUSIONS

This chapter has reviewed the necessary background to address three questions regarding the piping plover and interior least tern populations of the central and lower Platte River. First, the committee found that habitat conditions on the central and lower Platte River affect the likelihood of survival and recovery of the NGP population of piping plovers. The decline in the river’s plover population has been coincidental with the loss of its preferred habitat, especially in the central Platte River, where suppressed variability in flow has led to reductions in sandbars and beaches and indirectly to increased woodland and reduced open sandy areas. The piping plover population along the Platte River has consistently declined since 1996, and improvement in its numbers is likely to be closely tied to habitat conditions on the river that respond to hydrological adjustments. Breeding along the central Platte River is mostly in artificially created habitats that are not sustainable on a multidecade basis. Recovery requires a reversal of present trends by rejuvenation of a more natural regime of river flows, sediment processes, vegetation, and channel morphololgy.

Second, the committee found that, for the same reasons, the current habitat conditions on the Platte River are likely to affect the likelihood of survival and recovery of the interior least tern. The interior least tern has habitat preferences that are highly similar to those of the piping plover, so what affects one bird population affects the other. Because terns seek sandy, beach-like habitats and because these habitats are becoming less common along the Platte River as a result of flow, sediment, vegetation, and morphology changes, the tern population associated with the river is far below recovery goals. That sufficient suitable habitat stimulates a healthy tern population is demonstrated by components of the least tern population in other subunits of its range.

Third, the committee found that the USFWS designation of critical habitat for the piping plover is based on the best available scientific knowledge at the time of designation (2002). The agency followed reasonable and established procedures in the designation process, drawing on information from peer-reviewed journals and unpublished information, including agency reports and surveys. It consulted with practicing biologists who had specific useful information. USFWS adopted a prudent strategy in deciding on a general designation from Lexington to the confluence with the Missouri River in the absence of substantial time and support to refine the designation to a finer, parcel-by-parcel scale.

As in investigations related to the whooping crane, research into issues related to piping plovers and interior least terns revealed some gaps in data and knowledge. The integrated habitat use by plovers and terns shows the need for knowledge about how multiple species use the same habitat space

Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

and how they interact with other listed and nonlisted species. There is also a need to integrate the Platte River populations of plovers and particularly of terns with their larger populations. Platte River terns, for example, appear to range widely (up to at least 170 mi away from the river), so understanding of the Platte population cannot focus solely on the river. Finally—as is the case with conclusions related to time trends in climatic data, hydrological data, and crane population numbers—conclusions related to trends in the populations of plovers and terns must take into account the longest periods possible rather than focusing on a few years of record.

Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
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Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
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Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
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Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
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Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
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Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
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Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
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Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
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Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
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Suggested Citation:"6 Piping Plover and Interior Least Tern." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
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The tension between wildlife protection under the Endangered Species Act and water management in the Platte River Basin has existed for more than 25 years. The Platte River provides important habitat for migratory and breeding birds, including three endangered or threatened species: the whooping crane, the northern Great Plains population of the piping plover, and the interior least tern. The leading factors attributed to the decline of the cranes are historical overhunting and widespread habitat destruction and, for the plovers and terns, human interference during nesting and the loss of riverine nesting sites in open sandy areas that have been replaced with woodlands, sand and gravel mines, housing, and roadways. Extensive damming has disrupted passage of the endangered pallid sturgeon and resulted in less suitable habitat conditions such as cooler stream flows, less turbid waters, and inconsistent flow regimes. Commercial harvesting, now illegal, also contributed to the decline of the sturgeon.

Endangered and Threatened Species of the Platte River addresses the habitat requirements for these federally protected species. The book further examines the scientific aspects of the U.S. Fish and Wildlife Service’s instream-flow recommendations and habitat suitability guidelines and assesses the science concerning the connections among the physical systems of the river as they relate to species’ habitats.

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