Using Science to Improve the BLM Wild Horse and Burro Program A Way Forward (2013) / Chapter Skim
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2 Estimating Population Size and Growth Rates
2 Estimating Population Size and Growth Rates
Pages 31-60
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From page 31... ...
. This chapter responds to the BLM request for a review of free-ranging horse and burro population estimates, techniques to improve those estimates, and population growth rates.
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From page 32... ...
. The committee received unfavorable comments during the study process from many members of the public regard ing BLM's reports of equid population estimates and assumed or reported population growth rates.
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From page 33... ...
For example, if a BLM aerial survey counted 180 horses on an HMA and 90 percent of the animals were detected, the count was a reasonably accurate population estimate in that the true number of horses occupying the HMA was 200. However, if only 50 percent of the animals were detected, the count would represent a poor population estimate in that the true population size was actually 360 horses.
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From page 34... ...
, provide a flexible framework for implementing closedpopulation and open-population models in estimating abundance and related parameters. Whereas conventional capture-recapture methods for estimating population size (e.g., Otis et al., 1978; Williams et al., 2001)
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From page 35... ...
Two observers independently record the number of animals seen from a given location at the same time. Records are compared to inform population estimates by assessing how many animals or groups of animals are detected by both o bservers and how many are detected by only one observer or the other (Caughley, 1974a; Ransom, 2012b)
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From page 36... ...
Before correcting for detection probability, population size was biased (undercounted) by as much as 32 percent, but estimates accounting for heterogeneity of sighting probability (detection probability)
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From page 37... ...
have been used as genetic tags for individuals. With a capture-mark-recapture design, populations have been surveyed and the resulting data have been analyzed to estimate population sizes.
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From page 38... ...
The committee was informed that populations in years in which no counts were conducted were estimated by multiplying the previous year's population estimate by some assumed population growth rate until another count was conducted (Box 2-1; BLM, personal communication, December 2011)
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From page 39... ...
(1.20) 761 If a detection probability or growth rate is used to adjust counts without empirically measuring either quantity, the values may simply be assumptions or "best guesses," and the adjusted counts would be reported as population estimates with no associated measure of precision.
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From page 40... ...
the growth rate that was applied to obtain the population estimate with the removal data provided in a separate master database. The committee was informed by the national Wild Horse and Burro Program office that the HMA-specific data requested were not aggregated into a central database but were dispersed among the BLM field offices.
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From page 41... ...
Figure 2-2 New NOTE: Population estimates were based on survey records available as of February 2011. DATA SOURCE: Based on information provided to the committee by the Bureau of Land Management in December 2011.
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From page 42... ...
SOURCE: Survey response from the Bureau of Land Management, February 2012. Assuming that the reported data on the 40 sampled HMAs generally represent the procedures routinely used by BLM to enumerate horses on all HMAs, the committee made several generalizations about counts on all HMAs.2 Most counts are obtained with aerial surveys in which an entire area is surveyed in an effort to obtain a complete count of the horses occupying an HMA with no attempt to apply sampling methods or to estimate the proportion of animals that were undetected.
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From page 43... ...
TABLE 2-4 Example of Horse Inventory Data on Desatoya Herd Management Area in Nevada, Showing Irregular and Inconsistent Survey Methods and Incomplete Records Annual Percentage Population Population Date of of Area Adjustment Year Estimate Count Count Type of Craft Inventoried Method of Count 2000 304 August Jet Ranger 100 Direct 2001 294 December 1 80 Direct 2002 2003 2004 February 4 2005 2006 2007 238 April 7 Jet Ranger 100 Direct 2008 2009 2010 434 April 10 Jet Ranger 100 Direct 2011 543 July 11 Jet Ranger 100 Direct NOTE: These data provide an example of the difficulty of understanding how annual population estimates were derived from the survey count data. SOURCE: Survey response from the Bureau of Land Management, March 2012.
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From page 44... ...
The committee assumed that all population estimates were derived in some fashion from survey count data (as described and illustrated in Figure 2-1) , and the description of the process used to develop annual population estimates provided by the national Wild Horse and Burro Program office supports this assumption: When the data [annual HMA population estimates]
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From page 45... ...
As with detection probability, when it seemed plausible that a population growth rate was used to project population estimates for years in which no counts were conducted, the committee calculated the assumed growth rate by dividing the second year's annual population estimate by the previous year's annual population estimate to obtain an estimate of λ, that is, the population growth rate. The resulting values (reported as percent growth)
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From page 46... ...
The committee found that a substantial proportion of the HMA estimates published by the national office did not correspond to the ones the committee received from the field offices; discrepancies ranged from modest to many hundreds of animals. In addi tion, all HMAs in the reported national statistics had a population estimate for all years, whereas a substantial proportion of the HMA records that the committee received from the field offices had no population estimates reported for some of the years.
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From page 47... ...
Adherence to them will greatly improve the utility of equid population estimates. Second, in response to the widely held perception that free movement of animals among adjacent HMAs confounds inventory procedures and reduces the ability to interpret counts, managers have subjectively assessed their knowledge of equid movements among adjoining HMAs and aggregated 93 of 179 HMAs into HMA "complexes." Each complex is composed of two to six areas managed for equids; many HMAs are managed with adjacent
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From page 48... ...
Understanding growth rates is important for efficient and effective management of free-ranging equid populations. Knowing population growth rates gives managers the ability to project how quickly populations will increase and when management actions (such as removals or fertility treatments)
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From page 49... ...
Population Growth Rate Estimates Based on Counts The most direct method for estimating growth rate of a population is to obtain counts or population estimates over multiple years. If the population is growing at a relatively constant rate over the period for which counts or estimates of abundance are available, the abundance values, when log-transformed, will be approximately linear.
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From page 50... ...
. Population Growth Rate Estimates Based on Models A more indirect method for investigating population growth rates of free-ranging horse populations is the construction of population models that use age-specific estimates of horse survival and fecundity rates obtained from field studies.
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From page 51... ...
. Population Growth Rate Estimates Based on Horse Age-Structure Data Another source of data that was available to the committee to help in gaining insight into the average growth rates of free-ranging horse populations was the age structure of the horses captured and removed from western rangelands.
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From page 52... ...
The committee used a 5-year moving average with the 1989-2011 dataset of ages of captured and removed horses when calculating the index to have a large sample of captured and removed horses that would be characteristic of the diverse ecological settings of western rangelands and to reduce variation due to the particular subset of horse populations gathered in any given year. The growth rate index generally was 20-25 percent with some indication of a modest increase during the 1990s; but during the most recent decade, the growth rate index was relatively stable or perhaps experienced a slight decline (Figure 2-3)
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From page 53... ...
It was also stated that horses were selected for release to "maintain a diverse age structure." Thus, the committee found little evidence to suggest an overt and consistent bias in the age structure of horses that were removed from rangelands and concluded that the age-structure data can provide a reasonable assessment of the general growth rate of the free-ranging horse populations on public rangelands in the western United States. The committee concludes that the population growth rate index derived from the age structure of captured and removed horses is generally consistent with the herd-specific population growth rates reported in the literature.
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From page 54... ...
. The procedures used for developing annual HMA population estimates from counts are not standardized and often are not documented, but it seems clear that the national statistics are the product of many hundreds of subjective and probably independent judgments and assumptions by range managers and administrators about the proportions of horses counted in surveys, population growth rates, effects of management interventions, and potential animal movements between HMAs.
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From page 55... ...
It is more difficult to estimate the typical or average population growth rate in western horse populations inasmuch as such an assessment would require estimating growth rates in an adequate representative sample drawn from all horse populations managed by BLM. Although the literature provides a relatively large number of growth rate estimates, the studied populations constitute a sample of convenience in that they were selected simply because data for estimating growth rates were available or there was specific scientific or management interest in particular populations.
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From page 56... ...
Consequences for Management The committee's conclusions that there are substantially more horses on public rangelands than reported and that horse populations generally are experiencing high population growth rates have important consequences for management. Population growth rates of 20 percent a year would result in populations doubling in about 4 years and tripling in about 6 years.
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From page 57... ...
1986. Wild Horses of the Great Basin: Social Competition and Population Size.
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1991b. Growth rates of feral horse populations.
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1978. On the Question of Male-Limited Population Growth in Feral Horses (Equus caballus)
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From page 60... ...
2000. A demographic analysis, group dynamics, and genetic effective number in the Pryor Mountain wild horse population, 1992-1997.
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