Using Science to Improve the BLM Wild Horse and Burro Program A Way Forward (2013) / Chapter Skim
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3 Population Processes
3 Population Processes
Pages 61-92
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From page 61... ...
. That is, population processes -- such as population growth rates, age-specific survival rates, natality, and age of bearing first offspring (primiparity)
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From page 62... ...
, r is the intrinsic rate of natural increase, and K is the carrying capacity, that is, the maximum population size that the environment can support as affected by resource abundance. The discrete form of the equation defines the population increment over an interval of time, such as a year, and is expressed as Nt + 1 = Nt + R(Nt[K – Nt]
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From page 63... ...
Density independence is explained further below. Carrying capacity is a concept that has multiple definitions that depend on the situation.
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From page 64... ...
Population size N and the annual population increment ∆N are plotted against time. Point α is the inflection point, at which population growth begins to decrease as the population approaches K
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From page 65... ...
They noted that higher annual variation in juvenile survival as compared to adult survival can arise from multiple causes including increased vulnerability to predation, drought, harsh winters, and factors causing low birth weights and early growth rates. In an unmanaged horse population in Argentina that was approaching carrying capacity, fecundity was affected by density and rainfall, but adult, juvenile, and foal survival rates were not (Scorolli and Lopez Cazorla, 2010)
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From page 66... ...
study of the horse populations in the Pryor Mountains, whose results suggested that forage availability can affect mare condition and thus foaling rates. In addition to the total quantity of food, the quantity of high-quality food items may be diminished when populations are near carrying capacity.
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From page 67... ...
reported that social instability, specifically high rates of turnover among harem males, adversely affect female reproductive success and patterns of age-specific fecundity. He also indicated that increased levels of sexual harassment can lower female body condition and disrupt normal endocrine function.
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From page 68... ...
Thus, mortality was odeled as m a function of food per capita, and food supply was modeled as a function of rainfall. The model predicted a period of population growth following a period when population size was reduced below food-limited carrying capacity by rinderpest.2 Projected population d ynamics varied within a wide range as a result of rainfall and food-supply variation, but the projected population nevertheless reached maximal levels because of density-dependent feedback.
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From page 69... ...
ignored food limitations and carrying capacity in his individual-based model of the Pryor Mountain herd. He presumed that horse populations will be managed below food-limited carrying capacity and therefore not allowed to self-regulate.
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From page 70... ...
(2010) reported that foal survival rate in the Pryor Mountain Wild Horse Range was positively related to precipitation, probably because of the effects of variable forage production on mare condition.
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From page 71... ...
Ellis and Swift (1988) proposed that plant-herbivore systems in climatically variable environments are unlikely to be equilibrial and that traditional concepts of food-limited carrying capacity have relatively little value in predicting herbivore population sizes and dynamics in such environments.
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From page 72... ...
However, rainfall was the primary determinant of zebra population dynamics. In Serengeti National Park, Tanzania, zebra populations have remained roughly constant for decades, despite large changes in wildebeest and other bovid numbers caused by a rinderpest epidemic (Sinclair and Norton-Griffiths, 1982; Grange et al., 2004)
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From page 73... ...
used 11 years of data (1987-1997) to examine again the influence of mountain lions on the horse population in Montgomery Pass Wild Horse Territory.
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From page 74... ...
Another constraint is that among freeranging horse populations, foals are the usual prey, and predation on adults has rarely been documented until the recent studies in Nevada. Population size is not affected as much by foal survival as it is by adult survival (Eberhardt et al., 1982)
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From page 75... ...
. If resulting feedbacks to equid population growth are ineffective or if they have been disrupted by human activities, rangeland ecosystems can be pushed across thresholds into degraded states from which recovery is difficult or impossible (see Chapter 7)
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From page 76... ...
If animals are in poorer condition, mortality will be greater, particularly in times of food shortage resulting from drought or severe winter weather. Indeed, when population growth rate is zero, mortality must balance natality.
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From page 77... ...
Central Australia Berman (1991) studied populations of feral horses in central Australia.
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From page 78... ...
Shackleford Banks Shackleford Banks, a barrier island off the coast of North Carolina, supports a population of free-ranging horses that has experienced increases and decreases in population numbers in response to changes in carrying capacity resulting from management practices. Before the National Park Service (NPS)
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From page 79... ...
Although those changes helped to mediate some of the consequences of crowding while the population was in transition, in the end a new carrying capacity was reached and was accompanied by changes in behavior and vital rates. One of the biggest changes was a reduction in the stability of the harem.
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From page 80... ...
In an ecosystem modeling assessment (see Chapter 7) of the Pryor Mountain Wild Horse Range, it was possible to examine vegetation and animal responses to various horse densities, including the number present in 1970 (Coughenour, 1999, 2000, 2002)
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From page 81... ...
If there were no intervention, herds would reach food-limited carrying i capacity, body condition would decline, natality and survival rates would decline, and more animals would die of starvation. Removals are likely to keep the population at a size that maximizes population growth rate (see Figure 3-2B)
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From page 82... ...
If the population size is above the theoretical inflection point of the logistic growth trajectory (point α in Figure 3-2A) , reductions will increase the annual population growth increment.
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From page 83... ...
Thus, the favorable effects of increases in body condition, longevity, foal survival, and length of breeding season on population growth rate could offset to some extent the adverse effects of contraception on reproduction and population growth rate. That might be termed compensatory population growth; however, it is unlikely that the degree of compensation would be sufficient to overcome the degree to which contraception reduces reproduction and population growth.
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From page 84... ...
. In contrast, populations in two other reserves, which were near their food-limited carrying capacity before the drought, lacked such lightly grazed food reserves and suffered high mortality (Walker et al., 1987, as noted by Gasaway et al., 1996)
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From page 85... ...
Although it has been omitted from some models because populations were believed to be held below food-limited carrying capacity by management or other factors, it was found necessary to include in others when it was an important component of population dynamics, particularly in wild, unmanaged populations of equids. There are basically two approaches to modeling density dependence.
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From page 86... ...
The literature and the case studies show that although density dependence can regulate population sizes, responses will probably include increased numbers of animals in poor body condition and high numbers of animals dying from starvation. Those may be unacceptable outcomes for some stakeholders, particularly those who perceive that they result from human interference with natural processes of dispersal, access to key forage resources, or predation.
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From page 87... ...
A managerially important finding was that free-ranging horse populations are often limited by removals to levels below food-limited carrying capacity, so population growth rate could be increased by the removals through compensatory population growth related to decreased competition for forage. Thus, the number of animals that must be processed through holding facilities is probably increased by management.
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From page 88... ...
2002. Ecosystem modeling in support of the conservation of wild equids: The example of the Pryor Mountain Wild Horse Range.
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From page 89... ...
2000. A dynamic simulation model for evaluating effects of removal and contraception on genetic varia tion and demography of Pryor Mountain wild horses.
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From page 90... ...
1978. On the Question of Male-Limited Population Growth in Feral Horses (Equus caballus)
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From page 91... ...
2000. A demographic analysis, group ynamics, and d genetic effective number in the Pryor Mountain wild horse population, 1992-1997.
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