mule deer, bighorn sheep, and domestic animals are present. There is also some evidence that the magnitude of predation on horses by mountain lions may be related to the density of free-ranging horses, greater predation on horses occurring where densities of horses are higher (Andreasen, 2012).
The potential for mountain lions to affect the sizes of populations of free-ranging horses in North America is limited by the fact that most HMAs are in areas that have few mountain lions. The ranges of mountain lions tend to be concentrated in forested areas and at higher elevations (Kertson et al., 2011) and in areas that have mountainous or otherwise broken topography with limited viewsheds. In contrast, many horse populations favor habitats that have more extensive viewsheds. Mountain lions are ambush predators and require habitats that provide opportunities for stalking or finding prey without being seen. Other predators, such as wolves, are more cursorial—capable of pursuing prey across open habitats.
That a large predator, when abundant, can substantially influence the dynamics of free-ranging horses is not surprising inasmuch as black bears (Zager and Beecham, 2006), mountain lions (Wehausen, 1996), and other predators (Ballard et al., 2001; Boertje et al., 2010) have exerted strong influences on ungulate populations. However, the influence of predation on horses in the western United States is considerably limited by a lack of habitat overlap both with mountain lions and with wolves. Another constraint is that among free-ranging 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), and foal survival is strongly affected by other variables (such as weather).
If a population of herbivores were to self-limit, effects on the ecosystem would be expected. This section reviews the theory, expectations, and case-study examples of free-ranging horses in self-limiting circumstances.
Riney (1964) and Caughley (1970, 1976) proposed that, on introduction of a large herbivore into an ecosystem not previously occupied, there would be an initial irruption of the population that would lead to a decline in vegetation conditions, which would in turn lead to a decline in the herbivore population and allow partial vegetation recovery (Figure 3-4). The herbivore-vegetation system would then reach a new equilibrium between plant productivity and herbivore population density in which vegetation productivity and cover may be less than that in a system that does not have herbivores or in a system that is managed for maximal herbivore productivity. The resulting plant-herbivore system may be less productive, have less standing herbaceous biomass, and have a different plant species composition, but it may nevertheless be functional and sustainable. That conceptual model assumes that the vegetation-soil system has the capacity to persist in some form through and beyond the initial period after an introduction, in which it has been heavily used and reduced in function. It also assumes that surviving vegetation components would be adapted to withstand recurrent herbivory and would increase in relative abundance to form a plant community that is more adapted to withstand herbivory. As noted in Chapter 7, under some conditions, productivity of herbivory-adapted plant species may not be reduced by herbivory.