landscape, such as natural dry-season grazing reserves that define the dry-season bottlenecks and thus limit herbivore populations to a particular density. Density dependence therefore exists, but it is temporally variable inasmuch as food-limited carrying capacity varies with precipitation and, in seasonally cold environments, with snow cover.

EFFECTS OF PREDATION

Predators prey on wild equids; predation on onagers and zebras has been reported in Asia (Solomatin, 1973) and Africa (Kruuk, 1972; Schaller, 1972), respectively. In Africa, predation may limit some zebra populations (Sinclair and Norton-Griffiths, 1982; Mills et al., 1995). Zebras and other ungulates were not limited by food in Namibia but most likely by predation or disease (Gasaway et al., 1996). Zebra maintained excellent body condition during dry seasons and after droughts. Recruitment rates continued to be high, corresponding to those of a growing population. Such recruitment rates could be balanced only by high rates of yearling and adult mortality, which would presumably be caused by predation or disease. Predation was suspected of being a major population control in a collection of ungulate populations in Kruger National Park (Owen-Smith et al., 2005). Adult zebra survival was strongly related to increasing density, but the steepness of the response indicated that it was strongly affected by prey-switching by lions in response to decreased availability of alternative prey species. Mills et al. (1995) reported that zebra populations in Kruger were influenced by predation but to a smaller extent than wildebeest or buffalo. 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). Very low first-year survival limits the zebra population in the Serengeti, according to Grange et al. (2004), who found evidence that rates of predation on zebras were high and hypothesized that predation potentially holds the population in a “predator pit.” The principal predators, lion and spotted hyena, feed mainly on adult zebra, so it was not clear what the main sources of foal mortality were. Using data from 23 near-natural ecosystems in Africa, Grange and Duncan (2006) reported that zebra abundance relative to that of bovids is lower in ecosystems that have high lion densities and that zebra abundance is not as affected by forage abundance as bovid abundance; this suggests that zebras are more sensitive to predation than are bovids. Rubenstein (2010) reported that 73 percent of lion dung samples contained Grevy’s zebra and 53 percent contained plains zebra hair. One wildlife conservancy had high rates of lion predation on zebra.

Wolves are quite capable of preying on equids. In southern Europe, equids constituted 6.2 percent of wolf diets (range, 0-24 percent) (Meriggi and Lovari, 1996). In Abruzzo National Park, Italy, horses constituted 70 percent of wolf diets; however, unguarded horses are commonly hobbled in that area to prevent long-range movements (Patalano and Lovari, 1993, cited in Meriggi and Lovari, 1996). In northwestern Spain, a population of free-ranging ponies is heavily preyed on by wolves (Lagos and Barcena, 2012). Foal survival rate was very low (0.41), and 76 percent of foal carcasses found were killed by wolves. Van Duyne et al. (2009) reported that wild Przewalski’s horse foals were killed by wolves in Hustai National Park, Mongolia, and cautioned that predation could influence translocation efforts. However, those horses are sufficiently vigilant to survive and reproduce, so perhaps they have not lost essential skills (King and Gurnell, 2012). Wolves in a multiprey system have been reported to prey on feral horses in Alberta, Canada. Webb (2009) reported that one of 36 kills by wolves included a feral horse. Webb (2009) located 192 ungulates that had



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement