been killed by wolves in 11 packs from 2003 to 2006. Some 7 percent were feral horses, and they made up 12 percent of the total biomass consumed (0.01 ± 0.02 feral horse/pack per day). Despite evidence that wolves prey on equids elsewhere, the committee was unable to identify any examples of wolf predation on free-ranging equids in the United States.
Most predation on free-ranging equids in North America has been attributed to mountain lions. That has been reported by Robinette et al. (1959) and Ashman et al. (1983). Berger (1983c) cited an unpublished report of 21 cases of mountain lion predation on free-ranging horses in the Great Basin; those deaths spanned more than 20 years and had negligible effects on population growth. Feral (but not free-ranging) horses constituted 11 percent of mountain lion diets in Alberta (Knopff and Boyce, 2009). Horses constituted 10-13 percent of adult male lion diets, but female lion diets were almost devoid of horses (Knopff et al., 2010). Overall, mountain lion predation on free-ranging equids in North America is, with few exceptions, considered uncommon (Berger, 1986).
One of the exceptions is the free-ranging horse population on the central California-Nevada border. Turner et al. (1992) examined foal survival rates in the area (the Montgomery Pass Wild Horse Territory managed by the U.S. Forest Service) because there was a ban on mountain lion hunting in California and low hunting pressure in Nevada that led to a high density of mountain lions. The study was conducted from May 1986 to July 1991 by examining the horse and mountain lion populations and documenting deaths of horses. The average annual cohort of foals over the 5 years was 32. The annual survival rates were calculated for foals (0.27), yearlings (0.95), and adults (0.96). From 1987 to 1990, 48 foals were lost; 58 percent were located as carcasses and 82 percent of those were killed by mountain lions. The authors concluded that mountain lion predation had a substantial effect on the demography of that free-ranging horse population. The study was continued, and Turner and Morrison (2001) 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. Their results supported the earlier work of Turner et al. (1992): mountain lions were responsible for the deaths of 45 percent of the foals that were born. Mountain lion predation was also hypothesized as a major factor in limiting horse population growth in an area of southern Nevada where they use high-elevation forested habitats in summer (Greger and Romney, 1999). Those habitats are excellent for mountain lions because of their broken topography.
By and large, research that has addressed the question of predation on free-ranging equids in North America has been limited to anecdotal observations and a few published papers, but at the time of the committee’s review, studies at the University of Nevada, Reno, that should provide more quantitative data were under way. The work in several mountain ranges of western Nevada was examining predation by mountain lions in multi prey systems in which free-ranging horses had various densities. Diet data were being obtained by using information from GPS-collared mountain lions to investigate predation events; more than 700 predation events had been investigated as of June 2012. Ten of 13 collared mountain lions that had access to free-ranging horses regularly consumed horses as prey. Horses were documented to have been consumed as prey by collared mountain lions in eight mountain ranges throughout the study area in western Nevada (Virginia, Pah Rah, Fox, Lake, Wassuk, and Excelsior ranges and Virginia and Smoke Creek Mountains). Preliminary data suggest that in that study area, where free-ranging horses are available as prey, more than 50 percent of the diet of collared mountain lions is made up of horses when diet data on individual mountain lions are pooled. Preliminary results suggest that mountain lions in that multiprey system are generalists at the population level but that some diet specialization occurs at the individual level: some lions select for deer where horses are more abundant, and some select for horses to the near exclusion of other prey items where