Influence of predation by mountain lions on numbers and survivorship of a feral horse population

In an effort to expand our knowledge of the ecology of feral horses (Equus caballus), we initiated a study of the Montgomery Pass Wild Horse Territory (MPWHT), located along the California-Nevada border at the northern end of the White Mountains. We report on 11 years (1987-1997) of data on numbers, productivity, and survivorship of the feral horse population in the MPWHT. The majority of the MPWHT is located in pinyon-juniper (Pinus-Juniperus) w oodland. The adult horse population averaged 150 individuals, with a signif icant decrease occurring across the study. The number of foals born ranged between 29 and 35 through 1993, dropped to 22 to 24 for 1994-1996, and rebo unded to 31 in 1997. Although mule deer (Odocoileus hemionus) are the prima ry prey of mountain lions (Felis concolor), extensive predation on foals oc curred in MPWHT. The average number of foals killed each year by mountain l ions was 13.5 (45.1% of foals produced). There was a significant difference in the proportion of foals killed by coat color relative to the distributi on of colors born into the population. Annual survival (May to April) rate for foals averaged 0.32, ranging from a low of 0.23 during 1987-1988, to a high of 0.48 in 1996-1997. Yearling survival averaged 0.88, ranging from a low of 0.5 in 1994-1995, to a high of 1.0 in 5 of the annual periods. Adult survivorship averaged 0.92, ranging from a low of 0.81 in 1992-1993, to a high of 1.0 in 4 of the annual periods. The lion population was 4 to 5 from 1987 through 1991, increased to 8 in 1992, and then slowly decreased throu gh 1996. Number of lions dropped to 3 animals in 1997. The resident mountai n lion population is significantly influencing number of horses in the MPWH T, primarily through predation of foals. Increased foal survival during the latter part of our study, and especially during 1997, was apparently relat ed to a substantial decrease in number of lions.