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Analysis of mountain lion predation upon mule deer and elk in the Idaho Primitive area Hornocker, Maurice Gail

Abstract

This study was designed (1) to investigate the dynamics of a mountain lion population, and (2) to assess the impact of a population of lions on populations of big-game animals. The research was carried on in the Idaho Primitive area; intensive work was limited to the winter and early spring seasons. Lion population numbers were stable during the three-year study period, and available evidence indicates the present population level existed for some time prior to the start of the study-Intraspecific relationships, manifested through territoriality, acted to limit lion numbers and maintain population stability. Dispersal and mortality, particularly of young individuals, appeared to be important limiting mechanisms. The population was centered around a nucleus of mature individuals well-established on territories, but segments of the population were dynamic, exhibiting an inflow and outflow of individuals from season to season. These transients were composed predominantly of young animals. Strife appeared to be kept to a minimum by a "mutual avoidance" behavorial mechanism. Specific hunting territories were shared but appeared never to be used by more than one lion or family of lions at a time. Individuals, regardless of sex, appeared to respect the presence of another in a specific area. The "mutual avoidance" hypothesis is advanced as an important factor in the maintenance of lion populations. This mechanism provides for the distribution of lions in both space and time without costly-fighting. It also appears to insure greater success in securing large prey animals. Population size of prey species -- mule deer, elk, and bighorn sheep -- was established by making ground and aerial counts each year. Bighorn sheep numbers remained constant during the three-year period, but populations of deer and elk, the principal prey species, increased. The range was considered overstocked by deer and elk. Forty-four elk and 39 deer were recorded as definitely killed by lions during the three-year period. Only two kills of bighorn sheep were found; lion predation on this species appeared insignificant. Seventy-five percent of the elk killed by lions were 1% years old or less and 9% years or older; 57 percent of the deer kills were in these age classes. More "young" than "old" animals were killed. Lions were non-selective in their killing, except for "negative selectivity" in the case of mature bull elk. Factors acting separately or collectively to increase prey vulnerability included prey density, behavior, age, health, inter - and perhaps intraspecific strife, and the lion's predatory characteristics. It was concluded that elk and deer populations were limited by the winter food supply and that predation by lions was inconsequential in determining ultimate numbers of elk and deer. Lion predation, however, is a powerful force acting to dampen and protract severe prey oscillations and to distribute ungulates on restricted, critical range. From the theoretical standpoint, it also appears to be a strong evolutionary force, acting to remove less fit individuals from the population. The effects and influence of such predation are considered of great significance in the maintenance of ecologic stability in wilderness environments.

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