Influence of Coyote Predation on Black-Tailed Jackrabbit Populations in Utah

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The Journal of Wildlife Management







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This report presents data pointing to the possible influence of coyote (Canis latrans) predation on the latter years of decline and first 3 years of increase in a population of black-tailed jackrabbits (Lepus californicus) in northern Utah from 1962 to 1970. The study was conducted in the southern half of Curlew Valley, a 1,200-square-mile desert area dominated by sagebrush vegetation. The density index for jackrabbits in fall in Curlew Valley increased from 40 in 1962 to 61 in 1963, declined to 21 in 1967, and increased again to 185 in 1970. Eighty-five percent of the observed variation in the annual changes in density can be explained by the observed changes in October-to-October mortality rates of adults and by the birth-to-October mortality rates of juveniles. Mortality rates, calculated on a monthly basis, have been quite similar within years among the three life-history stages-juveniles from birth to October, adults from October to March, and adults from March to October-perhaps due to a common mortality factor operative throughout the year. These rates have undergone similar year-to-year variation, also possibly due to a common, extrinsic mortality source such as predation. Mortality rates have been correlated with coyote:rabbit ratios in the area. These correlations, plus extrapolations of the regression lines to zero coyote predation, suggest that coyote predation has been a major source of rabbit mortality from 1962 to 1970. This hypothesis is supported by telemetry data and speculative estimates on the proportion of jackrabbits taken by coyotes. The data indicate that 69 percent of the observed variation in rabbit numbers is associated with variation in the coyote:rabbit ratio. Accordingly, we postulate that coyote predation played an important role in the jackrabbit population trends from 1962 to 1970: hastening, if not primarily causing, the decline from 1963 to 1967 by its impact, and largely, or in part, permitting the increase in rabbits in 1968-70 by its relaxation. In the process, predation pressure on rabbits eased as both populations declined, because the coyotes declined more rapidly (coyote density decreased 87 percent, rabbit density 66 percent). This pattern may have followed a classical, Lotka-Volterra predator-prey oscillation during the studied phase of population change. We surmise that the Lotka-Volterra pattern will not hold for the initial population decline.