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

Abstract

This report presents data polinting 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 month]y 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, alsopossibly due to a common, extrinsiemortality 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 196S70 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 LotkaVolterra pattern will not hold for the initial population decline. PREDATION ON JACKRABBITS * Wagner and Stoddart 329 1963a. Food habits of the lynx in Newfoundland. J. Wildl. Mgmt. 27(3):384-390. 1963b. Movements and activiffes of the lynx in Newfoundland. J. Wildl. Mgmt. 27 (3):390-400. 1964. Physical characteristics of the Newfoundland lynx. J. Mammal. 45 ( 1 ): 3647. STONEBERG, R. P., AND C. J. JONKEL. 1966. Age determination of black bears by cementum layers. J. Wildl. Mgmt. 30 ( 2 ): 411414. VAN ZYLL DE JONG, C. G. 1966. Food habits of the lynx in Alberta and the Mackenzie District, N. W. T. Canadian Field-Naturalist 80 ( 1 ): 18-23. Received for pt4blication April 30, 1971. ONEBERG, R. P., AND C. J. JONKEL. 1966. Age The objective of this report is to present data gathered from 1962 to 1970 in northern Utah and southern Idaho data that provide evidence on the possible influence of coyote predation on black-tailed jackrabbit population trends during this period. Jackrabbit populations in this region fluctuate sharply through what local residents sometimes refer to as a 7-year cycle. However, there are no critical, long-term data ( spanning several decades) from which one can make judgments about the regularity or geographic synchrony of these fluctuations. The period of study herein reported witnessed the latter 4 or 5 years of a population decline (1962 to 1967) and 3 successive years of population increase. Subjective observations by local residents in the region place the last period of high population density approximately during the years This content downloaded from 157.55.39.159 on Sun, 18 Sep 2016 05:51:02 UTC All use subject to http://about.jstor.org/terms 330 Journal of Wildlife Management, Vol. 36, No. 27 Aptil 1972 1958 60. Thus, this study was initiated in about the second or third year of a protracted period of decline. Furthermore, the trends observed in our study area appear, again largely on the basis of subjective reports, to have been paralleled by jackrabbit populations over a broad area of southern Idaho, western Utah, and northern Nevada. French et al. (1965) observed a population high in southeastern Idaho in 1959. Synchrony has not been perfectv however, for we observed populations, which were 1 or 2 years out of phase with each other, in intermountain valleys in this region. The tentative hypotheses that we propose must relate only to the latter period of decline and to the first years of recovery, because our research was confined to this period. The relationships and ffie influential factors probably are different in the later years of increase and the initial years of decline. Hence, additional years of data are needed to disclose the processes under way in these unstudied periods of change and, in general, to test the hypoffieses herein proposed. Many individuals helped in these studies, including the crews of students who conducted the drive counts on the study area and those who aided in population collections and telemetry. C. R. Baird, H. G. Goulden, Betty L. Gross, D. Martinsen, R. A. Stefanski, and J. E. B. Stuart all served as laboratory or field technicians, or both. We acknowledge the financial support of the Environmental Sciences Branch, Division of Biology and Medicine of the U. S. Atomic Energy Commission through Contract No. ATf 11-1)-1329. We also acknowledge financial aid of National Institutes of Health Predoctoral Fellowship SFL GM 20803 02, and the Utah State University Research Council and Ecology Center. The coyote and raptor studies were supported by the Research Divison of the U. S. Bureau of Sport Fisheries and Wildlife and by the National Science Foundation Undergraduate Research Participation Program ( Project GY-6131, 1969), respectively.