Forage Interactions and Black-Tailed Jack Rabbit Population Dynamics: A Simulation Model

Abstract

We describe a simulation model that estimates the energy and protein demand, body weight fluctuations, and population density changes of black-tailed jack rabbits (Lepus californicus), and tests the hypothesis that dense populations are limited by forage resources. Using field and literature values for energy and nitrogen requirements for maintenance, growth, and reproduction, we estimated the demand for forage resources by fluctuating jack rabbit populations in Curlew Valley, Utah. The model produced body weight dynamics within error estimates of measured body weights of both sexes for 90% of an annual cycle. Simulated daily energy requirements (161.2 kcal/kg/day) and daily nitrogen requirements (353.6 mg/kg/day) were within 10% of reported empirical values. Sensitivity analysis of model parameters and variables suggests that output is most sensitive to variations in such inputs as digestive efficiency and maximum consumption rate. Simulated populations, at peak densities, consumed < 1% of available forage resources and there was no significant difference at different population levels in simulated energy or protein difference state. We conclude that the observed declines from population peaks, characteristic of this oscillatory population, must be caused by ecological mechanisms other than food resource depletion. J. WILDL. MANAGE. 46(4):1018-1035 Efforts to document and understand the population dynamics of consumers in ecosystems often have been founded upon demographic analyses of population changes. Competing explanations for population fluctuations of mammalian consumers have included both extrinsic and intrinsic mechanisms (see Lidicker 1978 for review). Population dynamics of a species may be related to many variables and dependent upon local conditions so that determining the ecological mechanisms causing population change is difficult. The problems are particularly difficult for medium-sized, free-ranging herbivores such as black-tailed jack rabbits where measurements of factors limiting population size are frequently limited by difficulties in isolating causal factors in the field. Black-tailed jack rabbit populations are known to fluctuate widely over much of the species' range. As a result of demographic studies of jack rabbits in Curlew Valley, northern Utah, Gross et al. (1974) concluded that these changes are of an oscillatory form. Density of jack rabbits in Curlew Valley has changed from 0.12 to 1.02 rabbits/ha, during a single fluctuation. Wagner and Stoddart (1972) showed that coyote (Canis latrans) predation influenced the oscillations in large part. H wever, because jack rabbits increase faster than coyotes, predation mortality is nversely density-dependent. Jack rabbits may be released from predation as the major source of mortality during popu ation increases. Other mortality sources--disease, exhaustion of the food supply or pituitary-adrenal stress, or both-will presumably be needed to reverse the population trend, induce the decline, and enable the now-abundant coyotes once again to assume dominance over the prey (Wagner and Stoddart 1972:341). Periodic food depletion has been suggested as an ecological mechanism that limits population size of many vertebrates (Lack 1954), including brown lemmings (Lemmus sibiricus) (Thompson 1955, Pitelka 1957, Collier et al. 1975), I Present address: Department of Animal Ecology, Iowa State University, Ames, IA 50011. 1018 J. Wildl. Manage. 46(4):1982 This content downloaded from 157.55.39.116 on Sun, 18 Sep 2016 06:26:10 UTC All use subject to http://about.jstor.org/terms SIMULATION OF JACK RABBIT NUTRITION* Clark and Innis 1019 wood-pigeons (Columba palumbus) (Murton et al. 1966), red grouse (Lagopus lagopus scoticus) (Miller et al. 1970, Watson and Moss 1971), and snowshoe hares (Lepus americanus) (Keith 1974). Keith (1974), Keith and Windberg (1978), Pease et al. (1979), and Vaughan and Keith (1981) concluded that food shortage initiates population declines of snowshoe hares. They observed a pattern of higher winter-spring weight loss, decreased reproduction, decreased juvenile growth rate and decreased juvenile survival over winter (Pease et al. 1979:58) that preceded the peak density and slowed the rate of increase of the population. The objective of our study was to examine a similar hypothesis regarding the role of forage availability in determining population change in black-tailed jack rabbits. Such a hypothesis involves complex dynamic relationships between forage availability and jack rabbit demographics. A simulation modeling approach is ideally suited for coping with the complexity and dynamic characteristics of such systems and identifies important relationships and weaknesses in the data (Forrester 1961, Wiens and Innis 1974). Portions of this study were supported by grants from the National Science Foundation (BMS-7420000) to G. S. Innis, the US/IBP Desert Biome (GB-15886) to F. H. Wagner, and the Energy and Research and Development Administration (E11-1-1329) to L. C. Stoddart. This work could not have been accomplished without the cooperation of L. C. Stoddart who provided access to unpublished data. We also thank R. B. Frederick, E. E. Klaas, W. P. Porter, and F. H. Wagner for reviewing a version of this manuscript.