Time Lags and Population Fluctuations in White-Tailed Deer

Abstract

We used cohort analysis to evaluate temporal variation in the population density of white-tailed deer (Odocoileus virginianus) at the Canonto Study Area in southeastern Ontario. During continuous study between 1953 and 1986, the population underwent 2 major fluctuations in density, ranging between 1.3 and 7.0 deer/km2. Variation in instantaneous exploitation rates was related linearly to hunting effort, as predicted by the simple random search model used in cohort analysis. Temporal variation in the annual rate of increase of the deer population was largely accounted for by variations in hunting effort and the per capita rate of recruitment. Recruitment rates and growth rates of juveniles showed evidence of delayed, rather than immediate, response to changes in population density. Dynamic interactions between deer and their food supply probably caused similar time lags in both growth and recruitment rates. Such time lags are likely to predispose the population to oscillations, and this tendency was possibly exacerbated by temporal variation in exploitation effort. J. WILDL. MANAGE. 55(3):377-385 Long-term studies indicate that some populations of ungulates exhibit pronounced fluctuations in abundance (Sauer and Boyce 1979, Peterson et al. 1984, Whyte and Joubert 1988). Such long-term fluctuations conceivably stem from a wide range of causes. In exploited populations, temporal variation in harvest rates can lead to pronounced population changes (McCullough 1979, Fryxell et al. 1988). Weather variation over time can also have a strong influence on demographic parameters (Sauer and Boyce 1979, Mech et al. 1987, Owen-Smith 1990). A variety of biotic factors might also contribute to pronounced population fluctuations. Trophic interactions could drive coupled oscillations in both ungulate and vegetation populations (Caughley 1976, McCullough 1979, Sauer and Boyce 1979). This could arise from either delays in recovery of vegetation abundance (Caughley 1976, McCullough 1979) or to induced chemical defenses by attacked plants (Cooper and OwenSmith 1985, Palo 1985, Robbins et al. 1987). Similarly, trophic interactions between ungulates and their predators could contribute to population fluctuations (Gasaway et al. 1983, Peterson et al. 1984). Finally, depressed early growth of ungulates can affect subsequent reproductive success (Albon et al. 1987), which could have a destabilizing influence on ungulate population dynamics. Each of these biotic factors could cause delayed demographic response to changes in population density. We examined the impact of changes in harvest rates and recruitment rates on a fluctuating population of white-tailed deer in southeastern Ontario. We then tested whether there were tim lags in the recruitment response to changes in po ulation density and considered whether d er-plant interactions, deer-predator interactions, or early growth effects could have caused time lags in deer recruitment. Empirical data were gathered by staff of the Ontario Ministry of Natural Resources, under the direction of the Wildlife Research Section, Wildlife Branch. Financial support for the data analysis was provided by the Natural Sciences and Engineering Research Council of Canada. We thank the numerous people who assisted with data collection over the past 3 decades, particularly R. L. Hepburn, who initiated and supervised the Canonto study for many years. We also express our appreciation to the Canonto hunters for their enthusiastic cooperation. D. M. Lavigne, T. D. Nudds, and D. R. Voigt provided useful comments on an earlier draft. This is Wildlife Research Section contribution 90-06 of the Ontario Ministry of Natural Resources.