Density dependence in northern ungulates: interactions with predation and resources
Variation in the abundance of animals has traditionally been explained as the outcome of endogenous forcing from density dependence and exogenous forcing arising from variation in weather and predation. Emerging evidence suggests that the effects of density dependence interact with external influences on population dynamics. In particular, spatial heterogeneity in resources and the presence of capable predators may weaken feedbacks from density dependence to growth of populations. We used the Kalman filter to analyze 23 time series of estimates of abundance of northern ungulate populations arrayed along a latitudinal gradient (latitude range of 40°–70°N) to evaluate the influence of spatial heterogeneity in resources and predation on density dependence. We also used contingency tables to test whether density dependence was independent of the presence of carnivores (our estimate of predation) and multiple regressions to determine the effects of spatial heterogeneity in resources, predation, and latitude on the strength of density dependence. Our results showed that the strength of density dependence of ungulate populations was low in the presence of large carnivores, particularly at northern latitudes with low primary productivity. We found that heterogeneity in elevation, which we assume acted as a surrogate for spatial heterogeneity in plant phenology, also reduced effects of density dependence. Thus, we show that external forces created by heterogeneity in resources and predation interact with internal feedbacks from population density to shape dynamics of populations of northern ungulates.
KeywordsGompertz model Kalman filter Large carnivores Local environmental variability Predation Spatial heterogeneity
This work was in part supported by the Large Mammalian Herbivore Dynamics Working Group supported by the National Center for Ecological Analysis and Synthesis, a center funded by the United States National Science Foundation (DEB-94-21535), the University of California, Santa Barbara, and the State of California. We are grateful to Dr. Marco Fest-Bianchet for making data on bighorn sheep in the Sheep River and Ram Mountain available for our analysis, to Ken Hamlin for data on elk of the Gravelly Mountain, Montana, and to Tim Clutton-Brock, Tim Coulson, and Josephine Pemberton, the Rum Red Deer Project and the St. Kilda Soay Sheep Project for allowing access to the data on red deer and Soay Sheep. We gratefully acknowledge support from the Ecological Biology Program of the National Science Foundation (Awards DEB 0119618 and DEB 0444711) to Colorado State University. Dr. Eric Post and two anonymous reviewers made helpful comments on our manuscript.
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