9th International Deer Biology Congress

AUTHORS: Floyd W. Weckerly, Department of Biology, Texas State University; Lisa J. Koetke, Department of Biology, Texas State University; Adam Duarte, Oregon Cooperative Fish and Wildlife Research Unit, Department of Fisheries and Wildlife, Oregon State University

ABSTRACT: Estimates of K carrying capacity are useful for conserving and managing large herbivores as this parameter is integral to understanding population dynamics and trophic interactions. Population growth models estimate parameters such as K carrying capacity from time series of abundances. Furthermore, when implemented using a state-space formulation these models can accommodate bias in population survey methods (observer error) so that estimated parameters are more accurate [1,2]. We estimated population parameters of five, non-migratory Roosevelt elk (Cervus elaphus roosevelti) herds in Redwood National and State Parks, California, USA, to assess variation in K and how variation in K can be used to understand population dynamics across herds [3]. Our objectives were: (1) evaluate three commonly used population growth models to understand Roosevelt elk herd dynamics in the parks, (2) estimate the maximum intrinsic rate of population increase (Rmax) and K of each of the five herds in the parks, and (3) assess how variation in K might affect strength of density dependence for each herd. We fitted the Gompertz, Ricker, and theta-logistic models to these data. Although these models indicated differing population dynamics and estimates of K for the same herd (Fig. 1), we based our inferences on the results from the Ricker model after considering elk life history, a comparison of the model estimates to the fit of the observed time series of abundances, and the precision of parameter estimates. Across herds estimates of K ranged over an order of magnitude from 22 to 246, yet Rmax was constant (0.21). As such, the strength of density dependence should be inversely related to K [4], a pattern that has been detected at larger regional scales, but not at a landscape scale. The strong density dependence of smaller herds might also explain why one of the herds that originated 27 years ago and has experienced changes in available forage resources has yet to establish density-dependent population growth, resulting in imprecise estimates of K.