Factors influencing long-term population dynamics of pronghorn (Antilocapra americana): Evidence of an Allee effect

Abstract

Populations of pronghorn (Antilocapra americana) are subjected to multiple forms of density-dependent and density-independent regulation. Little is known about the combined effects of these variables across multiple populations throughout the landscape. The objectives of this study were to examine long-term trends in density and recruitment in pronghorn and to assess how different forms of regulation influence these trends. We used multiple density-dependent and density-independent explanatory variables in a model selection process to explain variation in pronghorn density and July fawndoe ratios from 1955 to 1993 in 4 pronghorn management units in Nebraska. We also investigated levels of density-dependent feedback in each management unit. Examination of long-term population trends suggested that pronghorn populations in Nebraska exhibited an Allee effect (inverse density dependence), in which population growth decreases as density decreases. We suggest that variation in rangeland condition and presence of adequate forage could explain the presence of the Allee effect. Both density-dependent and density-independent variables affected pronghorn populations, with density of cattle, spring precipitation, abundance of winter wheat, and hunting harvest most important for explaining fluctuations in pronghorn densities. Snow depth and density of cattle best explained yearly July fawndoe ratios. The importance of each factor was variable across the different management units. Management objectives aimed at improving access to and abundance of food resources during times when rangeland condition is poor could prevent drastic declines in pronghorn populations. © 2010 American Society of Mammalogists.