Large-scale spatial gradients in herbivore population dynamics

Spatial gradients in density dependence and cyclicity are familiar features of the population dynamics of small mammals, particularly Fennoscandian rodents. The most well-documented of such gradients is a weakening of direct density dependence and an increase in the tendency of populations to cycle the farther north they occur, a phenomenon that has been attributed to gradients in predation and seasonality. Among large mammals, however, for which evidence of cyclicity is less clear, geographic gradients in population dynamics are limited to spatial variation in the strength of density independence. The population dynamics of caribou and muskoxen in Greenland, for example, display latitudinal gradients in the response of populations to large-scale climatic fluctuation. To my knowledge, the existence of spatial gradients in density dependence has not been explicitly investigated in large mammals. Here I present an analysis of the dynamics of 27 populations of caribou and reindeer in Greenland, Finland, and Russia, spanning 21 degrees of latitude (51.7°-72.7° N) and 215 degrees of longitude (56.4° W-159.5° E), to identify spatial gradients in density dependence and independence. Results of autoregressive time series analysis show a clear gradient in the strength of direct density dependence exhibited by these populations that declines from southern to northern latitudes. Although this pattern mirrors the latitudinal gradient evident in Fennoscandian rodent dynamics, an analysis of the dimensionality of these time series suggests that few, if any, of the populations are limited by predators. The existence of an inverse latitudinal gradient in the magnitude of the influence of large-scale climate on the dynamics of these populations suggests there may be a tension in the strength of density-dependent vs. density-independent limitation experienced by them.