The role of climate and species interactions in determining the distribution of two elevationally segregated species of small mammals through time

Abstract

The relative importance of abiotic and biotic factors in determining species distributions has long been of interest to ecologists but is often difficult to assess due to the lack of spatially and temporally robust occurrence records. Furthermore, locating places where potentially highly competitive species co-occur may be challenging but would provide critical knowledge into the effects of competition on species ranges. We built species distribution models for two closely related species of small mammals (Neotoma) that are largely parapatric along mountainsides throughout the Great Basin Desert, USA using extensive modern occurrence records. We hindcasted these models to the mid-Holocene to compare the response of each species to dramatic climatic change and used paleontological records to validate our models. Model results showed species co-occurrence at mid-elevations along select mountain ranges in this region. We confirmed our model results with fine-scale field surveys in a single mountain range containing one of the most extensive survey datasets across an elevational gradient in the Great Basin. We found close alignment of realized distributions to the respective abiotic species distribution model predictions, despite the presence of the congener, indicating that climate may be more influential than competition in shaping distribution at the scale of a single mountain range. Our models also predict differential species responses to historic climate change, leading to reduced probability of species interactions during warmer and dryer climatic conditions. Our results emphasize the utility of examining species distributions with regard to both abiotic variables and species interactions and at various spatial scales to make inferences about the mechanisms underlying distributional limits.