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

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 a 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.