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.

Methods
Occurrence records for species distribution models came from the Global Biodiversity Information Facility (GBIF) (https://www.gbif.org/, accessed January 2022) and VertNet (http://vertnet.org/, accessed January 2022), and data we collected from surveys throughout the Great Basin in the Summer and Fall of 2021. We cropped all records to the Great Basin ecoregion, the boundary of which was obtained from the United States Geological Survey (USGS) database. We constrained records to dates on or after 1950, and with less than or equal to one kilometer coordinate uncertainty to reflect the resolution of the data layers. We used the spatial analysis georeferencing accuracy (SAGA) protocol to georeference data with no recorded coordinate uncertainty (Bloom et al. 2018). We thinned locality data to 1km raster cells.

From April through October 2022, we conducted surveys (24 sites, 6555 trap nights) for woodrats in the southern Snake Range of eastern Nevada.

We also obtained mid-Holocene fossil and midden records assembled from the Neotoma Paleoecology Database (http://www.neotoma.db.org; January 2024) (Williams, Grimm et al. 2018), and primary literature (Grayson 1985, Terry et al. 2011) to validate mid-Holocene model projections. We filtered records by excluding records with uncertain identification, restricting to only those with a calibrated median age between 4,500 and 7,500 years before the present and max age <11,700 (Grayson 2011), and trimming all records to our Great Basin extent. All paleontological records included were morphologically identified to species.