Drivers of species distribution in woodrats (Neotoma) of the Great Basin Desert
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Authors
Coconis, Alexandra Cecilia
Issue Date
2025
Type
Dissertation
Language
en_US
Keywords
diet , elevational gradient , microbiome
Alternative Title
Abstract
The drivers that determine species distributions have long been of interest to ecologists and are highly variable across space and time. Interactions between abiotic and biotic variables may differentially shift fitness outcomes between individuals of different species, and in turn move species’ boundaries and alter community assemblies. Understanding a species’ climatic niche and plastic capacity in response to shifting conditions are important in predicting whether or not they can withstand rapid environmental changes. However, a species’ distribution is also influenced by biotic interactions such as competition, the strength of which is dependent upon factors such as climate and resource availability. Additionally, spatial scale and geographic context affect the relative importance of drivers of distribution and may differ from the regional to the local scale or from one location to another. I studied the distribution across space and time of two species of woodrat (Neotoma) that co-occur across the Great Basin of western North America. I built region-wide species distribution models and conducted extensive field surveys across a single elevational gradient, validating these models and locating previously undiscovered areas of sympatry between the two species studied. I then hindcasted their distributions into the mid-Holocene time period to confirm fossil and midden evidence indicating little to no interactions between the two species over time. I found that the factors most important in determining their distributions are highly variable regionally, but that abiotic factors appear more important than biotic factors, notably the presence of a congener in determining their range limits when their distributions come into proximity of one another. I also show a reduced potential for interaction during the mid-Holocene in comparison to today. I then built generalized linear models to test for both biotic and abiotic drivers of relative abundance across a single mountain range and to measure differences in niche breadth across variables and within each species. I found that temperature is a limiting factor for both species, but that for the smaller-bodied species, an interaction between temperature and the presence of a congener most influenced relative abundance. I also show that the factors that most differentiate the niche of these species include their thermal niche and use of rock structure, presumably for thermal buffering. Finally, to understand diet and microbial niche differences between these species and intraspecific diet and microbiome shifts across this mountain range, I sequenced DNA from fecal pellets collected from both species across sites of allopatry and sympatry. I found that at sites of sympatry both species consumed more juniper, a common plant with toxic plant specialized compounds, but that N. lepida consumed more juniper than N. cinerea. I also found a smaller total dietary niche breadth for N. lepida at sites of sympatry versus sites of allopatry, indicating possible effects of interspecific competition in contact zones. Finally, I found the most significant effect on gut microbial diversity was host species identity, aligning with previous studies of mammalian gut microbiomes. I also found small effects of dietary diversity on gut microbial diversity, indicating that as diet becomes more complex, so too does the gut microbiome.