Rebecca C. Terry, Megan E. Guerre, David S. Taylor

How species respond to environmental change is shaped in part by what they eat.  Dietary specialists consume a narrow range of resources, often exhibiting morphological and behavioral adaptations to their diet resulting in reduced niche flexibility.  Thus ongoing environmental change can place dietary specialists at particularly high risk.

In the Great Basin of western North America, the Chisel-toothed Kangaroo Rat, Dipodomys microps, is a leaf-eater that specializes on the desert shrub Shadscale, Atriplex confertifolia.  Shadscale is a highly unpalatable salt-rich plant, but D. microps has broad flattened incisors that allow it to scrape off the salty leaf coating to access the succulent tissues underneath.  Feeding trials in the 1970s showed D. microps perished without access to shadscale, highlighting its presumed importance.  Yet at the Last Glacial Maximum (~21,000 years ago), shadscale was not present in the northern Great Basin, but D. microps was, raising the question of how did the rats persist?

We analyzed 13C and 15N stable isotopes in fossil and modern D. microps bones to quantify its dietary niche over the last 8,000 years at a locality in northwestern Nevada consisting of pellet deposits below a long-term owl roost.  Shadscale is an isotopically distinct shrub due to its C4 photosynthetic pathway (other Great Basin plants are C3), allowing us to assess its contribution to D. microps’ diet.

Contrary to expectation, δ13C values indicate that D. microps at this site consumes a variety of plants other than Shadscale, and has done so for millennia, despite Shadscale’s availability.  The proportion of Shadscale in the diet was only ~35% at its highest, ~7000 years ago, and has been declining, especially over the last 30 years.  Furthermore, δ15N values are consistently elevated, indicating that this population may be supplementing their diet by consuming insects.

Our results challenge assumptions about specialization, and suggest that individual populations of D. microps may actually have greater niche flexibility than typically assumed.  This, in turn, may buffer the species as a whole against environmental change, promoting persistence through time.  Finally, our study highlights the unique contribution that paleontological data can make to understanding the behavior of species and ecosystems today.

Photo credit: Jim Anderson

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