HOW SPECIALIZED IS A DIET SPECIALIST? NICHE FLEXIBILITY AND LOCAL PERSISTENCE THROUGH TIME OF THE CHISEL-TOOTHED KANGAROO RAT

Rapid changes in global climate have put many species at risk, particularly niche specialists. In the face of such change, consumers can shift their geographic or elevational ranges, or remain in place by taking advantage of new resources. In the Great Basin of western North America, the Chisel-toothed kangaroo rat, Dipodomys microps, is a specialized folivore thought to primarily consume the desert shrub Atriplex confertifolia. Because of its association with A. confertifolia, D. microps is presumed to have tracked the shrub as it moved south during the last glacial maximum. However, recent phylogeographic evidence indicates that D. microps did not shift and/or contract its range to a southern refugium. Here we use analyses of stable isotope ratios to examine the history of niche variation within D. microps from the end of the early Holocene to the present at a single site in the northwestern Great Basin. We thereby evaluate the role that niche flexibility may have played in allowing this presumed dietary specialist to cope with a changing environment.

We measured δ¹³C and δ¹⁵N in D. microps bone collagen from fossil specimens spanning the last 8,000 years at Two Ledges Chamber in Northwestern Nevada. Mean δ¹³C values through time indicate that, contrary to expectation, D. microps consumes a diversity of plants other than A. confertifolia (an isotopically distinct C4 shrub), and has done so for millennia. Analyses of isotopic mixing models suggest that the proportion of C4 in this species’ diet was highest in the middle Holocene, and has declined towards the recent, especially in the last century, even after Seuss effect correction. Mean δ¹⁵N values are consistently high, indicating D. microps could also be consuming a high proportion of insect protein in the past and today. Our results imply that this “dietary specialist” may actually have greater niche flexibility than previously assumed, and that this flexibility may have allowed D. microps to persist in place during past episodes of major environmental change. Understanding which species have responded to changing conditions in the past via niche shifts, as opposed to or in addition to range shifts, will allow us to better predict the impacts of future environmental change.