Spatial and temporal environmental heterogeneity drive ecological adaptation. Environmental change influences organismal response across these scales. Range edges provide ideal locations to study how populations of species respond, and adapt, to different environmental conditions and stressors. Dietary specialization is one mode of environmental adaptation, particularly for herbivores that may be exposed to potentially toxic plant compounds. Dietary adaptations of herbivores provide an opportunity to investigate patterns of genotype by environment interactions within natural populations. Particularly, the study of closely related and hybridizing populations in secondary contact provides unique insight into the genetic and environmental mechanisms that help shape species adaptation. We studied ecological and genetic interactions of desert woodrats (Neotoma lepida), Bryant’s woodrat (Neotoma bryanti) and respective hybrids in a ~60 ha study site characterized by a sharp ecological gradient between habitats and distinctive vegetative communities in Kern Co., California. We used capture-mark-recapture methods to sample populations, collect tissue samples for genetic analysis, and estimate population density among the two habitats using a Bayesian modeling approach. Individuals were genotyped at 17 microsatellite loci. We used a 2-choice cafeteria experiment to test for dietary preference between these two species.
Results/Conclusions
We captured and uniquely marked 169 woodrats from May-August, 2016 within adjacent habitats (“Hill” and “Flat”). We identified 62 N. bryanti, 69 N. lepida, and 31 hybrid individuals. We detected spatial segregation of genotypic classes; N. bryanti were primarily found in the hill habitat which is characterized by large boulders and California chaparral. Neotoma lepida were found in the flat habitat characterized by sandy washes and Mojave desert scrub. Density estimates for the two habitats were significantly different, with the hill habitat estimated to have ~ 4 times the number of woodrats per hectare than the flat habitat. Two-way choice trials showed a difference in dietary preference for Rhamnus sp. (associated with hill habitat) and Prunus fasciculata (associated with flat habitat) between individuals of each genotypic class (ANOVA, F = 5.1076, P = 0.016, df = 2). Because these two plants differ substantially in their chemical composition, our data suggest differential dietary and detoxification adaptations between these species. This system provides an opportunity to further understand how dietary segregation contributes to ecological adaptation and the maintenance of species boundaries.