Anthropogenic disturbance alters the isotopic niche and interspecific interactions of two sympatric small mammals

Background/Question/Methods

The isotope ratios of all potential food sources in an area allow for estimation of the diet of consumers within that area. In addition to diet selection, foraging strategy and habitat use may also affect the δ¹³C and δ¹⁵N values of consumers. Small mammals in Southeast Alaska live in a landscape of habitat patches comprised of old-growth, young-growth, thinned and clearcut forest stands. Keen’s mice (Peromyscus keeni), an omnivore and dietary generalist, and dusky shrews (Sorex monticolus), an insectivore specialist, occupy all habitat types on Prince of Wales Island (POW), Alaska. Our objective was to determine how landscape heterogeneity and foraging strategy affect the isotopic niche of small mammal consumers. We hypothesized that: 1) mice and shrews will be isotopically distinct due to differences in diet; and 2) mice and shrews will exhibit relatively narrow isotopic niches representative of their respective foraging strategies. We used stable carbon and nitrogen isotopes to estimate the diets of sympatric mice and shrews on POW across these four habitat types in summers from 2010-2012. We also collected and analyzed all potential prey items. Multi-source dual-isotope mixing models were used to determine the range of possible contributions of each food source. 

Results/Conclusions

Mice exhibited variation of up to 7 ‰ in δ¹³C and 11 ‰ in δ¹⁵N between habitats and across the landscape. In contrast, shrews exhibited a relatively narrow niche. The average isotope ratios of mice and shrews overlap slightly in δ¹³C and completely in δ¹⁵N but the degree of overlap is habitat specific. Mixing models indicate the highest overlap in diet selection in young-growth stands. Following a population crash of mice in 2012, shrews exhibited a significant shift in δ¹³C values, indicating that the presence of mice influences shrew diet. The relatively wide isotopic niche of mice suggests specialized foraging behavior of individuals within a generalist population. This type of individual resource use affects competition, social interactions, and risk of predation. This study highlights the importance of considering habitat use and foraging strategy as well as diet selection when examining isotopic patterns of consumers.