Annual and seasonal changes in diets of martens: evidence from stable isotope analysis

Abstract

Theory predicts that generalist predators will switch to alternative prey when preferred foods are not readily available. Studies on the feeding ecology of the American marten (Martes americana) throughout North America suggest that this mustelid is a generalist predator feeding largely on voles (Microtus sp.; Clethrionomys sp.). We investigated seasonal and annual changes in diets of martens in response to the changing abundance of small rodents (Peromyscus keeni, and Microtus longicaudus) on Chichagof Island, Southeast Alaska, using stable isotope analysis. We hypothesized that martens would feed primarily on small rodents during years with high abundance of these prey species, whereas during years of low abundance of prey, martens would switch to feed primarily on the seasonally available carcasses of salmon. We also hypothesized that home-range location on the landscape (i.e., access to salmon streams) would determine the type of food consumed by martens, and martens feeding on preferred prey would exhibit better body condition than those feeding on other foods. We live-captured 75 martens repeatedly, from mid-February to mid-December 1992-1994. We also obtained marten carcasses from trappers during late autumn 1991 and 1992, from which we randomly sub-sampled 165 individuals. Using stable isotope ratios and a multiple-source mixing model, we inferred that salmon carcasses composed a large portion of the diet of martens in autumn during years of low abundance of rodents (1991 and 1992). When small rodents were available in high numbers (1993 and 1994), they composed the bulk of the diet of martens in autumn, despite salmon carcasses being equally available in all years. Selection for small rodents occurred only in seasons in which abundance of small rodents was low. Logistic regression revealed that individuals with access to salmon streams were more likely to incorporate salmon carcasses in their diet during years of low abundance of small rodents. Using stable isotope analysis on repeated samples from the same individuals, we explored some of the factors underlying feeding habits of individuals under variable ecological conditions. We were unable to demonstrate that body weights of live-captured male and female martens differed significantly between individuals feeding on marine-derived or terrestrial diets. Therefore, martens, as true generalist predators, switched to alternative prey when their principal food was not readily available on a seasonal or annual basis. Although salmon carcasses were not a preferred food for martens, they provided a suitable alternative to maintain body condition during years when small rodents were not readily available.

Evidence and implications of recent and projected climate change in Alaska's forest ecosystems

“…However, it will affect a wide range of low-elevation animals indirectly through effects on stream-flow and the production and availability of salmon (Oncorhynchus gorbuscha, O. keta, O. kisutch, O. nerka, and O. tshawytscha), which are a major summer food resource for a diverse group of mammals, birds, and insects (Gende et al 2002). Salmon play a critical role in body size, population density, and productivity of brown bear (Ursus americanus; Hilderbrand et al 1999), nesting success and productivity of bald eagle (Haliaeetus leucocephalus; Hansen 1987), timing and success of reproduction in mink (Ben-David 1997, Ben-David et al 1997a, and body condition and survival of American marten (Martes americana; Ben-David et al 1997b). Due to the complex life cycles of salmon, the difficulty of establishing quantitative relationships between the supporting services of river systems and salmon returns has been noted (Chittenden et al 2009).…”

Golden eagles, feral pigs, and insular carnivores: How exotic species turn native predators into prey

“…We used stable isotope ratios (␦ 13 C and ␦ 15 N) as an index of prey consumption by eagles, foxes, and skunks (21). We analyzed golden eagle breast feathers, plasma blood samples (fox, skunk, and pig), whole arthropods, rodent tails, and fruits from Santa Cruz Island.…”

Marine mammals and the community structure of the Estuary and Gulf of St Lawrence, Canada: evidence from stable isotope analysis

“…Creating models that estimate the relative contribution of different prey to the diet of a consumer requires some basic knowledge of potential prey species. Owing to the rapidly increasing complexity of the calculations, these models generally assume that no more than 2 or 3 prey species are important (Gearing 1991, Ben-David et al 1997. The stable isotope approach also requires determination of the enrichment in 13 C and 15 N at each trophic level (trophic enrichment factor, TEF).…”