ANNUAL AND SEASONAL-CHANGES IN DIETS OF MARTENS - EVIDENCE FROM STABLE-ISOTOPE ANALYSIS

Theory predicts that generalist predators will switch to alternative p rey when preferred foods are not readily available. Studies on the fee ding ecology of the American marten (Martes americana) throughout Nort h America suggest that this mustelid is a generalist predator feeding largely on voles (Microtus sp.; Clethrionomys sp,). We investigated se asonal and annual changes In diets of martens in response to the chang ing abundance of small rodents (Peromyscus keeni, and Microtus longica udus) on Chichagof Island, Southeast Alaska, using stable isotope anal ysis. We hypothesized that martens would feed primarily on small roden ts during years with high abundance of these prey species, whereas dur ing years of low abundance of prey, martens would switch to feed prima rily on the seasonally available carcasses of salmon. We also hypothes ized that home-range location on the landscape (i,e., access to salmon streams) would determine the type of food consumed by martens, and ma rtens feeding on preferred prey would exhibit better body condition th an those feeding on other foods. We live-captured 75 martens repeatedl y, from mid-February to mid-December 1992-1994. We also obtained marte n 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 t he diet of martens in autumn, despite salmon carcasses being equally a vailable in all years. Selection for small rodents occurred only in se asons ill which abundance of small rodents was low. Logistic regressio n revealed that individuals with access to salmon streams were more li kely to incorporate salmon carcasses is 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 un derlying feeding habits of individuals under variable ecological condi tions. We were unable to demonstrate that body weights of live-capture d male and female martens differed significantly between individuals f eeding on marine-derived or terrestrial diets. Therefore, martens, as true generalist predators, switched to alternative prey when their pri ncipal food was not readily available on a seasonal or annual basis. A lthough salmon carcasses were nut a preferred food for martens, they p rovided a suitable alternative to maintain body condition during years when small rodents were not readily available.