Populations and communities affect individual dietary patterns of bullfrogs, Lithobates catesbeianus

Background/Question/Methods

Ecologists often overlook individual patterns of diet in predators that are considered generalists.  However, many species with broad diets on the species level may be composed of individuals that specialize. A large body of research has shown that individual specialization (IS) can influence population and community dynamics, for example by decreasing intraspecific competition, increasing species persistence, and increasing species coexistence.  However, less work has focused on how population and community dynamics affect IS.  Here, we describe diets from individual bullfrogs, Lithobates catesbeianus, using stable isotope analysis (SIA) on whole blood.  With that information, we A) measured IS using the proportional similarity (PS) index; and B) tested the hypotheses that intraspecific competition (i.e., high bullfrog densities) and resource diversity both increase IS. We measured density by conducting 20-minute scan searches of ponds shortly after dark and then dividing the number of bullfrogs by the area of the pond. We used a linear model to test the effects of density on IS. We collected potential prey with sweep-nets and pitfall traps, and we measured resource diversity with the Shannon-Weaver index.  We used a linear permutation model to test the effects of resource diversity on IS.

Results/Conclusions

Lithobates catesbeianus individuals varied widely in PS values from 0.65 to 0.97 and were statistically different from one (t=7.55, df=15, p<0.001), indicating strong IS. We found support for both our hypotheses regarding the effect of populations and communities on IS.  Frogs at ponds with a greater density of conspecifics showed more IS (F=5.946, df= 1,14, p=0.02868, R²=0.248).  Frogs captured at ponds with higher resource diversity also had more IS (F=8.587, df=1,14, p=0.01096, R²=0.3359). Furthermore, we showed that IS can be influenced at the population level by intraspecific competition and at the community level by resource diversity. This is the first study to directly test the influence of resource diversity on IS. While previous studies on IS have shown that individuals can affect populations and communities, our study demonstrates that populations and communities can affect individual dietary patterns.  This suggests potential feedbacks between individual actions and higher levels of ecological organization and that IS needs to be increasingly included in studies of population dynamics and ecological communities.