Individual specialization among a guild of generalist predators

Background/Question/Methods

Many ecological studies consider individuals equivalently, thus overlooking individual diet patterns within generalist predators.  However, many individuals use only a subset of their population’s diet spectrum, a phenomenon known as individual specialization (IS).  Individual specialization has important ecological consequences, for example stabilizing population dynamics and increasing species coexistence.  However, most studies on IS have focused on a single species and have not investigated patterns of IS in a group of ecologically and evolutionarily related species.  Here we measured IS in four species of Anura: Lithobates catesbeianus, L. clamitans, L. sphenocephalus, and Anaxyrus americanus.  We tested the hypotheses that IS is greater in species with larger ranges of snout-vent lengths and wider niches.  We collected frogs and toads at Bernheim Arboretum and Research Forest in central Kentucky.  To measure diets we used stable isotopes from whole blood.  We used stable isotope analysis in R to determine the proportion of diet types for each individual of each species.  We used proportional dissimilarity (PD) indices and between-individual component (BIC) of the total niche width to measure the amount of IS in each species. We used linear models to test if species with larger niches and greater range in body size had more IS.

Results/Conclusions

Lithobates catesbeianus had the most IS, with a mean PD of 0.17 and BIC of 0.055.  Anaxyrus americanus had the second most IS, with a mean PD of 0.14 and BIC of 0.043.  Finally, both L. sphenocephalus and L. clamitans had a mean PD of 0.08 and BIC of 0.019.  Species with larger ranges of body size did not generally have more IS (t=1.542, df=1,2, p=0.263, R²=0.3147).  Although the high range of PD values in L. catesbeianus may be attributable to its large variation in body size, A. americanus had similar PD values but less variation in body size.  Species with larger total niches did not have higher BIC (t=0.531, df=1,2, p=0.648, R²=0.1237).  Our study demonstrates that IS can vary both within and between families, and thus that IS may be a trait that can easily evolve or disappear.  Causes of IS may vary, and more studies are needed to elucidate why certain species have greater IS than other, closely related species.  Finally, our study contradicts others’ findings in that species with a wider total niche width did not have a larger BIC, demonstrating that individuals can utilize a wide niche.