Niche partitioning and the role of intrapopulation niche variation in structuring a guild of generalist anurans

Background/Question/Methods

Niche partitioning is fundamental to ecological communities. Studies of this phenomenon traditionally treat individuals as identical even though recent research has demonstrated the importance of intrapopulation niche variation in populations. We propose that intrapopulation niche variation can support species coexistence by decreasing the interaction strength between competing heterospecifics. We tested for niche partitioning among five species of frogs and toads and whether between-individual variation in diets aided niche partitioning. We determined differences in diet with both stable isotopes and the stomach contents, and we also analyzed morphology and habitat choice.  We used PCA to determine important morphological traits and included those traits in a MANCOVA and ANCOVAs for the isotopic data. We used ANOVAs or the nonparametric equivalent to determine whether species differed in morphological traits and in distance to pond. We used an ANOVA with a null distribution built from a MCMC bootstrapping approach to test for differences across species in the stomach content data.

Results/Conclusions

Species differed significantly in isotope values. Values of δ¹⁵N indicated L. catesbeianus foraged at the highest trophic level amongst the considered species, and A. fowleri foraged at the lowest. Anaxyrus americanus, L. clamitans, and L. sphenocephalus had intermediate δ¹⁵N values. Within most species, larger individuals had higher δ¹⁵N values. The δ¹⁵N values of large A. americanus individuals were similar to those of small L. clamitans and L. sphenocephalus individuals. The δ¹⁵N values of large L. clamitans individuals were similar to those of small L. catesbeianus individuals. Values of δ¹³C indicated that A. fowleri, A. americanus, and L. sphenocephalus foraged in primarily terrestrial habitats while L. catesbeianus and L. clamitans foraged closer to ponds and on more aquatic prey. Species differed in the prey found in their stomachs. Both A. fowleri and A. americanus took significantly more ants than did the other species, Lithobates sphenocephalus took significantly more orthopterans. All species preyed upon beetles more than would be expected based on environmental availability, and all species took non-flying prey in greater proportion than non-flying prey. Our results emphasize the importance of considering individual-level variation when investigating niche partitioning, as such variation can affect the degree of niche overlap among species.