Feeding limitations in temperate anurans and the niche variation hypothesis. Supplementary Material

The niche variation hypothesis (NVH) states that populations with wider niches are more phenotypically variable. The NVH has important ecological and evolutionary implications but has been controversial since its inception. Recent interpretations have supported the NVH by directly comparing among-individual diet variation with population dietary niche breadth. Traditional studies of the NVH focused on morphological traits as proxies of niche variation, with contradictory results. Gape-limited predators may be relatively likely to show effects of morphological variation on diet breadth because gape size can strongly limit diet. We used five anurans to test NVH predictions, including three true frogs, <i>Rana catesbeiana</i>, <i>R. clamitans</i>, and <i>R. sphenocephala</i>, and two toads, <i>Anaxyrus americanus </i>and <i>A. fowleri</i>. We combined recent and traditional approaches by comparing both individual variation in diet and variation in gape width with dietary niche breadth. We found support for the NVH within two species of the three true frogs but not for either toad species, a difference likely driven by greater strength of the feeding limitation caused by gape width in the frogs. Toads had higher gape width to snout-vent length ratios, reducing the strength of the feeding limitation imposed by gape width. We found strong support for the NVH among species; species with more among-individual variation in diet and species with more variation in gape width had broader niches. Our results highlight the circumstances under which the NVH is applicable and demonstrate an example in which the NVH is supported through both traditional and recent interpretations

The supplementary material provides additional information regarding number of frogs and toads captured in different regions, a map of the field site, model selection analysis for the GLMMs performed for each isotope type, additional stomach content analysis and cluster analysis on the stable isotopes. from Niche partitioning and the role of intraspecific niche variation in structuring a guild of generalist anurans

Intra-population niche differences in generalist foragers have captured the interest of ecologists, because such individuality can have important ecological and evolutionary implications. Few researchers have investigated how these differences affect the relationships among ecologically similar, sympatric species. Using stable isotopes, stomach contents, morphology and habitat preference, we examined niche partitioning within a group of five anurans and determined whether variation within species could facilitate resource partitioning. Species partitioned their niches by trophic level and by foraging habitat. However, there was considerable intraspecific variation in trophic level, with larger individuals generally feeding at higher trophic levels. For species at intermediate trophic levels, smaller individuals overlapped in trophic level with individuals of smaller species and larger individuals overlapped with the smallest individuals from larger species. Species varied in carbon isotopes; species with enriched carbon isotope ratios foraged farther from ponds, whereas species with depleted carbon isotope values foraged closer to ponds. Our study shows that these species partition their niches by feeding at different trophic levels and foraging at different distances from ponds. The intraspecific variation in trophic level decreased the number of individuals from each species that overlapped in trophic level with individuals from other species, which can facilitate species coexistence