Feeding limitations in temperate anurans and the niche variation hypothesis

in Amphibia-Reptilia
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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, Rana catesbeiana, R. clamitans, and R. sphenocephala, and two toads, Anaxyrus americanus and A. fowleri. 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.

Feeding limitations in temperate anurans and the niche variation hypothesis

in Amphibia-Reptilia



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    Isotope biplots for A) toads and B) true frogs. Crosses represent standard deviations for carbon (horizontal) and nitrogen (vertical). Solid boxes represent mean isotope values of prey with crosses representing one standard deviation of the isotope values.

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    Niche breadth versus proportional dissimilarity. Niche breadth is measured with SIBER (Jackson et al., 2011). (A) Rana catesbeiana, (B) R. clamitans, (C) R. sphenocephala, (D) Anaxyrus americanus, and (E) A. fowleri. Filled circles and the solid lines are real data and the regression model from the real data. Open squares and dashed lines are null data and the regression models from the null data.

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    Boxplot of gape width to snout vent length ratios among species.

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    Niche breadth versus proportional dissimilarity among species. Each point represents a species. □ = Anaxyrus americanus, ○ = A. fowleri, △ = Rana catesbeiana, + = R. clamitans, × = R. sphenocephala.

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    Coefficient of variation for gape width versus proportional dissimilarity among species. Each point represents a species. See fig. 4 for symbol legend.

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