Ontogenetic and individual diet variation in amphibian larvae across an environmental gradient

1. Variation among individuals within size or age classes can have profound effects on community dynamics and food-web structure. We investigated the potential influence of habitat disturbance on intrapopulation niche variation. 2. Amphibians occupy a range of lentic habitats from short-hydroperiod intermittent ponds to long-hydroperiod permanent ponds. We quantified ontogenetic diet variation and individual specialisation in wood frog tadpoles (Lithobates sylvaticus) and blue-spotted salamander larvae (Ambystoma laterale) to investigate the influence of hydroperiod on population niche width across a natural hydroperiod gradient using stable isotope and gut content analyses. In one of the few tests using larval forms, we tested the niche variation hypothesis, which predicts that populations with larger niche widths also have increased individual variation. 3. Our results support the niche variation hypothesis, indicating that more generalised populations exhibit higher within-individual diet variation. We report gradual changes in the relative importance of diet items, decreased dietary overlap and increased trophic position in L. sylvaticus throughout development. A. laterale became more enriched in delta C-13 and increased in delta N-15 throughout its larval period. We did not find a relationship between hydroperiod and niche parameters, indicating that niches are conserved across heterogeneous habitats. In contrast to most documented cases, we estimated low levels of individual specialisation in amphibian larvae. 4. Amphibians are an important link between aquatic and terrestrial ecosystems, whereby diet shifts can influence food-web structure by altering energy flow pathways and the trophic position of higher consumers, ultimately changing food-chain length.