Species richness and abundance, as well as trophic relationships, are affected by habitat configuration. Smaller habitat patches suffer greater external interference, being more susceptible to diversity loss and could also receive more trophic subsidies from outside or matrix, which can favour the maintenance of populations even in small patches. Natural mosaics of forest patches in a grassland matrix in southern Brazil are good places to analyse such a process. Our objective was to analyse the effect of habitat patching on trophic relationships based on the evaluation of two forest amphibian species (Physalaemus lisei and P. carrizorum) and compare those from habitat patches versus core/contiguous habitat. We compared isotopic carbon (d 13 C) and nitrogen ratios (d 15 N) in two anuran species as well the variation in their diet at different forest patch sizes. We chose carbon isotopes since they act as a proxy for inferences on the original habitat from which the matter flows towards the anurans, while nitrogen reveals their tropic level. Our results revealed that trophic sources for these amphibians derives predominantly from primary forest productivity. However, Physalaemus lisei d 13 C values indicated that the contribution of grassland trophic sources in the diet was higher in frogs from forests patches (<100 ha) than in those from core forest habitat. In smaller patches, P. lisei consumed predominantly ants, which was the most abundant prey on habitat. On the other hand, in the core forest, where isopods were the most abundant prey, most of its consumption was beetles, spiders and ants. In contrast, P. carrizorum showed no significant changes in isotopic ratios or diet across habitats. Isotopic data suggested that both species occupied lower trophic levels in smaller forests than in core forests. Our data showed a tendency towards the increase of grassland contribution and decrease of amphibian trophic position in patched habitats in comparison to continuous habitat.
