Predation, Palatability and the Distribution of Tadpoles in the Amazon Rainforest

Abstract

A variety of aquatic habitats with different levels of potential predators are available to larval amphibians in Central Amazon rainforest. The anuran community at Reserva Florestal Adolfo Ducke, 25 km east of Manaus, Amazonas, Brasil, was studied to determine which species have eggs and/or larvae in water and how those larvae are distributed in time and space. The temporal and spatial distribution of potential predators as well as abiotic characteristics of these waterbodies were determined simultaneously to test for correlations with the distribution of tadpoles. The distribution of tadpoles was strongly related to fish predation pressure. Several tadpole species were found only in waterbodies with high fish abundance and thus have the ability to survive with fish. Most of these tadpoles were found to be unpalatable in controlled experiments. Unpalatability is the major adaptation allowing the coexistence of tadpoles and fish and is thus a major factor affecting tadpole community composition in this system. Controlled experiments showed that fish do not eat anuran eggs while the tadpoles of Leptodacrylus knudseni and Osteocephalus taurinus ate all types of eggs offered. The percentage of anurans with aquatic oviposition was positively related to fish abundance and negatively related to the occurrence of species of tadpole that ate eggs in experiments. These findings suggest that the present patterns of anuran distribution represent an evolutionary response to predation on the eggs and larvae. Contrary to the models of Heyer et al. (1975) and Wilbur (1984), desiccation and predation-pressure were not the major factors affecting species richness within waterbodies of the RFAD rainforest. In support of the model of Heyer et al. (1975), anuran species richness was correlated with the size of the waterbody. This could be because the size of the waterbody is related to increased complexity and availability of microhabitats. The range of volumes of waterbodies was also found to directly affect species richness of the RFAD community. While predation appeared to have a limited effect on species richness of individual ponds, predation-pressure was found to have a major influence on species composition. Anuran eggs and larvae survived with specific predators by possessing particular survival-traits (e.g. unpalatability and oviposition strategies). However, survival-traits were not effective against all predators in all habitats. The distribution of different predators among ponds provides a patchy environment on a local scale (i.e. within ponds). When combined with the variety of survival-traits exhibited by the anuran species, this spatial patchiness in predation contributes towards species richness within the anuran community of the RFAD rainforest.