STRENGTH AND MECHANISMS OF COMPETITION BETWEEN COMMON AND ENDANGERED ANURANS

Abstract

We investigated the strength and mechanisms of asymmetric competition between the larvae of an endangered British anuran (Bufo calamita) and those of two competitive superiors (Bufo bufo and Rana temporaria). The former species is normally allotopic with the latter two but has recently suffered competition from them following successional changes on sand-dune habitats that have rendered all three species syntopic. In a replicated cage experiment using natural densities of larvae, survivorship of B. calamita was reduced more strongly by interspecific than by intraspecific competitors. Predation did not relieve interspecific competition and in some treatments further increased the mortality of B. calamita. Survival was highly correlated with growth rate, and the negative effects of interspecific competition on growth were much stronger than those of intraspecific competition. Feeding-niche overlaps among all three species were high, and growth rates of B. calamita were related to total algal cell numbers available in the treatment cages. However, larvae in treatments exhibiting interspecific competition also generated large numbers of Anurofeca (= Prototheca) richardsi. This unicellular organism is implicated in interference competition between anurans. B. calamita growth also correlated inversely with A. richardsi numbers. Taken together the data demonstrate strong adverse effects of interspecific competition on the endangered B. calamita and indicate that both resource and interference mechanisms were probably operative, though the former was more important than the latter. Methods for alleviating the destructive impact of competition on the endangered species are discussed.