Interspecific competition between larvae is not an important strncturing force in mixed communities of Rana and Bufo on an English sand-dune system

We investigated the roles of competition and predation in the structuring of mixed anuran communities ( Rana temporaria and Bufo bufo ) in an English sand-dune system. Rana temporaria breeds earlier in spring than B. bufo. but the latter species was numerically superior in all ponds at the study site. We measured survival and growth rates of larvae in three duplicate sets of natural ponds. In one set we removed B. bufo spawn, leaving only R. temporaria^: in a second sel we removed R. temporaria spawn, leaving only B. bufo: and in a third set we left boih together. The third set also contained substantial numbers of predatory fish ( Perea fluviatilis and Gasterosteus aeuleatus ). Rana temporaria survival was lower than that of B, bufo in all ponds, on average by sixfold. Rana temporaria fared better on its own than in the mixed-species ponds, but lower survival in the latter ponds was probably due to fish predation rather than to Inierspecific competition. By eonlrast, B. bufo survival was sevenfold higher in mixed-species compared with single-species ponds, probably because they are distasteful to fish and thus benefited indirectly from fish-predation of invertebrates, Protothieca rieharsi. a mediator of interference competition between anuran larvae under laboratory conditions, was present in some of the dune ponds but was not correlated with larval growth inhibition. Taken together the evidence implicated predation of larvae and intraspecific competition, rather than interspecific competition, as the dominant structuring forces in this community.