Non-behavioural interference competition between anuran larvae under semi-natural conditions.

Two major types of indirect competition have been recognised in natural communities, notably scramble for resources and interference to gain advantage by various alternative mechanisms. Interference effects are however often difficult to demonstrate in the field and their significance in nature requires more extensive study. Larvae of the anurans Bufo bufo and Bufo calamita compete strongly both in the field and in the laboratory, with the former species generally superior to the latter. Under laboratory conditions an interference component to this competition is readily demonstrable so we carried out an experiment to determine whether interference effects between the larvae of these species are also detectable in natural ponds. Larvae were reared at natural densities in cages immersed in a sand dune pool. Survival and growth rates were measured under conditions of no interaction, partial interaction (permitting interference but not resource competition) and full interaction between the species. For partial interaction, larvae were separated by mesh such that water and faeces could pass between compartments but the animals could not. Numbers of Anurofeca [=Prototheca] richardsi, a mediator of interference competition between these anurans under laboratory conditions, food acquisition and food availability were also determined during the course of the experiment. Asymmetric interspecific competition, manifested as reduced survival and growth rates of B. calamita, occurred under conditions of both full and partial interactions between the species though overall competition strength was much greater under fully interacting compared with partially interacting conditions. Interference competition, probably mediated by A. richardsi, was implicated under the partially interacting conditions in which resource competition was prevented.