INTERSPECIFIC COMPETITION BETWEEN BUFO LARVAE UNDER CONDITIONS OF COMMUNITY TRANSITION

Progressive stabilization (''fixation'') of a sand dune system by vege tational succession has allowed one anuran species (Bufo bufo) to encr oach and breed in areas formerly the sole preserve of another (B. cala mita). We investigated interactions and the potential for competition between larvae of these two species under conditions of community tran sition. Survival, growth rates, and spatial and feeding niche overlaps of larvae as well as abundance of the unicellular microorganism Proto theca richasdsi (a mediator of interference competition between tadpol es) were measured in three duplicated sets of ponds in the same habita t. One set was used by B. bufo only, one by B. calamita only, and one by both species. B. bufo larvae survived and grew as well, or better, in mixed-species ponds as they did when alone. By contrast, B. calamit a larvae grew more slowly, suffered higher premetamorphic mortality, a nd emerged as smaller toadlets in mixed-species ponds, compared with s ingle-species ponds. B. calamita changed its spatial niche in mixed-sp ecies ponds much more markedly than did B. bufo, but spatial and feedi ng niche overlaps remained high. P. richardsi was rare in guts and fec es of larvae in single-species ponds, but it occurred in large numbers in guts and feces of larvae from mixed-species ponds. The data indica te that B. bufo is a superior competitor to B. calamita and that both resource and interference competition may occur between the two specie s in this transitional situation.