Spawn predation and larval growth inhibition as mechanisms for niche separation in anurans

We have investigated interspecific interactions between larvae of the 3 native British anurans, Rana temporaria, Bufo bufo and B. calamita in the field and in the laboratory. Over a 3 year period, predation of B. calamita spawn by the two competitively superior species accounted for only 0.3-3.0% of the total deposited in the dune pools of the study site. However, in ponds with marked niche overlap destruction of natterjack spawn was often 100%. Furthermore, growth rates of B. calamita larvae surviving to hatch were grossly inhibited when large numbers (>0.1 per l) of common frog or toad tadpoles were present. Overall survival to metamorphosis by the competitively inferior species was always negligible in these situations. These results were confirmed by experiments using caged larvae, and by movement of R. temporaria spawn between pools. Growth could be reduced to <25% of uninhibited controls, and time taken to achieve metamorphosis extended by at least 40% in the presence of larger larvae. Laboratory experiments confirmed that inhibitory effects of both R. temporaria and B. bufo larvae on growth by B. calamita tadpoles was demonstrable at tadpole densities comparable with those found in the field.