The basis of niche separation during terrestrial life between two species of toad (Bufo bufo and Bufo calamita): competition or specialisation?

Two congeneric anurans with highly overlapping geographical ranges, Bufo bufo and B. calamita, were investigated with respect to isolating mechanisms during the terrestrial phase of life in the three habitat types in which both species occur in Britain. Marked spatial niche separations were observed in all three habitats (coastal dunes, upper saltmarshes and lowland heaths). B. bufo was associated with complex, dense vegetation structures and had a relatively wide niche, while B. calamita had a narrower niche and occurred mostly on open, poorly-vegetated ground. Food niche overlap was high at all three study sites. Animals translocated into atypical habitats returned to their preferred type whenever possible, and suffered increased mortality or loss of condition if prevented from doing so. B. calamita operated at significantly higher body temperatures (by an average of 1.4° C) than B. bufo when hunting at night. Experiments in outdoor vivaria indicated that B. calamita survived in open habitats because individuals escaped desiccation in daytime by burrowing into the substrate, whereas B. bufo lacked this behaviour and perished on the inhospitable surface. By contrast, B. calamita individuals lost mass and became hyperactive in densely-vegetated conditions because their hunting efficiency was selectively reduced relative to that of B. bufo. Taken together, the data indicated that spatial niche separation between these species during the terrestrial phase of life was dictated primarily by behavioural and physiological factors, and not by competition.