Microhabitat and prey size among three species of Rana (Anura: Ranidae) sympatric in eastern Canada

Abstract

Microhabitat, prey size, and body morphology were examined in sympatric leopard frogs (Rana pipiens), green frogs (R. clamitans), and bullfrogs (R. catesbeiana) at a site in central New Brunswick. Ratios of head widths among species pairs were 1.23 (leopard and green frogs) and 1.51 (bullfrogs and green frogs), suggesting that leopard and green frogs may take prey of similar size. Analysis of stomach contents showed no significant differences in the mean prey sizes selected by leopard and green frogs. Rana sp. figured prominently in the diet of bullfrogs at the study site. Overlap among the species for prey length indicates the potential for competition between green and leopard frogs (Pij = 0.40 versus Pij = 1.41 for green frogs and bullfrogs). Structural aspects of habitat associated with vegetation were most important in discriminating microhabitat. Leopard frogs selected microhabitats that were much more densely vegetated than those of either bullfrogs or green frogs. In water, green frogs occupied sites that were closer to shore, were of lower water temperature, and had a higher vegetation canopy than sites occupied by bullfrogs. On land, green frogs selected microhabitats that were closer to water and less densely vegetated than those of leopard frogs. There was much greater microhabitat separation between leopard and green frogs than between green frogs and bullfrogs. There is some evidence that predation (bullfrogs preying on green frogs) and perhaps past competition (between green frogs and leopard frogs) have contributed to the patterns of resource use described. A number of other potential mechanisms (i.e., species phylogenies, inherent physiological or behavioural constraints acquired independently of interspecific interactions) remain unevaluated.