Asymmetric competition over calling sites in two closely related treefrog species.

Interspecific competition occurs when one species using a resource limits the use of that resource by another species. A dominance relationship between the species competing over a resource may result in asymmetric competition. Here, we tested the hypothesis that two sympatric treefrog species, the endangered Hyla suweonensis and the abundant H. japonica, compete with each other over calling sites. We observed the locations of calling individuals of the two treefrog species in rice paddies and tested whether removing one species affected the calling locations of the other species. Individuals of the two species were spatially isolated within rice paddies, with H. japonica at the edges and H. suweonensis in the interior. Male H. suweonensis moved towards the edges of rice paddies when male H. japonica were removed from the area, whereas male H. japonica hardly moved when male H. suweonensis were removed. The results of both studies are consistent with asymmetric interspecific competition, in which the calling locations of H. suweonensis are affected by the calling activity of H. japonica. In addition, H. japonica were found "sitting" on the substrate during call production, whereas H. suweonensis were "holding" onto vegetation. The difference in calling posture may represent an adaptive response to asymmetric interspecific competition.