Factors Controlling Tadpole Populations of the Chorus Frog (Pseudacris Triseriata) on Isle Royale, Michigan

At the northeast end of Isle Royale, the chorus frog, P. triseriata, breeds nearly exclusively in pools on the exposed rocky shores of Lake Superior. The persistence of these breeding pools increases with size and distance from the lake. Small pools and those next to the lake last less than the 55—83 d required for metamorphosis, and survivorship to metamorphosis in these pools is low. Large pools near the forest edge are permanent, but contain the predators Anax junius and Ambystoma laterale. Anax junius eliminates P. triseriata if they occur together in the same pool. P. triseriata sustains high densities only in pools at intermediate levels on the shore, where intraspecific competition is suggested by effects of density on initial growth rate and on the length of the larval period. A field experiment using randomized blocks in a factorial design showed significant effects of both food and density on survivorship and growth; in the experiment, tadpoles removed all supplemental food. The results demonstrate density dependence and indicate that food is limiting and in short supply in the natural breeding pools. Two features of P. triseriata population control may be common to other anurans of ephemeral habitats. First, partitioning of the larval habitat may allow predators to eliminate tadpoles from many pools, yet create refuges from predation in others. Second, in pools of predation, competition may be intense and exert control on recruitment.