Of frogs & ponds & peccaries

Natural History, July-August, 1998 by Clauda Gascon, Barbara Zimmerman

As we walked through a tract of Amazonian rainforest surveying frogs, we were startled by grunting noises louder than the granddaddy of all frogs could ever have produced. Braving the spiny palms and stinging ants of the undergrowth, we left the trail just in time to see a collared peccary running off. The wild pig had been rolling in a wallow, a muddy pond about six or seven feet in diameter. Since rainforests are dotted with mud wallows, which are also used by armadillos, we wondered whether these mammals might play a role in creating habitat for some of the forest's frogs.

Indeed, soon after we began looking for frogs in pig wallows, we found several species breeding there. Most often, the wallows provided tadpole habitat for tree frogs and two species of poison-arrow frogs, including one species we found breeding nowhere else. Although some wallow-breeding species use other kinds of pools as well, their overall survival may be tied to the habits of peccaries. We do not know whether these wallows are actually created by mud-loving mammals or are merely exploited by them; but peccaries and armadillos, by rolling and digging, enlarge the puddles, keeping them from filling up with debris and possibly preventing them from evaporating during the dry season. Therefore, frogs may breed in the pools year-round. And because the wallows are widely scattered, these frogs can occupy much larger areas of forest than frogs that rely on temporary ponds, which tend to be patchily distributed.

While logging and ranching were irrevocably altering the central Amazon, we were studying the distribution and abundance of frogs to determine their critical habitat requirements. Virtually all frogs need water to reproduce--or at least a wet place in which to spawn and fertilize their eggs. By hiking through the forest and identifying species by the males' mating calls, we learned that most species--about twenty-five of the forty kinds we encountered--depend upon ponds and pools to breed, an observation we confirmed by surveying tadpoles. Others called regularly from the banks or valleys of streams; ten or eleven of this group bred nowhere else. Sixteen species reproduced at upland ponds that were not connected to streams, and tadpoles of two species developed in tree holes continuing water. A few spawned in moist leaf litter, in self-generated foam, or within bromeliad plants. Interestingly, only seven kinds reproduced directly in streams, a finding that surprised us, since in the rainforests or Southeast Asia, most frog species do breed in streams.

Pools, ponds, and streams are not evenly distributed throughout the forest. Some areas contain many, while others have none. Wide streams flanked by broad, flooded areas provide more reproductive opportunities than small streams, but stillwater pools are the most important breeding habitat of Amazonian frogs. Therefore, to conserve many frog species, it would be better to protect 100 acres of forest containing quality breeding habitat man 500 acres without pools or wide streams.

After more than twelve years of research, we have learned that many frog populations have suffered from ranching and forest fragmentation, but not to the degree we expected. We had supposed that most of the forty forest species we encountered in undisturbed primary forest would disappear from the area after cutting. To our surprise, however, we began to observe several species of forest frogs calling at ponds in cleared pasture. Following up on these observations, Mandy Tocher, a graduate student from New Zealand, eventually found that twenty-six of the forest-dwelling frog species were now also living in pasture ponds. Remarkably, she discovered that two species rare in the virgin forest were living in the pastures.

We do not know, however, how many forest species can maintain themselves in the drastically different conditions of the pasture ponds. Since pastures contain no streams or large forest trees, frogs that are stream and tree-hole breeders cannot survive. We have found one species of tree frog, Phyllomedusa tarsius, successfully reproducing in pasture ponds. Although most of the other species we found there are breeding, we don't know if their eggs and larvae will survive or if their numbers are being maintained by immigration from nearby forest.

And what about the pig-wallow species? We placed large plastic tubs in forest patches and in pasture to see whether the frogs would colonize them. Sure enough, the same set of species we saw in the wallows appeared fairly quickly at the artificial ponds. These frogs apparently travel about looking for a pool. To find one, they not only search the forest floor but will also venture into cleared pasture. At least some of the forest frogs are more adaptable than anyone expected.