Brooding over Australian frogs.

Brooding over Australian frogs

In the last decade, about 20 of Australia's estimated 194 frog species have suffered serious local declines and at least two seem to have disappeared, reports zoologist Michael J. Tyler of the University of Adelaide in Australia. The declines echo unexplained losses of amphibian populations around the world (SN:2/24/90, p.116). One of the more bizarre species to become extinct anywhere is the Australian Reheobatrachus silus, o r gastric brooder, which Tyler identified in 1973. Adult females of this stream-dwelling species swallow their eggs, hatching and developing the young in their stomachs. Soon after the offspring lose their tadpole-like tails, the mothers eject the baby frogs by mouth. "Tyler's report of this unusual behavior made headlines in the mid-1970s, when the creatures were so abundant in the rain forest near Brisbane that researchers could observe 100 of them in a single night. But by 1981, the brooders had vanished.

One problem in tracing what happened, Tyler says, is that researchers did not immediately recognize the absence as an extinction because it occurred during winter, when observers would normally expect a temporary population decline.

"This phenomenon of apparently inexplicable decline or disappearance is [being] repeated time and again in eastern Australia," Tyler says. While he cannot offer an explanation for the trend, he suggests that important clues might emerge from an examination of sevreal other Australian species, such as the small marsh frog Limnodynastes tasmaniensis, that have thrived or even increased their numbers and range during the same time span. Oddly enough, the marsh frog may owe its good fortune to human tampering with the environment -- namely, road construction and excavation. During the rainy season, the pits created by workers quarrying for road materials make ideal breeding sites for the frogs, Tyler says. These temporary "water holes" cannot sustain fish and other predators of tadpoles, he adds. In more arid regions, excavation along the banks of streams has similarly boosted the population of other Australian frogs. "An appreciation of these success stories," Tyler says, "may contribute to an understanding of the modern failure."