Betsie Rothermel, a postdoctoral research associate at UGA’s Savannah River Ecology Laboratory, has published the first study ever to investigate whether juvenile amphibians possess an innate ability to orient themselves toward forest from a distance. Such an inborn talent might improve their chances of survival in habitats that have been fragmented by agriculture, development and other land uses.
Amphibians with complex life cycles-such as the spotted salamanders and American toads Rothermel studied-require aquatic habitats as larvae but are mostly terrestrial as juveniles and adults. Such amphibians are at risk both from shrinking wetland and vanishing terrestrial habitats.
To determine how many salamanders and toads could find their way to forest, Rothermel released both species from 18 artificial pools in pastures set up to replicate natural conditions. The pools were located from five to 50 meters away from the nearest forest edge. Drift fences were used to recapture the amphibians after they metamorphosed and embarked on their first migration into the terrestrial environment.
Rothermel found that most juveniles did not head for the nearest forest, suggesting they may not be able to detect forest from a distance and run a greater risk of perishing in unsuitable habitat. As the distance between the pool and the forest increased, fewer amphibians were able to reach the forest. At the 50-meter distance, less than 15 percent of either species made it to suitable habitat.
Adult amphibians sometimes lay their eggs in ponds located in fields and pastures. “Such breeding sites are disconnected from suitable terrestrial habitat,” Rothermel says, “and may be population sinks due to high mortality of juveniles during emigration.”
While such results may not be surprising, current wildlife management guidelines do not in fact take the terrestrial needs of many wetland animals into consideration. As suitable habitat becomes increasingly isolated, Rothermel says, an animal’s willingness and ability to cross open, disturbed areas is essential if the species is to survive. Yet for pond-breeding amphibians, which have limited mobility, movement behavior has been seldom studied. Such a lack of knowledge could leave these small vertebrates, which naturally experience high rates of extinction due to drought and other factors, especially vulnerable to habitat alteration.
Rothermel is not the first scientist to consider such “landscape complementation” as it applies to amphibian populations. Previous studies also suggested that aquatic breeding sites had to be connected to forest to maintain populations of forest-associated species like salamanders, Rothermel’s is the first study, however, to actually test the migratory ability of recently metamorphosed amphibians.
Land managers interested in restoring wetlands or conserving species of pond-breeding amphibians should find these results useful because they indicate how close wetlands and forest need to be to protect all life stages. Additional research is needed to determine whether some types of forest alteration can occur without disrupting connectivity for such migratory species. Rothermel plans to continue her work.
“I am now exploring similar questions in the context of the LEAP (Land-use Effects on Amphibian Populations) study here at the Savannah River Site,” she says. “Specifically, I am testing the migratory success of juvenile and adult amphibians in relation to different levels of habitat disturbance associated with forest management practices.”