| 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.
 |
| While Betsie Rothermel is not the first
scientist to consider “landscape complementation”
as it applies to amphibian populations, she is the first to
actually test the migratory ability of recently metamorphosed
amphibians. |
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.” |
