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Tuesday, May 8, 2012

Frogs Of The OSA


In a Costa Rica refuge, preservation tactics are leading to a stable ecosystem where amphibians are thriving

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Green-and-black poison dart frog (Dendrobates auratus). This female green-and-black was a little over an inch long. I set up my tripod spread low to the ground and placed the flash in a softbox to create this image in the dark conditions under the rain-forest canopy.
Amphibians are often referred to as the canaries of the ecological coalmine—important indicators of environmental health. Herpetologists are quick to note that scientific research on Osa amphibian populations is lacking. According to Dr. Bill Lamar of the University of Texas, most studies in the area have focused on "identification and range as opposed to abundance and ecology." Dr. Lamar adds that the numerous local microclimates make thorough study of amphibian populations challenging. "One of the problems is that there are distinct areas on the Osa and the adjacent mainland, at least in terms of humidity," he says.

This lack of detailed research makes it difficult to determine exactly how global warming and climate change—experienced so far as wetter rainy seasons and warmer, drier dry seasons—will impact amphibians here in the long term. Mulder points out that changing weather patterns will likely have mixed effects: "More rain during the wet season could be good for some frogs, but higher peak levels in the creeks would also wash out the leaf litter on the banks, and that's exactly the environment that Golfo Dulce poison dart frogs need to reproduce."

Over the last two decades, however, the experiences of local naturalists and visiting biologists suggest that population trends tend to be stable so far and may be increasing in some areas, particularly in secondary-growth forest that has been provided the protection it needs to recover.

Since the creation of the Osa refuge, a number of key threats that impact frog populations around the world have been locally eliminated or kept in check. Non-native species that might out-compete or prey upon native amphibians have yet to become a problem. The practice of poaching for the international exotic pet trade, once common, appears to have been reduced dramatically with the creation of new laws protecting wildlife and greater awareness of the economic value of local biodiversity. Commercial logging has ended within the refuge. The absence of mining or significant agriculture has left the surface water free of heavy metals or pesticides. Elsewhere, runoff of agricultural fertilizer and livestock waste into rivers and lakes leads to algae blooms and a resulting explosion in populations of aquatic snails that are a key vector for trematode worms responsible for debilitating deformities in frogs.

The Osa frogs obviously benefit from the health of the ecosystem, in general, but Conservación Osa and local land owners also have taken measures to encourage favorable conditions. "My impression is that we've been seeing greater numbers and diversity of amphibians at El Remanso in part due to various practices we've put in place," says Daniel Gehring, who has landscaped the area around the lodge with native plants favorable to the frogs and has created a small frog pond. Of course, the healthy rain-forest ecosystem does the bulk of the work, and for any visitor here it's an inescapable fact that the frog species are present in great abundance.

Within two days of my first outing with Rinaldo Mulder, I had encountered a dozen different frog species on my walks through the forest. Within five days, I had found 15 Golfo Dulce poison dart frogs on the banks of the creeks.

"We used to see them only in one particular spot, but now we find them at various locations in several creeks," says Mulder. He added that a biologist who visited recently managed to document 30 different amphibian species during a single nighttime walk. Some of the native species include the "gaudy" red-eyed tree frog, masked tree frog, gladiator tree frog, cane toad, rain frog, glass frog and two more species of colorful poison dart frog—the green-and-black and the granular, the latter of which is often described as a red frog wearing green tights. By the way, poison dart frogs don't bite and are perfectly safe to photograph up close, but you wouldn't want to lick one.

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