Kenneth H. Thomas/Photo Researchers On the move in coming decades: the American bullfrog.The consequences of climate change for animals can seem very direct, as with polar bears in a warming Arctic. Others involve leaps, like the case of an invasive bullfrog: by 2080, it could splash into some of South America’s most ecologically rich protected areas, disrupting unique hotbeds of biodiversity. At least, that’s the prediction of a new study in the journal PLoS One.
Worldwide, researchers have increasingly been focusing on how a changing climate has altered or is likely to alter migration patterns and the habitats that different species may find hospitable.
For example, one recent study suggests that more than a million giant king crabs have ventured into the warming waters of Palmer Deep in the Antarctic shelf in recent decades, destroying native sea life. (Colder waters may have kept these “skeleton-crushing predators” at bay for more than 14 million years, the report said.)
Another, a meta-analysis published in the journal Science, found that a host of animal and plant species are moving to cooler, higher altitudes at a striking speed (an average of eight inches per hour). They have moved farthest in regions where the most warming has occurred, the report said.
And then there’s Lithobates catesbeianus (or Rana catesbeianus), commonly known as the American bullfrog.
Working out of universities in Argentina, Brazil and Colombia, the authors of this latest study have mapped species distribution models against climate models, information about biological preserves and sites where the species currently lives.
The result is a prediction of what places are more likely to be invaded by 2080. If the climate changes as anticipated, it appears that a bullfrog invasion will subside in portions of central western Brazil, Argentina, Paraguay and Bolivia. But it will increase in parts of northern Brazil, southeastern Colombia, eastern Peru and southern Venezuela, the researchers project.
The American bullfrog is a particularly vexing trespasser. “Bullfrogs are superfrogs, very adaptable and seemingly immune to most of the causes of amphibian decline,” said Peter B. Moyle, associate director of the Center for Watershed Sciences at the University of California, Davis. “They live in a wide variety of habitats, colonize new ones readily, and eat everything that fits into their mouths.”
If that seems like an overstatement, consider the United States Geological Survey’s summary of the species’ diet, which includes birds, rodents, frogs, snakes, turtles, lizards, and bats. In short, “they are voracious eaters who will also prey on their own young,” the survey says,
Dr. Moyle noted that the American bullfrog gets so big that people around the world have embraced them for culinary purposes (frog legs), even in Europe, “home of the original edible frog.”
Endemic to the eastern United States and Canada, the species has been introduced in more than 40 countries and four continents, including more than 75 percent of South America.
As areas where the American bullfrog has already taken up residence become less hospitable in a changing climate, the researchers, led by Javier Nori of the Universidad de Córdoba, anticipate that protected forest areas will become more suitable for the species.
Unless steps are taken to prevent the invasion, the authors write, climate change could enable the American bullfrog to thrive in areas of the Andean-Patagonian forest, eastern Paragua and northwestern Bolivia, where the species has not yet been reported.
The authors’ concern is especially high for the Atlantic Forest, a biodiversity hot spot in tropical South America. “Continuous monitoring of the native biodiversity in this biome should be a priority since L. castesbeianus is likely to colonize reserves more efficiently under climate changes,” they write.
Dr. Moyle said the study is of the sort that should be carried out for other species worldwide because “it demonstrates that we can predict alien invasions.”
The American bullfrog has a record that even Cortés might envy. But beyond its history, there are additional reasons that the species seems likely to colonize new territory.
“Amphibians rely on external temperatures, moisture levels, rainfall to regulate their own conditions,” said Robin Moore, an amphibian conservation officer for Conservation International. “They have semi-permeable skin, so even slight changes in rainfall can really affect them. Given their reliance on external temperatures and climate, as the climate changes, they are going to move.”
For native species, the arrival of the American bullfrog often means new competition, predation and the rapid spread of deadly disease among amphibians.The bullfrog is a carrier of amphibian chytrid fungus, Dr. Moore said, “possibly the most devastating disease to affect an entire class of animals, certainly vertebrates.”
According to the International Union for Conservation of Nature, based near Geneva, nearly half of all amphibians are now at risk of extinction for many reasons, climate change among them. Dr. Moore described it as an enormous conservation challenge.
Dr. Moyle said the latest study was quite thorough in its climate modeling and predictions and suggested that this could make it more convincing for policymakers.
But even good models have their limitations.“It is important to keep in mind that these are predictions,” he cautioned, noting that they are based in part on a big assumption: that the areas that conservationists need to protect in 50 years will be the same areas protected for their biodiversity today. “It is unrealistic to assume no change,” he said.
In an e-mail, the study’s authors also emphasized, for example, that their models do not take into account the dispersal capability of the bullfrog and its interactions with other organisms. So while this type of modeling is useful for pinpointing areas prone to invasion and providing management guidelines, Dr. Nori’s team wrote, more work is needed.
The scientists called on governments to redouble their efforts to collaborate with universities, research institutions and environmental groups to address “an imminent biological invasion of the bullfrog in the continent.”
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