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What's Causing Bird and Amphibian Decline?

Cameron Walker
for National Geographic News
November 8, 2004
 
Species in the air and on the ground are in deep trouble, according to two recent studies tallying declines in bird and amphibian populations.

While many factors may be causing this, climate change could be behind amphibians' decline, while habitat loss is whittling away bird populations.



Nearly 1 in 3 of the 5,743 described amphibian species are in decline, according to survey results released last month. At least nine species have disappeared since 1980.

"It was much worse than we expected," said Simon Stuart, leader of the Global Amphibian Assessment organized by the World Conservation Union (IUCN), Conservation International, and the Virginia-based conservation nonprofit NatureServe.

"The evidence of amphibian decline is getting clearer and clearer," said Stuart Pimm, a conservation biologist at Duke University in Durham, North Carolina. In a group of 435 species suffering rapid declines, nearly half face neither vanishing habitat nor overexploitation. "It's beyond the normal cause, habitat destruction," Pimm said.

Birds were added to the lineup of species in trouble when the U.S. National Audubon Society released its first State of the Birds report, also last month, documenting the decline of nearly 30 percent of North America's birds.

From past bird surveys, researchers had known the extent of the problem, said Greg Butcher, director of bird conservation for the Audubon Society. "This is just a way of laying it out in black and white," said Butcher, who led the State of the Birds project.

Even though scientists have identified only one-tenth of the estimated 15 million species of birds worldwide, Pimm said, it's clear that species have gone missing. "There's a bunch of things that just aren't there anymore," he said.

Amphibians in Trouble

The amphibian study identified a group of species that is in decline for no readily obvious reason. These "enigmatic decline" species may be suffering from a combination of infectious disease and a changing climate, said Stuart, the amphibian study leader.

Amphibians have extremely thin skin, which makes them sensitive to slight changes in temperature, humidity, and air and water quality. It's also made them highly susceptible to chytridiomycosis, a fungal disease that appeared in the last two decades, first in Australia and Central America. The fungus has now spread to amphibians worldwide, and at least eight species have likely vanished as a result; an additional 113 species have not been found in the wild in recent years and may also have disappeared.

A warming climate could have encouraged the fungus to appear and spread, Stuart said. "It is an extremely high research priority to find out how to control the disease in the wild," he said.

The golden toad (Bufo periglenes), once a flagship species of Costa Rica's Monteverde Cloud Forest Reserve, was last seen in 1989. Extensive searches have failed to uncover the golden toad. It's thought the species may have succumbed to a combination of climate change, fungal disease, and perhaps other factors.

Whether or not their declines are understood, amphibians may serve as a warning for less-sensitive species. "The risk is, what happens with amphibians now could be what happens with other species in the future," Stuart said.

Worldwide concern for amphibians sprung from the first World Congress on Herpetology in 1989, when researchers armed with isolated studies and anecdotes from scattered research sites began to piece together a larger puzzle.

Throughout the 1990s more studies pointed to losses too big to be explained away by the natural ups and downs of amphibian populations, Stuart said.

David Bradford, a Las Vegas, Nevada-based herpetologist for the U.S. Environmental Protection Agency (EPA), said that while the issue of natural population fluctuations hasn't yet been completely resolved, the widespread species decline is convincing evidence that this isn't a natural phenomenon. "You've just got huge areas where amphibians have disappeared," he said. "State-sized areas do not normally vanish [in a natural cycle.]"

Grassland Birds Vulnerable

Audubon's report compiled more than 30 years of bird-count records, taken from the U.S. Geological Survey's annual breeding-bird survey, which monitors bird populations each spring at 3,000 roadside survey routes around the country.

Grassland birds fared the worst of the lot, with 70 percent of species in decline—a decline that started even earlier than the breeding bird survey, said Robert Askins, an ornithologist at Connecticut College in New London, Connecticut.

After World War II a shift to more intensive types of agriculture chipped away at bird habitat, Askins said. Traditional farming included fallow fields and wide grass hedges that birds could call home. But mechanized farming allowed farmers to use larger areas of land and plant them intensively, leaving birds out of luck.

"A lot of these species use very specific types of habitats," said Askins, who has studied the effect of fragmented habitat on birds in the northeast U.S. and Japan. When grasslands are turned into cropland or suburban developments, grassland birds like the eastern meadowlark (Sturnella magna), which has had its numbers drop by two-thirds since 1966, are shut out from their previous nesting areas.

As with amphibians, diseases are a growing problem for birds. The West Nile virus is beginning to infect birds such as U.S. crows and owls, Askins said, but no one is certain how widespread its effect will be.

DDT Ban Helps Birds

While the birds seem to be declining, some improvements hint at hope that populations can bounce back.

"One of the neatest things is that we've seen the recovery of bird-eating and fish-eating birds," Butcher said. Bald eagle, peregrine falcon, pelican, and osprey populations have been slowly soaring since the EPA banned the pesticide DDT in 1972. DDT had the side effect of thinning birds' eggshells.

"We have good examples that things can improve," he said.

While a handful of amphibians have started to recover with reintroduction programs, such as the Mallorcan midwife toad (Alytes muletensis), the prospects may be more dire for amphibians than for birds. The problems facing amphibians can't just be attacked locally, Stuart said. "We have to focus on climate change and really start talking about how to stop this incredible experiment with the planet we're doing," he said.

A worldwide effort may be needed to stem the tide, but local groups can assist scientists' efforts in species monitoring—a key to learning more about population health. In South Africa a volunteer force recorded frog and toad calls, leading to an extremely comprehensive species' atlas, Stuart said.

Many experienced bird-watchers have volunteered throughout the years to monitor North America's birds during the Breeding Bird Survey, which provided the data for the State of the Birds report.

Even small actions, like providing backyard bird feeders and fountains, can boost bird survival, Butcher said. "Many of these birds are just here making a stop on their way south," he said "If we take care good care of the birds when they're with us, it would help them all across their range."

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