By PAUL RECER
AP Science Writer
March 18, 2004,
9:50 PM EST
WASHINGTON

A steep decline in birds, butterflies and native plants in Britain supports the theory that humans are pushing the natural world into the Earth's sixth big extinction event and the future may see more and more animal species disappearing.

In an effort that sent more than 20,000 volunteers into every corner of England, Scotland and Wales to survey wildlife and plants, researchers found that many native populations are in big trouble and some are gone altogether.

"This is the first time, for instance, that we can answer the question, 'Have butterflies declined as badly as birds?'" said Jeremy A. Thomas, an ecologist with the National Environment Research Council in Dorchester, England, and the first author of a study appearing in the journal Science.

A survey of 58 butterfly species found that some had experienced a 71 percent population swoon since similar surveys taken from 1970 through 1982.

Some 201 bird species were tracked between 1968 and 1971, and then again from 1988 to 1991, with a population decline of about 54 percent. Two surveys of 1,254 native plant species showed a decrease of about 28 percent over 40 years.

Thomas said that other scientists, noting losses of mammals and other animals, have speculated about the loss of insects, but the British butterfly study is the first to actually document over decades such a steep decline.

"Population extinctions were recorded in all the main ecosystems of Britain," Thomas and his co-authors wrote.

This supports the theory, they said, that "the biological world is approaching the sixth major extinction event in its history."

Thomas said that some past extinctions have killed off more than 90 percent of all life forms and "nobody is suggesting we are at that point." But, he said, "if this goes on for the foreseeable future then within a short period in geological time we will be getting toward the level of a major extinction."

Scott Miller, a biologist with the Smithsonian's National Museum of Natural History, said the British study was impressive in its thoroughness. He said, "They may not be representative of the world as a whole, but they have the best data."

The data support the idea that the rise of humans over tens of thousands of years -- along with climate changes -- is reshaping the natural world in ways that aren't thoroughly understood.

Scientists have identified five extinction events in Earth's history, with some so severe that more than 90 percent of all life forms died.

The last and most famous extinction was the Cretaceous-Tertiary event some 63 million years ago that killed the dinosaurs and allowed the rise of mammals. It is thought to have been caused by an asteroid hitting Earth.

"We are in the middle of a sixth extinction event that began about 50,000 years ago" with the expanding role in the world of human beings, said Paul S. Martin, a zoologist and geochemist at the University of Arizona in Tucson. "It's happening, but it's slower and it is not clear it will be as severe as some of the others."

Stuart Pimm, an ecologist at Duke University, said in Science that the British study results "show that we have likely underestimated the magnitude of the pending extinctions."

Miller and Martin both point to the hundreds of species, mostly large animals and birds, that already are gone, some wiped out directly through human action.

Martin said the fossil records show that the disappearance of many animals in Australia, Madagascar and North America started about the time that humans arrived.

Gone from the natural North American environment, for instance, are mammoths, camels, giant sloths and saber-toothed tigers. The causes of the other extinctions are not well understood.

The largest ended the Permian Period some 250 million years ago. All but about 4 percent of all species disappeared then.

There were three other lesser-known events in the Ordovician (435 million years ago), the Devonian (357 million years ago) and the Triassic (198 million years ago) periods.