Plant, bird and butterfly species dwindling in the United Kingdom, researchers report

03/12/04

Results bolster the 'Sixth Extinction' hypothesis, Science authors say

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Two new studies of U.K. flora and fauna offer some of the first comprehensive evidence that species diversity is decreasing in the United Kingdom. The findings support the hypothesis that the world is experiencing a mass extinction on par with the other five mass extinctions that have punctuated the history of life.

Until now, this hypothesis has rested on data representing a relatively small portion of the world's plants and animals. Population information about insects, which make up approximately 50 percent of all known species, has been particularly sparse.

"One of the very obvious flaws in this argument was that it was an enormous extrapolation based on the best available evidence at the time," said author Jeremy Thomas of the Natural Environment Research Council (NERC) Centre for Ecology and Hydrology in Dorchester.

The two reports, which used data collected by scientists and thousands of volunteers scouring the U.K. countryside, now provide a thorough census for much of U.K. wildlife. The studies appear in the journal Science, published by AAAS, the nonprofit science society.

"These results are by far the most detailed estimates we have for declines in the distributions of multiple species from major different groups of organisms," said Andrew Sugden, an ecology expert and Science's international managing editor.

In their study, Thomas and his colleagues analyzed six surveys covering virtually all of the United Kingdom's native plant, bird and butterfly populations over the last 40 years. Although the results varied for individual species, each group of organisms showed some overall declines.

Butterflies have fared particularly poorly, the authors found. Over 20 years, the ranges of approximately 70 percent of all the butterfly species in the United Kingdom declined to some degree -- from a relatively small number of regional disappearances for some species to nation-wide extinctions for a few others.

Overall, these insects have disappeared from 13 percent, on average, of areas they once occupied, the authors report.

"That's the opposite of what people thought 20 years ago: that insects were much more resilient because they could fly about; so that changes our priorities in the United Kingdom," Thomas said.

If butterflies prove to be representative of insects as a whole, then "the world is indeed experiencing the extinction crisis many people have been suggesting and talking about for years," Thomas said.

For each of the three types of organisms, Thomas and his colleagues analyzed one set of population data from 20 to 40 years ago and another set collected more recently. For all the datasets, the researchers divided Great Britain up into squares ten kilometers across and recorded the number of species spotted at least once in each square.

One-third of all the species recorded disappeared from at least one of the squares they had occupied 20 or 40 years ago. That group includes 70 percent of the butterfly species, as well as 28 percent of native plant species and 54 percent of native bird species.

It should be "harder for policymakers or decision-makers to pooh-pooh this idea that extinction rates are real, if they see this evidence," said Thomas. "It strengthens the case for those arguing for policies nationally and globally to mitigate the effects that man is having on the environment."

In a second report, Carly Stevens, a Ph.D. student at the Open University in Milton Keynes and the NERC Centre for Ecology and Hydrology in Huntingdon, and her colleagues have implicated nitrogen pollution as the most likely reason for reduced grassland species richness in parts of the United Kingdom, and possibly elsewhere in Europe.

Excess nitrogen can allow a few species, especially grasses, to grow fast and crowd or shade out their neighbors. The nitrogen is deposited from the atmosphere as the result of agricultural fertilization and fossil fuel combustion.

"When species are lost through pollution or habitat fragmentation, it is often the native ones that disappear first," said coauthor Nancy Dise of the Open University and Villanova University in the United States.

"Maybe they are insects that are rare and endangered, maybe plants of untapped future value to medicine, or maybe just wildflowers or butterflies that make our hillsides beautiful to look at. And these may be just the showy members of a whole, functioning ecosystem that is slowly degraded," said Dise.

"We don't know how great the implications for consequential loss of other species, which rely on particular plant species, might be. Our results support the idea that pollution should be reduced and soon," Stevens added.

Stevens and her colleagues recorded the presence and abundance of plant species in 68 Agrostis-Festuca grasslands, which are typical of temperate grasslands in Europe and elsewhere.

The researchers then analyzed twenty different environmental factors to see which could best explain the variability in species richness from one site to the next.

Their results showed that the effects of nitrogen deposition could account for more than half of the variation in plant species richness. The relationship was linear, meaning that every additional amount of nitrogen deposited on a site over many years corresponded to an incremental decline in species richness.

The authors estimate that Agrostis-Festuca grasslands receiving an average amount of nitrogen deposition in the United Kingdom or central Europe may have already lost more than 20 percent of their species richness. Even though the rate of nitrogen deposition is beginning to decline in many areas of Europe and North America, recovery will likely be very slow, according to the scientists.

"The data suggest that it's taken around 40 years of high nitrogen deposition to get to this state, so it may take some time for species to return," said Dise. "And some of the changes may be irreversible."

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