March 19, 2004
An accelerating decline in species, in particular a fall in butterflies, provides the first hard evidence that the Earth is on the verge of a sixth mass extinction.
There have been at least five over the past 500 million years, with the biggest occurring 250 million years ago.
Scientists report today that species diversity is falling fast and, contrary to current opinion, insects are particularly hard hit. This indicates that scientists may have underestimated the magnitude of the pending extinction.
If they are correct, the Earth is heading for the first global wipeout with an organic cause, with humans the dominant agent of destruction. Earlier extinctions were triggered by volcanism, cosmic impacts and other physical causes.
"The warning is there for all to see - we are poised on the verge of the sixth extinction crisis," said Dr Sandy Knapp of the Natural History Museum. "Britain, by virtue of its well-known and well-studied biodiversity, is the canary for the rest of the globe."
Around 28 per cent of our 1,254 native plant species have significantly fallen in abundance in the past 40 years, 54 per cent of the 201 native bird species over two decades, and 71 per cent of our 58 butterfly species over the same period, according to the milestone comparative study published in the journal Science, led by Dr Jeremy Thomas of the Natural Environment Research Council's Centre for Ecology and Hydrology, Dorchester.
"One form of life has become so dominant on Earth that, through its
over-exploitation and waste, it eats, destroys or poisons the others," he said. "It is accelerating, this decline, and we are going to lose more than we lost in the past 20 years."
The team's study used data collected by scientists and 20,000 volunteers scouring the countryside and could only have been done here, where more is known about diversity than anywhere else.
Until now, the idea that the world is undergoing a sixth mass extinction, with the loss of species rising to 100 times normal rates, has rested on studies of a relatively small portion of the world's plants and animals.
Population information about insects, which make up approximately half of all known species on Earth, has been particularly sparse and talk of a mass extinction was "an enormous extrapolation", said Dr Thomas.
Now this gap has been filled, as Dr Thomas and colleagues analysed six surveys covering virtually all of the native plant, bird and butterfly populations over the last 40 years, including one he helped to conduct 25 years ago, revealing the impact of pollution, habitat loss and degradation.
Dr Thomas said his team was surprised butterflies had fared so poorly, a discovery with global implications. "This provides the first objective support, for any group of insects, for the hypothesis that the world is experiencing the sixth major extinction event."
Over 20 years, the ranges of approximately 70 per cent of all butterfly species declined to some degree, many severely. Shadier woodland floors, resulting from changing management, have harmed caterpillars of the high brown fritillary, causing a "frightening" 71 per cent drop, for example. Loss of grasslands have harmed the blues, such as the Large Blue.
On average, these insects disappeared from 13 per cent of areas they once occupied. "That's the opposite of what people thought 20 years ago: that insects were much more resilient because they could fly about," Dr Thomas said.
In a second study published today in Science, Carly Stevens, a doctoral student at the Open University and the NERC Centre for Ecology and Hydrology in Huntingdon, and colleagues recorded the abundance of plant species in 68 grasslands in upland areas.
She reports "strong evidence" of a decline in species richness, for instance in species such as heather, harebells and eye-bright.
Nitrogen pollution is the most likely cause: excess nitrogen can allow a few species, especially grasses, to grow fast and crowd or shade out their neighbours. The nitrogen is the result of agricultural fertilisation and fossil fuel combustion.