Earth faces sixth mass extinction
The Earth may be on the brink of a sixth mass extinction
on a par with the five others that have punctuated its history, suggests the strongest evidence yet.
Butterflies in Britain are going extinct at an even
greater rate than birds, according to the most comprehensive study ever of butterflies, birds, and plants.
There is growing concern over the rate at which species
of plants and animals are disappearing around the world. But until now the evidence for such extinctions has mainly come from
studies of birds. "The doubters could always turn around and say that there's something peculiar about birds that makes them
susceptible to the impact of man on the environment," says Jeremy Greenwood of the British Trust for Ornithology in Norfolk,
and one of the research team.
Now there is concrete evidence that insects - which account
for more than half the described species on Earth, are disappearing faster than birds.
"If we can extrapolate that pattern of the British butterflies
to other British insects, and indeed to invertebrates across the planet, we are obviously looking at a very serious bio-diversity
crisis," says team member Mark Telfer of the Royal Society for the Protection of Birds in Bedfordshire,
Six large sets of data collected over the past 20 to 40
years in England, Wales, and Scotland
were analyzed by Jeremy Thomas of the Natural Environment Research Council Centre for Ecology and Hydrology in Dorchester, UK and
colleagues. More than 20,000 volunteers submitted over 15 million records of species.
The researchers found that populations of 71 percent of
the butterfly species have decreased over the last 20 years, compared to 56 percent for birds and 28 percent for plants. Two
butterfly species (3.4 percent of total) became extinct, compared to six (0.4 percent) of the plant species surveyed. None
of the native breeding birds went extinct in the last 20 years.
Crucially, the decline in populations happened in all
the major ecosystems and was distributed evenly across Britain,
rather than in just a few heavily degraded regions.
The crisis could be foreshadowing a sixth mass extinction,
warn the researchers. Life on Earth has already seen five mass extinctions in its four billion year old history. The last
one, which wiped out the dinosaurs, happened 65 million years ago at the end of the Cretaceous period and was possibly caused
by a giant meteor collision.
The current extinction is being precipitated by the widespread
loss of habitats because of human activity, according to Tefler. The remaining habitats are small and fragmented, and their
quality has been degraded because of pollution.
This claim is strongly supported, at least for plants,
by a second study published alongside Thomas' paper in Science. Carly Stevens of the Open University in Milton
Keynes, UK, and her colleagues studied the diversity of plants
in 68 grassland sites in the UK. The number
of species in each site varied greatly, from a mean of 7.2 to 27.6 species per site. Nitrogen pollution was found to blame
for this variability.
"We found strong evidence that the decline in the species
richness of grasslands within the UK was
linked to nitrogen pollution," says Stevens. "In areas of high nitrogen pollution the species richness was much lower than
in areas of low pollution, such as the Scottish highlands."
Atmospheric nitrogen pollution is caused mainly by the
burning of fossil fuels and from intensive agriculture, especially from the volatilisation of animal waste. This nitrogen
is deposited on the soil, favouring the growth of some species to the cost of others.
"Evidence of a global extinction crisis has come into
stark focus with these important results," comments Mark Collins of the United Nations Environment Program's World Conservation
Monitoring Centre in Cambridge, UK.
Journal reference: Science (vol 303, p 1879)