Dawn Levy writes that since 1960, biologist Paul Ehrlich and his research group have been conducting a study of the population of Jasper Ridge's Bay checkerspot butterflies. But now they won't be able to continue their study, because the last two Jasper Ridge populations went extinct in 1991 and 1998. After examining 70 years of rainfall and population data, the researchers conclude that extreme swings in regional climate hastened the extinction of the butterflies.
"This is the first time anyone has documented the effects of weather variation linked to climate change on extinction of populations," says Ehrlich. Previous studies have looked at shifts in average conditions, such as whether the climate has become drier or wetter in general, or the effects of a single extreme climate event, such as a hard freeze. The Stanford study instead examined the effect of climate variability - swings in rainfall highs and lows - on extinction, and tied the extinctions to this. "People have long been aware of the problem of species extinctions, but are just beginning to realize the importance of population extinctions," Ehrlich says.
"What we've done is develop a model describing how precipitation drives the changes in population numbers of the butterfly," says Carol Boggs, director of the Center for Conservation Biology. "We looked at how precipitation has changed locally in the last 70 years and found that the variability in precipitation had increased since 1971." The drastic differences in rainfall created pressures the checkerspots simply could not overcome, she says. "Population extinction in these insects can happen very fast - in a matter of decades - if you have high variation in rainfall."
The Jasper Ridge checkerspots weren't the last checkerspots on Earth. Several other populations of the butterflies are known to exist, but many of these are under pressure as well. "Three hundred years ago, the checkerspots were almost certainly widespread in California," Ehrlich says. But when the Spaniards settled the state in the 1700s and 1800s, in the hay they fed their cattle they inadvertently imported Eurasian grasses that competed with the native plants upon which the butterflies fed.
If populations become too small or isolated, fairly rapid extinction is inevitable. Small populations have trouble riding out the changes that can sharply reduce their numbers. Human population growth can bring urban sprawl and plant invasions that isolate one population "island" from another and create insurmountable barriers to migration and repopulation.
Rainfall is the biggest factor in determining checkerspot numbers. Caterpillars hatch from eggs in April but will starve if they can't get big enough before the summer drought comes and their seasonal food plants perish. The longer those plants stay alive, the more time the caterpillars have to get fat enough to reach their fourth instar (the stage between molts). At that point, the caterpillars can go into a resting stage called diapause, when their metabolism shuts down for the summer. "The biggest source of mortality for the butterflies is failure to get big enough to go into diapause before the plant vanishes," Boggs says.
When the rains come in November, the caterpillars begin feeding again. Now they are only half an inch long, but they must multiply their weight many times before they become chrysalids (pupae), the stage at which caterpillars metamorphose into butterflies. "This happens in late winter," Ehrlich says. "It takes about 10 days for the feeding-growing machine, the caterpillar, to be converted into the reproducing-dispersing machine, the adult."
"Population extinctions, like species extinctions, have been going on ever since life first evolved," Ehrlich points out. "What's different now is the rate of extinction is higher than any time in the last 65 million years - since the dinosaurs went extinct." The Jasper Ridge checkerspots were going to go extinct anyway, according to Boggs and Ehrlich. But something happened in 1971 that increased the speed of the extinction: Annual rainfall began to vary wildly. Boggs says, "We have no data to indicate the cause."
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Global warming will wipe out wild polar bears within 60 years, according to a report from the Polar Bear Institute. By 2060, climate experts believe Arctic pack ice will have melted so much that all of the existing population of 22,000 polar bears will starve because the animals they feed on, such as seals, will be difficult to find.
The World Wide Fund for Nature (WWF) drew attention to the bears' plight in 1999, and says they are "living on borrowed time" and climate change is their main risk.
Polar bears need the spring pack ice as support for their dens and to enable them to travel large distances in search of prey, particularly seals. But that ice has thinned by 40 per cent in 20 years, from an average thickness 16 feet to 9 feet, says Peter Wadhams, professor of ocean physics at Cambridge University.
As the pack ice thins it reduces the areas bears can hunt. Thin or broken ice is more tiring to walk over, so they have to travel further to get the same amount of food. For cubs, that can be the difference between life and death.
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Almost a quarter of the world's mammals face extinction within 30 years, according to a United Nations report on the state of the global environment. The destruction of habitats and the introduction of alien species from one part of the world to another are blamed for the threatened loss to biodiversity.
The United Nations Environment Program (Unep) report identifies more than 11,000 endangered animal and plant species - including more than 1,000 mammals, nearly a quarter of the world's total. One in eight bird species is also in danger of extinction, and more than 5,000 different plants.
The species that will vanish within three decades include well-publicized cases such as the black rhinoceros and the Siberian tiger, as well as less well-known animals such as the Philippine eagle and the Asian Amur leopard.
Human settlement in wilderness regions, as well as rainforest and wetlands destruction and the impact of industry, have had a dramatic impact on the survival of animals and plants.
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