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The world's most well-studied population of butterflies has not gone extinct.
At least not yet.
For 38 years, during the brief spring butterfly flight season, students and scientists working with biological sciences Professor Paul Ehrlich have searched the rocky grasslands of Jasper Ridge Biological Preserve for Euphydryas editha bayensis Bay Checkerspot butterflies. Quick with the nets and gentle with their hands, the hunters have caught, marked and released hundreds, sometimes thousands of red, black and cream-spotted butterflies each season. Last year, in the spring of '96, they found none.
So famous is the Jasper Ridge population of checkerspots that ecologists around the nation lamented their passing. "It's the loss of a symbol," Florida State University ecologist Daniel Simberloff told a reporter for Science magazine early this year. The Science news article stated that Stanford biologists had decided to stand by and watch the butterflies die off. A reader fired off a letter of white-hot protest: No scientific goal could possibly justify such callousness, the letter-writer said.
"People thought we were heartless and strange," says research scientist Alan Launer of Stanford's Center for Conservation Biology, headquarters for the butterfly population studies. In reality, Launer said, the sudden drop in butterfly numbers was a surprise: In previous years, the checkerspots appeared to be slowly increasing. Postdoctoral fellow Stuart Weiss said there was very little that could be done to save the butterflies at this location and a great deal to be learned by watching and waiting.
So Launer and Weiss had mixed emotions when butterfly hunter Craig Fee came in from Jasper Ridge late in March with good news: That morning, he had caught and released five male checkerspots in prime condition. Rumors of the checkerspot's extinction were turning out to be premature.
Later in the week, hunters found another checkerspot, a single female. In theory, these six stubborn butterflies could repopulate the ridge. "In reality," said Launer, "this is probably the flicker on the edge of extinction [at this location]." The problem for the scientists: They must be certain that the Jasper Ridge population is truly extinct before they can re-introduce new colonies of Bay Checkerspots from nearby preserves. Now, that work will not begin until at least the year 2000.
How species go extinct
Weiss' and Launer's work is a continuation of one of the most comprehensive studies ever conducted of the rise and fall of a population. "Few other animals have been studied over long periods as intensively as the butterflies of Jasper Ridge," said Ehrlich, the Bing Professor of Population Studies.
"You have to go to Homo sapiens, a mammal, to get anything like comparable information," he said. "I think in some ways we understand more about the dynamics, the changes in population size, of Euphydryas than we do about humans."
By watching this butterfly, Ehrlich and generations of graduate students have exploded some long-held beliefs about how populations grow and how they sometimes disappear. Their research has helped to frame ecologists' understanding of co-evolution how plants and animals evolve depending on each other. The checkerspots have helped farmers understand why some plant-eating insects become crop pests and others do not. And they may offer an early warning of the effects of global climate change.
Ehrlich laments the lack of more long-term studies to compare the checkerspots with the population dynamics of other species. But he said some general conclusions can be drawn from the butterfly research:
* A species may not be made up of one large population but many smaller groups, separated by geographic or other barriers. By marking and tracking nearly every butterfly every year, the Jasper Ridge researchers learned that checkerspots on one sunny slope could be thriving while another sub-population was dying out less than a few hundred yards away. Over the years, populations on three separate spots rose and fell and were re-colonized. Finally, a long drought followed by heavy rains wiped out all but a few individuals.
As more and more sub-populations reach extinction, an entire species can die out in stages. Listed as "threatened" in the Endangered Species Act, and protected in large numbers at two other Bay Area preserves in addition to Jasper Ridge, the Bay Checkerspot has a fairly good chance of survival. But the same sorts of dynamics apply to other species as well.
For example, Peruvian anchovy fishermen learned that fish also have distinct sub-populations and that each group must be managed separately to save their fishery from an irrecoverable collapse.
At Jasper Ridge, the butterflies thrived on three distinct patches of serpentine, none larger than a few football fields. The checkerspot extinctions show that even a protected area with the right soil and plants may not be enough to save a population. Weiss believes the extra saving factor is topoclimate.
A "topoclimate" can be as large as a hillside, as small as a hummock: It's a location where the sun hits the ground at a certain angle on a certain day of the year. Working with an algorithm developed by civil engineering Professor Gil Masters to evaluate solar energy collectors, Weiss has developed a way to characterize warm and cool patches of terrain where butterflies are likely to prosper.
Checkerspot caterpillars need cool north and east-facing slopes on serpentine soil, because plantago and owl's clover stay green there longer. When they emerge as butterflies, they need sunny south and west-facing slopes where more flowers bloom. Weiss has shown that on a rough hillside with many warm and cool slopes right next to one another, butterflies are more likely to survive each year even as the weather fluctuates from cold and wet to hot and dry.
At Jasper Ridge, each of the butterfly habitats had slopes advantageous for some seasons but not for others. Fortunately for their long-term survival, one of the preserves set aside for the Bay Checkerspots has just the right sort of rough terrain. At Kirby Canyon in San Jose, a preserve managed by the city and a landfill company, Waste Management Inc., provides what Weiss calls an ideal checkerspot habitat.
Butterflies and climate change
Evidence from the checkerspots and their host plants contributed to the concept of co-evolution: that many species evolved dependent on one another. As pollution, invading species, housing projects and other incursions disrupt some species, others that depend on them must move on or perish.
The next big disrupter may be climate change. Most scientists now agree that human activities are making the earth warmer; ecologists are worried that warming may occur so fast that living things will not adapt successfully.
"There's a phrase, 'phrenological uncoupling,' " Weiss says. If the seasonal growth cycles of plants and animals go out of phase, one or both species can disappear. "It's scary. You can watch it happen with the butterflies." Since large organisms, like trees and animals, will take years to react to any change in climate, Weiss and Ehrlich say the butterflies at Jasper Ridge and around the world may serve as indicators for climate change. They could be figurative canaries in mineshafts, warning of effects that, years later, could harm forests or crops or ecosystems that humans depend on.
When checkerspots are re-introduced to the preserve, Weiss's topoclimate analyses will be used to chart how well they adapt to the traditional habitats. The re-introduction won't "save" the Bay Checkerspot; while it is still threatened, the ecologists say that careful management at the Kirby Canyon refuge should support thousands of the subspecies each year. The Jasper Ridge test will offer an example of how and whether a species can recover from loss of some of its sub-populations.
For more about Jasper Ridge Biological Preserve on the World Wide Web, go to: http://jasper.stanford.edu/jrbp/WWW/. For the Center for Conservation Biology, go to: http://www.stanford.edu/group/CCB/.
By Janet Basu