Author:Barry A. Palevitz
Camille Parmesan , assistant professor of integrative biology at the University of Texas in Austin, cares about butterflies, so she's concerned: Global warming is bugging her beloved insects. Three years ago, Parmesan reported that Edith's checkerspot butterfly had moved northward along the west coast of North America over the past century.1 Specifically, local populations were four times more likely to go extinct in Mexico, at the southern end of Edith's range, than at the northern end in Canada, as predicted if global warming were at work.
Alaskan glaciers and Greenland ice sheets are melting in the face of record warmth, much of it caused by human activity,2 but how global warming affects biology is uncertain. That's why Edith's checkerspot was a proverbial coal mine canary. "Dozens of different papers ... at some level have shown distribution changes," says Parmesan. Her study documented climate-induced change in a species over its entire range. According to Ronald Pulliam, ecology professor at the University of Georgia and former head of the National Biological Survey, "She showed that you can bring historical data to bear on the question and get reasonable answers. She set an example."
Science always hungers for more data, and if climate change is so significant, Parmesan's findings should hold true for other organisms. As Parmesan herself put it, "conclusive evidence for or against the existence of predicted biological effects of climate change will come ... from replication of this type of study with additional taxa in other regions."1
Parmesan didn't have long to wait--research on the biological effects of climate change is hot: Witness a journal started in 1995 called Global Change Biology, published by Blackwell Science. A variety of data now say that climate change has an impact on organisms.
Chris Thomasand Jack Lennon of the Centre for Biodiversity and Conservation at the University of Leeds in England recently compared successive censuses of British birds covering a 20-year period ending in 1991.3 They calculated that the northern margin of southern species shifted northward by 19.4 kilometers, while the southern margin of northern species remained relatively stable. The numbers were for species with no net change in range size, which helps eliminate potential errors in interpretation. The simplest explanation is climate change, especially since Britain warmed during the 20-year period.
Could climate be altering bird reproduction? The answer is yes, but any connection to range isn't obvious, according to Humphrey Crick, senior ecologist and head of the demography unit of the British Trust for Ornithology in Norfolk. In a study published two years ago, Crick and coworkers found that egg laying for 20 British birds advanced by nine days between 1971 and 1995, though factors besides temperature and precipitation may have contributed.4
Together with Timothy Sparks, biostatistician at the Institute of Terrestrial Ecology in Huntingdon, U.K., Crick again dipped into Trust nesting records, this time back to 1939, and found significant long-term trends in egg laying for 19 of 36 species.5 The birds started families later in the 1960s and '70s, but laid eggs earlier in the '80s and '90s. The pattern reflects a similar trend in temperature, which cooled just after midcentury, then warmed. Higher spring temperatures were especially significant, just as they were for early laying of the British great tit.6
At least one North American bird is also breeding earlier.7 A team led by Jerram Brown, professor of biological sciences at the State University of New York at Albany, found that Mexican jays in the Chiricahua Mountains of Arizona are laying eggs about 10 days earlier since 1971, and the change correlates with increasing minimum temperatures in March and April, when laying begins. According to Brown, "Ours was the first report outside Britain and still the only one I know of, though there are rumors of another."
Climate varies with altitude much as it does with latitude, and species composition responds in parallel. So a team led by J. Alan Pounds of the department of biology, University of Miami, looked at the effects of global warming on high, in Costa Rican cloud forests. Theory predicts that warming should raise cloud levels on these mountains, thus depriving ecosystems of critical mist precipitation, particularly during dry periods.8 According to Pound's team, less mist means declining populations of cloud forest lizards and frogs.9
Parmesan also checked elevation. Populations of Edith's checkerspot above 2,400 meters persisted more than those lower down.1 The effects of global warming depend on locale and species.
And not to neglect oceans, a team led by John McGowan at the Scripps Institute of Oceanography, University of California, San Diego, recently related Pacific sea surface temperatures and global warming to disturbances in a host of marine organisms.10
While birds and butterflies moved north and laid eggs sooner, global warming lengthened the growing season of plants. A collaboration led by Ranga Myneni, associate professor in Boston University's department of geography, used satellite data to calculate a normalized difference vegetation index, or NDVI, for the entire northern hemisphere.11 NDVI is related to the amount of photosynthetically active radiation absorbed by plants, and between 1981 and 1991 it increased north of 45 degrees N latitude, indicative of a 12-day longer growing season. In effect, spring started eight days earlier and fall ended four days later during the 1980s.
Earth-bound observations at a network of European monitoring stations called the International Phenological Gardens support the satellite data. Using information on spring and fall events, including leaf unfolding, flowering, and fall coloration, Annette Menzel and Peter Fabian of the department of bioclimatology and pollution research at the University of Munich concluded that despite some local differences, spring advanced by six days and autumn fell behind five days between 1959 and 1993.12 The total increase in growing season of nearly 11 days is remarkably close to Myneni's estimate from satellite data.
It's also close to the 8-10 day speed-up in egg laying reported by the Crick and Brown groups. Says Crick, "I am pretty sure that birds are responding to the environment via food supplies, rather than to temperatures per se. Earlier leafing is likely to lead to earlier insect emergence ...." Higher minimum temperatures also help birds conserve energy. The same factors may be speeding up reproduction in the Mexican jay, according to Brown: "Probably part of the cause of advanced laying comes from the food supply, and this includes a variety of arthropods, especially caterpillars. The other part seems to come from a direct effect of temperature on calories consumed at night."
Now Parmesan is back on stage with a look at butterflies from Europe to north Africa.13 Says Parmesan, "There isn't anywhere else in the world [but Europe] where there are as good historical records and current censuses." Leading a collaboration of 13 scientists, she found that in the last 30-100 years the northern boundary of 65 percent of 52 nonmigratory species advanced northward, while it withdrew southward in only 2 percent and remained stable in 34 percent. On the other hand, the southern boundary stayed the same in 72 percent of 40 species examined. Where data were available for both northern and southern boundaries, 63 percent of the species shifted northward, 6 percent moved southward, 23 percent were stable, and 3 percent expanded in both directions. Net northward movement of butterflies correlates with a northward shift in climatic isotherms (lines of equal temperature) in Europe and North America.
Movement of northern edges isn't surprising, but stability on the south side is a mystery. Says Parmesan, "I haven't really a clue." It could be due to other factors, including habitat loss. But if anything, habitat loss should have moved butterflies southward, given patterns of regional land use. Parmesan also excluded habitat destruction in her earlier study of Edith's checkerspot, as did Brown for Mexican jays.
Still, Parmesan is worried. Butterflies and birds are free to move around; less-mobile species such as beetles and frogs, which can't cross barriers posed by an increasingly fragmented landscape, may be at even greater risk. The decline in cloud forest frogs and lizards bears that out.
From butterflies to plants, the data agree: Earth has a fever, and its inhabitants are taking notice. Barry A. Palevitz (firstname.lastname@example.org) is a contributing editor for The Scientist.