Wildlife on the Hot Seat

What happens to the natural world when the Earth warms

  • Sharon Levy
  • Aug 01, 2000
ONE SPRING DAY in 1989, Camille Parmesan hiked with colleagues into a high meadow in California's Sequoia National Forest hoping to observe newly emerged Edith's checkerspot butterflies in flight. Instead, she found a hillside littered with the bright wings of dead butterflies. "It was spooky, the weirdest thing any of us had ever seen," says Parmesan, an ecologist at the University of Texas at Austin. "And one person there had worked on butterflies for 40 years."


The butterflies Parmesan found were not victims of poisoning or disease. They had simply gotten out of sync with the plants they depend on.

That year in the Sierra Nevada mountains, the snowpack melted early, and warm temperatures caused the butterflies to emerge early from their cocoons. But the plants that produce the nectar the insects feed on were not as sensitive to the change and had not yet bloomed. So the butterflies starved. That one small local population is now extinct, a sign that seemingly minor changes in climate--like a springtime temperature increase of less than one degree F.--can powerfully affect wildlife.

In the past decade, other scientists too have tracked impacts of global warming on life in a variety of habitats in North America and elsewhere--from Arctic sea ice to the arid U.S. Southwest. For some species, warming appears to be bad news; for others, the consequences are mixed or even positive. Some creatures are shifting their ranges north or bearing young earlier. In the Arctic, where the increase in temperature is the greatest, the changes appear to be causing a feast for some animals and famine for others. "The biological data cannot settle the debate over whether the current warming trend is due to human activities or not," says Parmesan. "What we can say is that a 0.7 degree Fahrenheit warming is important to wildlife." And biologists are concerned at indications that warming of more than a couple of degrees more could be catastrophic for animals and plants--particularly for rare or endangered species or wildlife restricted to small areas.

There's little question that the planet's atmosphere and oceans are warming up, and the signals from wildlife add to the evidence. A panel of experts at the National Academy of Sciences reported in January that global warming over the last two decades "is undoubtedly real and is substantially greater than the average rate of warming during the twentieth century." Mean global temperatures have increased 0.72 to 1.44 degrees F. over the past century and 0.45 to 0.72 degrees in the last 20 years. Many experts think the trend is caused by human use of fossil fuels that release carbon dioxide and other greenhouse gases. A few others still argue it is part of long-term variations in climate that have affected the planet for millennia.

But one thing is becoming clear: Rising temperatures are having real effects on wildlife of all kinds. In April, for example, scientists at a meeting of the Ecological Society of America reported that yellow-bellied marmots are emerging from hibernation 38 days earlier than they did 23 years ago, possibly making it more difficult for the animals to find food in areas still covered with snow. And American robins are beginning to migrate earlier in the year, perhaps because of warming temperatures at low altitudes. But at high altitudes, according to measurements at a study site in the Rocky Mountains, seasonal temperature ranges have not changed in 25 years. And that means robins arriving early there are finding more snow and waiting longer for it to melt than in the past.

Parmesan's ground-breaking study of the Edith's checkerspot was the first to examine the effects of long-term warming on a species over its entire range. First she used decades-old records in museums and private butterfly collections to patch together a map of the insect's historical distribution. Then she set out to census populations of the species from Baja California in Mexico into Canada.

After four years of fieldwork, Parmesan came up with a dramatic result: Her study subject was moving north and into the mountains as temperatures in those locations warmed. "The average location of a butterfly population moved 92 kilometers farther north and 124 meters up in altitude," she says. Parmesan also collaborated with colleagues to do a sweeping study of butterfly ranges in Europe, and the team reported last year that the same pattern holds there. "Two thirds of the species we studied were shifting north, which is very significant," she says. "If the movement were just random, you'd expect half the species that moved to go south and half to go north."

In other work, Stanford University doctoral candidate Jessica Hellman is studying how warming may affect the Bay checkerspot, a subspecies of Edith's checkerspot found in grasslands around San Francisco Bay. She is one of eight graduate students whose studies of wildlife and climate change are supported by fellowships from the National Wildlife Federation.

Hellman set up a series of greenhouse experiments in which caterpillars and their host plants were subjected to different temperatures. The point was to mimic conditions that already exist during warmer, dryer years, eliminating other factors that could influence the study subjects in the wild. Her preliminary results suggest that fewer caterpillars survive to become adults when warmer temperatures arrive earlier in the summer and cause their food sources of plant leaves to dry up earlier in the year.

"The long-term prospects for the Bay checkerspot are not good," Hellman says. "These insects are already threatened with extinction from habitat loss and human development, and my work suggests that climate change is likely to make the situation worse." Warmer temperatures could drive butterflies into areas where habitat has been depleted. Says Parmesan, "When that happens, we're suddenly going to get a lot of extinctions."

Meanwhile, a study of Mexican jays has found that some species are able to change the timing of their annual cycles to adapt to warming. That finding was reported last year by biologist Jerram Brown of the State University of New York at Albany, who began studying cooperative behavior in an Arizona population of the birds more than 30 years ago. Looking back at decades of data, he realized that as the area warmed, the jays began a gradual trend of breeding earlier each spring. By 1997, the birds were laying their first eggs an average of 10 days earlier than they did in 1971. For insect-eating birds--including Mexican jays--such flexibility may be critical for keeping in step with important food sources, such as caterpillars that tend to emerge earlier in warmer weather.

A previous study in England found that many birds in the United Kingdom have begun laying their eggs earlier during the past 25 years. Using data on 65 species of breeding birds compiled by volunteers from 1971 to 1995, scientists found that 20 species were laying significantly sooner in the spring. Only one species was laying significantly later. Says Brown, "The same pattern occurring in England and Arizona makes it more plausible that it's a global phenomenon." The British researchers recently hypothesized that more juveniles are likely to survive if they are born earlier in the spring and have more time to grow and fatten up before winter.

Rising temperatures are changing life in the oceans, too. Marine ecologist Rafe Sagarin, a doctoral student at the University of California at Santa Barbara, is using a National Wildlife Federation fellowship to study creatures in the intertidal zone of central California's Monterey Bay, where the water has warmed by about 1 degree C. over the last 6 decades. Focusing on a section of the bay that was studied in the 1930s, Sagarin has noticed a pattern: Species that were once more common south of Monterey Bay increased while those that had been more common to the north declined. "For instance," he says, "a tube snail that in the 1930s was abundant in Southern California but rare in Monterey Bay is now one of the most dominant intertidal species. And a northern species of limpet decreased significantly in abundance."

The take-home lesson, according to Sagarin, is that the intertidal community is dynamic and very likely responding to warming temperatures right now. His study area has been protected as a scientific refuge since the first surveys 60 years ago, so the changes are not due to direct human disturbances like fishing. And the move north of southern species fits the model of temperature-driven changes.

Some of the most dramatic impacts of warming on wildlife are taking place in the Arctic. Take the example of polar bears in Canada's western Hudson Bay. The bears rely on sea ice as a platform from which they hunt their favorite prey, ringed seals. In spring lots of newly weaned seal pups swim the waters of the bay, and the bears have a chance to fatten up before the sea ice melts. Once it does, they must go ashore and fast for as long as four months. Pregnant females fast for eight months and must use their fat reserves to carry and feed their cubs until they can return to the ice.

Biologists Ian Stirling and Nicholas Lunn of the Canadian Wildlife Service have found that the polar bears of western Hudson Bay have on average weighed less and have been giving birth to fewer cubs since 1981. During that time, rising temperatures have caused the sea ice to break up earlier in the spring, giving the bears less time to hunt. "Sea ice on Hudson Bay has been breaking up two to three weeks earlier in the 1990s than it did in the 1980s," says Lunn. The link between earlier breakup of the sea ice and the decline in the polar bears' condition, he adds, "seems to represent the best explanation for what we have observed over the past 20 years."

If the trend continues, the number of western Hudson Bay bears may start to drop, and the bears are likely to have more run-ins with people if they are forced to forage on land. As for other polar bears, communities and individuals scattered throughout the Arctic all face their own unique circumstances. Some populations of the bears live their whole lives on permanent sea ice, never coming ashore, and are less likely to be affected by warming in the near future--although the possible prospect of massive ice melting could spell serious trouble for the species.

Lunn points out that ice conditions can vary significantly from one year to the next, and that different parts of the Arctic can have very different microclimates: In eastern Hudson Bay, spring air temperatures are cooling rather than warming, sea ice breaks up at the same time it did 20 years ago, and the bears are fatter and healthier than their relatives in the western end of the bay.

The impact of changing ice conditions can create a ripple effect through the ecosystem. "Populations that are closely tied to snow and ice will be the first affected by global warming," says ornithologist George Divoky of the University of Alaska at Fairbanks. Since 1975, when he started studying a small group of black guillemots that had just begun nesting on Cooper Island near Barrow, Alaska, Divoky has seen those ties sustain and then starve the colony.

The seabirds first nested in boxes and other debris left behind by a Navy installation. Divoky built more nest boxes, and the guillemot population grew, peaking at 225 nesting pairs. Then the numbers began to drop. The population is now down to 110 pairs. Since 1990, he explains, rising temperatures have caused sea ice to recede farther from the island each spring, before the guillemots' chicks have fledged. The birds feed on Arctic cod that thrive beneath the ice. Parents with chicks to feed can't fly farther than about 20 miles from the nest to forage. In recent years the ice has melted back to 150 miles from the island while chicks are still in the nest.

This summer may be different, however. Meteorologist Russell Page of the National Weather Service's Alaska region has predicted that sea ice in the Barrow area would be thicker and break up later than usual this year. Page and many other climate researchers believe that this year may mark the beginning of a phase of colder weather in the North Pacific, caused by a cyclic climate pattern.

Whether that would be good news for the Cooper Island guillemots depends on how cold and snowy it gets. To nest, the birds need areas that are free of snow for at least 80 days. Before higher spring and summer temperatures arrived in the 1970s, the birds did not breed on Alaska's North Slope.

Climate change also may be affecting the Porcupine caribou herd that calves on the coastal plain of Alaska's Arctic National Wildlife Refuge. There summer temperatures have risen about two degrees F. over the last 30 years. Using satellite images, biologist Brad Griffith of the U.S. Geological Survey has found that since 1985, the amount of greenery on the calving grounds has increased as the temperature has increased. "That means there's more for a caribou cow to eat, more milk she can produce," he says. "So the calves grow faster and are healthier, and calf survival has gone up." The vegetation has also been greening up earlier, and the mass calving--in which 50,000 calves may arrive in a single week--has been taking place about one week earlier in the warmer springs.

But in the midst of all this plenty, the herd has declined. The problem may be unrelated to warming: For example, the creatures may have depleted the lichen food source in their winter range, or caribou predation may have increased. But the weather could be implicated. To know for sure, biologists need detailed studies of conditions on the herd's winter range. "If it's warmer in the winter, the snow may melt and freeze more often than if it were colder, and that may make it harder for the animals to access forage," says Griffith. That's one theory of Canadian biologists who study Peary caribou, which have suffered huge population losses in the Northwest Territories, perhaps from winters with unusual warm spells.

All of these studies--whether they reveal the impact of rising temperatures on birds, snails or bears--depend on knowledge of biological history. Knowing the past, say the scientists who study warming's effect on wildlife, is the only way to understand changes that may take place in the future. Or even in the present. "We should not expect the effects of global warming to take place 50 years in the future," says Sagarin. "It's happening right now."

California journalist Sharon Levy wrote about biological clocks in the December/January issue.

NWF Priority: Curbing Climate Change

Recognizing the serious threat that global warming poses to Earth's ecological systems, the staff of NWF's Climate Change and Wildlife Program is working with U.S. policymakers and the public to find reasonable, effective strategies to help solve the problem. As part of this program, NWF has funded several new scientific studies on the potential impacts of climate change on U.S. ecosystems and wildlife, including the grizzly bear. NWF also recently published The Toll from Coal, a report describing how the nation's coal-fired power plants are contributing to global warming, and what actions people can take to seek reductions in emissions from older plants. For more information, visit our Web site at www.nwf.org/climate.

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