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The Climate Conundrum
by Mari N. Jensen

Beak pointed at the sky, an alarmed great bittern (Botaurus stellaris) sways with the surrounding reeds, relying on its motion and reed-mimicking stripes to blend in. But trying to become one with the marsh cannot allay all danger. In fact, the bittern's dependence on reed beds means its existence is threatened by forces no amount of graceful bending in the breeze will deter: The bird's seaside haunts in Britain may simply vanish as global warming causes oceans to rise.

Across the Atlantic in Delaware Bay's marshes, coastal plains swamp sparrows (Melospiza georgiana nigrescens) hop about near the high-tide line in a band of marshes only about a football field or two wide. This subspecies of sparrow, found only in tidal marshes from New Jersey to Maryland, is also threatened by sea level rise, says Russell Greenberg, an ornithologist who heads the Migratory Bird Center at the Smithsonian's National Zoo. Although the bird inhabits a very populated region of the world, little is known about exactly where it lives. Greenberg just completed the first global survey of the sparrow's haunts this summer. The sparrow, he found, has strict habitat requirements. Although some of the birds now live in protected areas, he says, "It wouldn't take a whole lot of shifting of sea level to put the entire population off of any protected land."

Global warming is affecting far more than just a few rare seaside birds. The more scientists look, the more they are linking climate change to changes in the animal world. Shrinking sea ice makes hunting harder for polar bears and Adlie penguins but is a boon for chinstrap penguins. In Europe, many butterfly species are flitting farther north than ever before as northern climes become warmer. With the milder spring temperatures, birds breed earlier. But not all animals and plants respond the same way to the changes, so interdependent species may get out of synch. And areas like parks and nature preserves may become unsuitable for the very animals they were designed to protect.

Researchers are now beginning to document species at peril from global warming. From rare gelada baboons, whose cool mountain-top retreat in the Ethiopian highlands may shrink, to Bengal tigers, whose coastal habitat in the Sundarbans mangrove forest along the India–Bangladesh border could become submerged, climate-change effects on wildlife may be fully understood only after it’s too late.

Fewer Feast Days in the Frozen North

In winter, the ice covering western Hudson Bay is thick enough to land a helicopter on. Polar bears (Ursus maritimus) roam the ice, primarily feeding on ringed seals as they surface at breathing holes in the ice. By March, the bears are at their leanest.

But a bonanza of food is just around the corner. Ringed seals (Phoca hispida) give birth in April, providing a banquet for the 1,200 bears that make up the western Hudson Bay population, one of the southernmost of Canada’s 14 populations of polar bears. From then until the ice breaks up in late July, the bears gorge themselves on seal pups, according to Nick Lunn, a Canadian Wildlife Service research scientist who works on the Polar Bear Project. It’s crucial that the polar bears pack on the pounds, because when they come ashore after the ice breaks up they subsist on stored fat until the ice forms again in late November.

However, global warming is cutting the bears’ feasts short. Spring air temperatures in western Hudson Bay have warmed by nearly four degrees Fahrenheit since 1950. Now, Lunn says, the ice breaks up two to three weeks earlier, forcing the bears ashore sooner. It’s a double whammy for the bears—they have less time to put on fat, but the fat they do store must last three weeks longer than in olden days. For the last 20 years, Lunn and his colleagues have seen the bears’ physical condition—as determined by an index of various body measurements reflecting how well each bear is doing—decline by about 15 to 20 percent.

The situation is even worse for pregnant females, Lunn says. Rather than go back out on the ice in November, a pregnant female digs an earthen den in the fall and holes up for the winter. She gives birth to anywhere from one to three cubs in late November. Snug in their den, the new family cocoons together until February or March. All that time, mom’s stored fat must not only sustain her, but provide the energy needed to make milk for nursing cubs. Even in good years, by the time she and her cubs emerge to go back on the ice and hunt for seals, she’s “very thin,” Lunn says. “The hide is just sort of hanging on [her].”

Researchers at the Polar Bear Project have noticed that the bears are starting to have fewer cubs. At present, the western Hudson Bay population of polar bears is stable, Lunn says, but he adds, “If the trend continues as it is, eventually this population will start to decline.” Although he cautions that other forces could be at play, he argues that global warming right now seems the best explanation for what researchers see happening to the bears.

It fits with predictions, he points out. Biologists have suggested that, for creatures adapted to Arctic conditions, those animals living farther south will be the first to feel the effects of global warming. The western Hudson Bay population indeed lives farther south than other polar bears. And in regions where the ice persists as long as it always has, biologists who study polar bears haven’t seen the decline in condition that Lunn and his colleagues have noticed. That could change. Lunn says, “If global warming occurs throughout the Arctic, you’re likely to see these things happen elsewhere.”

Penguins Paddling Farther for Food

In a comparably harsh environment at the other end of the globe, William Fraser has seen Adlie penguin (Pygoscelis adeliae) numbers plummet in the last quarter century from 15,200 breeding pairs to about 8,000 pairs. He too attributes the animals’ plight to shrinking sea ice and other environmental changes caused by global warming.

Fraser, an ecologist at Montana State University in Bozeman, does his research at Palmer Station, a U.S. research base on the western Antarctic peninsula. In the summer, the ocean comes right up to his research site, and Teton-like mountains form the backdrop for the region’s tremendous diversity of wildlife, which includes vast breeding colonies of giant petrels, South Polar skuas, elephant and fur seals, and Adlie, chinstrap, and gentoo penguins.

In about 1988, Fraser and his colleagues began to notice unaccountable alterations in the Adlie penguin populations. There was no indication that the food web was changing, but penguins weren’t doing so well. The team realized the penguins’ winter habitat—the sea ice—had been shrinking as winter temperatures rose. The seven- to nine-degree Fahrenheit increase in midwinter temperatures that has occurred in the region, he says, is “one of the largest increases on the planet.”

For Adlies, the sea ice isn’t just a place to hang out. It serves as a platform for feeding in the food-rich areas of the ocean. In mid-July, the height of the Antarctic winter, the penguins have only a few hours of daylight to forage under the ice for krill, the tiny shrimp-like animals at the base of Antarctic food chains. When the sun limps over the horizon at about 11 a.m., the birds all pop into the water. They stay submerged until about 1:30 p.m. or so, but all jump back out well before dark, which occurs around 2:30 in the afternoon. During those brief feeding bouts, the penguins need to chow down more than enough calories for that day; they must also store up fat for the breeding season ahead.

Fraser suspects that as the ice recedes, it no longer covers the best feeding grounds. Adlies, rather than just plopping into the water right by the best grocery store in town, now have to either swim some distance to find good feeding grounds or forage in the less productive areas right on the ice’s edge. Either way, the birds are likely to be getting less food—and expending more energy to get it—than they once did. Fraser doesn’t see skinnier birds returning in the spring, but he is seeing fewer of them. The winter season, he thinks, is do or die for the penguins: Birds that don’t get enough to eat just don’t live. This year he plans to outfit about 25 Adlie penguins with satellite tags so he can track the movement of the birds throughout the winter to see how close they stay to the ice.

Depending on where their traditional breeding grounds are, the birds that return may run into additional difficulties. As the globe warms, the poles are getting more winter precipitation. For most Adlies, that’s not a problem. But those who nest in places where the wind piles up snow encounter much more snow than in past years. Undeterred, the birds burrow down into the snowdrifts and lay eggs. But if the birds don’t reach bare, gravelly soil and instead lay their eggs on the snow itself, the spring thaw renders the nest site an icy puddle.

“There’s six inches of water, and the birds are just sitting on the eggs,” Fraser says. If the eggs are flooded too long, the embryos die. Adlies seem to be hardwired to begin breeding at a certain time on the calendar, he says, so waiting until the snow melts just isn’t an option for them. “Historically, there was probably little or no snow that time of year,” says Fraser.

Fraser attributes the drop in Adlie penguin numbers to these two global-warming-induced environmental changes—the retreat of the sea ice and the flooding of nest sites. “The big picture is that the Adlie penguins are retreating,” he says. “But there’s a limit to how far [south] they can go—they can’t get below the Antarctic Circle because they’ll run out of light in the wintertime.”

In the same location, as the sea ice retreats, an ice-avoiding species, the chinstrap penguin (Pygoscelis antarctica), is expanding southward. In 1975 there were eight or nine breeding pairs at Fraser’s field site; now there are 300 to 400. “Ecologically, they are the exact opposite of Adlies,” he says. During winter, chinstrap penguins hang out on icebergs in the open sea. And although the chinstraps breed side by side with the Adlies, the chinstraps start about three weeks later, meaning they start after the snow melts.

“As one environment replaces another, the chinstraps are doing well—and the Adlies are not,” says Fraser. For species like Adlies, whose breeding behavior seems triggered by a time on the calendar rather than by environmental conditions, Fraser predicts the speed of climatic change will continue to exceed the rate at which species can adapt.

Swallows and Swallowtails Confounded by Climate

But not all birds are tied so tightly to the calendar. By looking at years of nesting records for 65 species of British birds, ornithologist Humphrey Crick of the British Trust for Ornithology in Thetford, England, reported in 1997 that about one-third of these species were laying their eggs an average of nine days earlier than a quarter century ago. The finding fit in nicely with reports that trees are leafing out and insects emerging earlier in Britain.

The British Isles aren’t the only place the rites of spring are starting sooner. Upon learning of Crick’s work, University of Wisconsin at Milwaukee behavioral ecologist Peter Dunn realized he could do a similar analysis with nest box records for tree swallows (Tachycineta bicolor) in North America.

For years, North American birders have gathered information about nesting birds such as the species of bird, nest location, and number of eggs. Birdwatchers then send the data off to central record-keeping agencies—either Cornell University in Ithaca, New York, for U.S. nests, or Toronto’s Royal Ontario Museum for Canadian nests. Records on tree swallows are particularly common, Dunn says, because of the hundreds of thousands of bird enthusiasts all over North America who put up nest boxes for bluebirds. “Most of the time they get tree swallows.”

Dunn and his Cornell colleague David Winkler found that from 1959 to 1991 the archive had 3,450 good nest records on tree swallows. Unlike Crick’s data, which covered a relatively small, homogenous region, the tree swallow nest records spanned North America—stretching from California to North Carolina, and north as far as Nova Scotia.

Because the records covered such a large geographic area, Dunn didn’t expect to find a very clear pattern. He was wrong. Like Crick, he found the birds were laying their eggs nine days earlier than 30 years before. That finding, Dunn says, provides a strong argument that the birds are indeed responding to a worldwide phenomenon like global warming, rather than some local climatic variations in the British Isles or in one part of North America. Other bird researchers are finding similar changes: Mexican jays (Aphelocoma ultramarina) in Arizona’s Chiricahua Mountains are laying eggs 10 days earlier than in 1971, reports Jerram Brown, a biologist at the State University of New York in Albany.

To figure out when to breed, birds probably use changes in day length as an initial cue, but the bird’s own body condition determines the fine-tuning, Dunn says. Food availability is probably key for determining laying dates. Tree swallows fly around like “little vacuum cleaners” sucking insects out of the air, Dunn says, and appear to breed only after having consumed a certain threshold amount of insects. Dunn is now collecting information to see whether North American insects are indeed emerging earlier than they have in the past.

Tree swallows aren’t the only animals in which scientists have seen continent-wide shifts in behavior that seem to track global warming. By comparing information from museum records, private collections, and researchers’ field notes with current censuses of butterfly populations, Camille Parmesan, a University of Texas at Austin ecologist, showed that the southernmost populations of Edith’s checkerspot (Euphydryas editha)—a butterfly that ranges from Mexico north into Canada—were winking out faster than those farthest north. This pattern of extirpation, she reported in 1996, would be expected if global warming was driving the changes in the butterfly’s distribution.

Not content with seeing the pattern in a single species, Parmesan wanted to know whether a similar global warming signal could be observed for many such species across an entire continent. So she chose Europe, knowing that specimens and records from several centuries of butterfly collecting could be found there. For two years she drove all over Europe, visiting museum scientists and butterfly collectors from Helsinki to Barcelona and enlisting their help. She and her colleagues spent weeks poring over butterfly specimens and field notes to tease out the data showing where Europe’s 440 species of butterflies used to live and where they live now.

Parmesan wanted to include only butterfly species with large geographic ranges, those found as far north as Britain, Sweden, and Finland while simultaneously ranging south to France and even North Africa. Of the 183 species with such wide distributions, only 35 species ended up as good candidates for the analysis. When the results were in, the researchers saw a clear pattern: Two-thirds of the 35 species had shifted their ranges north in the last 100 years, some as much as 150 miles. “The sheer number of species shifting north is incredible,” she says, adding that it’s not just the big, beefy fliers like swallowtails that are trekking north generation by generation; fragile little butterflies called “blues” and “coppers” are also making the journey.

Seeing changes in distribution at the north or south edges of a species’ range is exactly what biologists have predicted will happen with global warming. Researchers were careful to screen out any species that might be affected by other factors like habitat loss. “It’s the best data set to date on a multi-species system showing the kind of response we expect from climate change,” says Parmesan.

Mountain Dwellers Shifting Schedules

Some animals just seem to find the whole thing confusing. At Rocky Mountain Biological Laboratory in Gothic, Colorado, Billy Barr has been recording the first date each spring that he sees a particular animal species. An avalanche watcher for the U.S. Forest Service and business manager for the lab, he lives at the 9,500-foot elevation site year-round. His records show that, although yellow-bellied marmots (Marmota flaviventris) are coming out of hibernation 38 days earlier than they did in 1973, least chipmunks (Eutamias minimus) are snoozing 11 days longer, and golden-mantled ground squirrels (Spermophilus lateralis) 26 days longer.

Although April air temperatures have been getting warmer, the snowpack lasts just as long as it did in the past—late May or early June—because recent winters have dumped much more snow on Gothic. A bit like Punxsutawney Phil, marmots do come out from time to time, apparently testing the air temperature in order to decide whether to stay topside. If the slumbering chipmunks and squirrels use a different wake-up cue, like the amount of snow over their burrows, that might account for their lolling about below ground so much longer, says Rocky Mountain Biological Laboratory and University of Maryland at College Park ecologist David Inouye.

Animals may be getting out of synch with their food supply, he says. Plants don’t start growing until the snowpack melts, which means these days marmots fast an additional month after they emerge. And the squirrels and chipmunks, who stash food in their burrows for bedside snacks, may find the cupboard getting bare. “The animals are either going to have to change their behavior or go locally extinct.” (That’s exactly what happened to one population of Edith’s checkerspot butterfly in California’s Sequoia National Forest. The butterflies emerged a week earlier than their nectar plants—and they all died. Parmesan says the dead butterflies made “a hillside of bright orange wings.”)

Extinctions may also occur because high-altitude animals’ mountain-top habitats just get too toasty. Translucent-winged apollo butterflies (Parnassius apollo) flit about high-elevation habitats throughout much of Europe. Lepidopterist Henri Descimon has been watching populations in France’s Jura mountains for the last 40 years, according to Parmesan, and he reports that on every single mountain under 2,750 feet the apollo butterflies have disappeared. On those smaller mountains, the butterflies cannot go any higher in elevation in search of cooler climes, so they “get bumped off the mountain.” Parmesan says similar things are happening to endemic populations of Erebia butterflies in the Swiss Alps.

Climate Changing Conservation

Climate change is even messing up conservationists’s best laid plans. Parks and natural areas set aside to protect a specific endangered species or ecosystem may no longer fulfill that purpose as the globe warms. Biologists stress that climate change is altering environmental conditions faster than most plants and animals can adapt. Although people can just crank up the air-conditioning, the birds and the bees won’t have that option.

Some creatures, like the chinstrap penguins or the butterflies Parmesan studied, are already taking advantage of the climatic changes to move into new territories. Others, like the Adlie penguins, are finding themselves with nowhere to go. Parmesan recommends that conservationists and park managers link preserves in ways that let animals move as the climate warms, such as by providing north-south corridors or corridors that provide a range of elevations.

—Mari N. Jensen is a freelance science writer who lives in Tucson, Arizona.

ZooGoer 30(2) 2001. Copyright 2001 Friends of the National Zoo. All rights reserved.