From the issue dated October 18, 2002
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Trapped by EvolutionAnimals' instincts lead them astray in modern, much-altered environmentsBy LILA GUTERMAN Pity the male buprestid beetle. Evolution taught him a lesson that, until only
Unfortunately, some beer bottles exhibit the same characteristics. Scientists discovered nearly 20 years ago that the two-inch-long male Julodimorpha bakewelli beetles in Australia are fooled by stray bottles and try to inseminate them. In some instances, the mistake means not only sexual frustration but also death: Ants attack the beetle as he mounts the bottle, an act he won't interrupt even as his genitals are bitten. In a paper in this month's Trends in Ecology & Evolution, three ecologists say the beetle's behavior is just one example of a common, but underappreciated, phenomenon. When humans al-ter the environment, they often cause problems more subtle than simply destroying habitat, the researchers argue. The changes can create a situation in which an animal's evolved behavior hurts its chances of surviving or reproducing, which in turn can send the species downhill, and fast. "If you're dealing with a population that's in serious trouble, this is something you ought to consider," says Paul W. Sherman, a professor of animal behavior at Cornell University, who is one of the authors of the paper. "Evolved behaviors are there for adaptive reasons. If we [disrupt] the normal environment, we can drive a population right to extinction." The Wrong Cue Mr. Sherman and his two colleagues call the mechanism an "evolutionary trap." Their new term expands on the concept of the ecological trap, which describes why species bypass better habitats to live in less suitable ones. Animals go to those poorer sites because they are led astray by environmental cues that no longer help in an environment altered by humans. More than 20 years ago, two ecologists described an ecological trap that appears in forests fragmented into patches by activities like logging, road-building, agriculture, and development. Birds from 21 species were putting nests near forest edges, even though predators frequently ate their eggs and their nestlings there. The researchers suggested that the birds had evolved to prefer sites with a variety of vegetation types because, in the past, that had meant better foraging and good protection against predators. But the cue -- heterogeneous plant life -- no longer indicated a better habitat, because fragmentation had produced more forest edges with such a variety, and an accompanying surge in the number of predators there. Scientists have discovered many other ecological traps since then. For example, some grassland birds choose to nest in pastures, based on an evolutionary preference for low vegetation. But when the pastures are mown, young chicks are unable to fly away to escape. Mammals, too, fall into ecological traps. In the past 50 years, manatees have been found farther north in Florida's waters, because of warm water discharged from power plants. But whenever a plant is turned off, for maintenance or other reasons, the manatees encounter water too cold for them. The concept of ecological traps has always applied to habitat choice. But now Mr. Sherman, Martin A. Schlaepfer, and Michael C. Runge point out that changes in the environment can cause formerly positive behavior to be harmful in other ways, interfering with breeding, migration, and feeding, for example. Mr. Sherman began thinking about that broader possibility when he was studying wood ducks. Normally the ducks, Aix sponsa, nest in cavities of dead trees, laying 10 to 12 eggs at a time. For about 50 years, wildlife managers have erected nesting boxes for wood ducks to help them reproduce. Strangely, though, the boxes often caused more harm than good. The Cornell professor studied the birds for about 15 years to figure out why. Female wood ducks have adapted to a dearth of appropriate nest sites by trying to follow other ducks to their nest cavities to lay an egg or two there, or even take over the cavity. The nesting boxes were simply too conspicuous -- ducks were laying 30 to 50 eggs in a single box. That's too many eggs for one duck to incubate; many nests were then abandoned. Making the boxes easy to find was an attempt to assist the ducks, Mr. Sherman says. But it also helped female ducks to locate others' nests rather than find their own. "The solution is a real simple one," he says. "Hide the boxes back in the woods." Mr. Sherman also saw hints of a trap in the behavior of a rodent that has "just about winked out from the earth," in the words of Mr. Runge, an ecologist at the U.S. Geological Service's Patuxent Wildlife Research Center, in Maryland. Perhaps as few as 350 individuals remain of the northern Idaho ground squirrel, Spermophilus brunneus brunneus. Mr. Sherman has studied one population in Adams County, Idaho, where from 1987 to 1999 the squirrels' numbers declined from 272 to 10. He and other scientists wanted to know why. The answer may be as simple as not having enough food. The squirrels' habitat has been transformed because people have prevented fires from occurring, allowing pine trees to invade what used to be open meadows. Shrubs have elbowed out the grasses and herbs that the squirrels feed on. But Mr. Sherman and Mr. Runge thought more might be going on. The small, short-tailed squirrels hibernate for eight months of the year. During the late spring and summer, they must eat enough to triple their body weight to survive the winter. They also must reproduce -- or not even try. Though this behavior has never been studied in northern Idaho ground squirrels, a close relative's reproductive efforts each spring are dictated by how much food is available. "That's a very smart evolutionary adaptation," says Mr. Runge. "Why have pups when you know you're not going to be able to feed them?" He and Mr. Sherman speculate in a paper due to appear in Ecology that the squirrels may be fooled by the greenery present in the altered habitat when they emerge from hibernation. Expecting food throughout the summer, they invest energy in mating. But not enough food becomes available late in the summer for them to fatten up sufficiently. "The animals have gone ahead and reproduced and put themselves in jeopardy, but there's no payback, and they starve over the winter," says Mr. Sherman. Though it hasn't been proved, the idea of an evolutionary trap threatening the squirrels is intriguing, says Eric Yensen, a professor of biology at Albertson College, in Caldwell, Idaho, who also studies the northern Idaho ground squirrel. "It gives us some things we can test. ... Somebody's going to have to do a behavioral study to figure out just what they're cueing on." In the meantime, because of limited funds and the desperately small squirrel populations, wildlife managers with Idaho's Department of Fish and Game and the U.S. Forest Service have begun prescribed burns and supplemental feeding to try to save the rodents without fully understanding what is threatening them. Bruce A. Haak, a state biologist, says some of the populations are already rebounding. Wayward Lizards While Mr. Sherman and Mr. Runge contemplated trapped squirrels, Mr. Schlaep-fer, then a graduate student in natural resources at Cornell and now a postdoctoral fellow there, was thousands of miles away in Costa Rica, thinking along the same lines about reptiles. He was studying the lizard Norops polylepis, which normally lives in cool, shady areas. But the lizards had been found laying eggs in pastures that people had cleared in the forest. The problem, he found, is that the adults do not survive well in the pastures. He has not figured out why. "These lizards may never have seen a pasture in their evolutionary past," Mr. Schlaepfer says. He wondered if they had evolved to lay eggs in the sunniest areas of the forest, since those were the warmest. "They might be drawn to pastures in search of good egg-laying sites, unaware that the adults were going to be hammered." Back in Ithaca, Mr. Sherman was serving on Mr. Schlaepfer's dissertation committee. They discovered that they and Mr. Runge had all been thinking about how animals get trapped by their adaptive behaviors when they are in human-influenced environments. When Mr. Schlaepfer started looking for examples of other evolutionary traps described in the scientific literature, he found numerous cases. In the mid-'90s, sea-turtle hatchlings born on Florida beaches were dying because, after emerging from their eggs, they turned inland instead of heading out to sea. Scientists discovered that hatchlings normally rely on light on the horizon over the ocean to decide which direction to migrate. But the light from nearby homes and hotels was fooling them. Florida has protected the turtles by requiring building owners either to use light of a different wavelength or to shield the lights from the beach. Other examples came up: Because of global warming, the insects that are fed on by great tits -- small, yellow-and-black birds found in Europe -- appear earlier in the season than they used to. But length of daylight, not temperature, determines when the birds lay their eggs. So the young hatch after the bulk of their food has come and gone. Leatherback turtles sometimes eat plastic bags, presumably because of their similarity in appearance to a favorite food, jellyfish. Mr. Runge says that all four animals he's currently studying -- manatees, grassland birds, pintail ducks, and the ground squirrels -- may be stuck in evolutionary traps. "It's just a coincidence that they're all facing this. Or maybe it's not. It could be a ubiquitous mechanism." Hanna Kokko, an assistant professor of ecology at the University of Jyväskylä, in Finland, has done theoretical work on ecological traps. She suspects that evolutionary traps, too, are probably very common. "I would expect that now there is a name for this phenomenon, we might pay a lot more attention and find a lot more," she says. Finding the problem sometimes gets you most of the way toward solving it, as was the case with the sea turtles and the wood ducks. But too often, conservation biologists and wildlife managers don't look to behavior for causes of population declines, say Mr. Sherman and his colleagues. "If we hear about a declining population, the typical response is: We've got to buy more of its habitat and save the habitat it's living in," he says. "With the wood ducks, we could have bought lots more habitat and put out more nesting boxes, but that wouldn't have solved the problem." MISLED ANIMALS Evolved behavior that helps species survive in natural environments can harm the animals in human-altered ones.
SOURCE: Martin A. Schlaepfer, Michael C. Runge, and Paul W. Sherman, Trends in Ecology & Evolution, October 2002
http://chronicle.com Section: Research & Publishing Volume 49, Issue 8, Page A19
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Copyright © 2002 by The Chronicle of Higher Education