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Field & Street 

Living on the edge is stylish among some privileged groups of humans. It is reality for the rest of creation. The hepaticas and bloodroots whose blossoms are opening now may be frozen by a spring snowstorm. But if they don't flower until May they will have too many competitors, and the trees that surround them will cut off their sunlight. The trick is getting the timing just right.

Migratory birds fly thousands of miles over deserts of high-rises and plowed fields. If they are lucky they find a rich patch of forest to land in, a place where they can feed and rest before the next leg of their flight. If their luck is bad they touch down on acres of asphalt where there is nothing to eat.

The dangers continue. The successful trillium--its flowers thoroughly pollinated, its seeds maturing--is bitten off at the ground by a white-tailed deer. A wood thrush's eggs--nearly ready to hatch--are a meal for a raccoon. The raccoon becomes a meal for crows after misjudging the strength of a branch in a tall red oak.

The story of life is about who makes it through these hazards and who doesn't. The difference is a question of statistics. If 20 percent of the eggs in a bird's nest survive to adulthood, the species thrives. Fifteen percent survival may mean extinction. Nature must address its creator in the words of the old horseplayer's prayer: "Lord, let me break even. I need the money."

In recent years apparent declines in the populations of many different kinds of animals and plants have stimulated research into the question of just what breaking even means. Some of this population decline is obviously attributable to habitat loss. You don't have to be an ecologist to guess that fewer forests mean fewer forest animals or that draining wetlands reduces frog populations.

The scary evidence is about declines that go beyond habitat reduction. Suppose we find 100 pairs of ovenbirds in a 1,000-acre forest.

That averages out to one pair for every ten acres, so a 100-acre forest ought to have ten pairs. But a lot of 100-acre woods have no ovenbirds at all. Why not?

That question seemed to be answered by the concept of area sensitivity: some species will live only in large blocks of their favored habitat, even though they may use only a tiny fraction of that block. That answer inspired yet more questions: Are area sensitive species kept out just by their preferences, or are other factors at work that make small blocks of habitat unsuitable for them?

Scott Robinson of the Illinois Natural History Survey addressed this question in several years of research in the Shawnee National Forest. He found that in the fragmented landscape of southern Illinois, where blocks of forest are scattered among farm fields, the brown-headed cowbird, a brood parasite that lays its eggs in the nests of other birds, was present in such numbers that forest birds such as the wood thrush were able to produce very few young. The wood thrushes were basically raising cowbirds rather than wood thrushes. Added to this was a very high rate of nest predation by raccoons and other animals.

Robinson's conclusion was that the Shawnee was essentially a population sink for many forest birds--birds were moving in and dying without reproducing. Excess birds from somewhere else--perhaps the Ozarks--were looking for good nesting locations, but instead wound up rearing cowbirds.

Here in the Chicago area Christopher Whelan of the Morton Arboretum began to look into the nesting success of forest birds with a study that began in 1989. Whelan didn't have the armies of grad students available to Robinson, so he decided to concentrate his efforts on just two species: American robins and wood thrushes. Both of these closely related birds nest in the arboretum's east woods. Both birds seek out similar nesting sites in tall shrubs and small trees. But they have one key difference: robins remove cowbird eggs from their nests, and wood thrushes do not. Robins nest in deep woods, but in presettlement times they also nested along woodland edges and in scattered trees on open ground. This preference has opened up large amounts of nesting territory for them in neighborhoods, cemeteries, and parks, and around golf courses. It also suggests that they have a long evolutionary history with cowbirds, since these birds generally prefer similar habitats. Whelan has examined more than 800 robins' nests since his study began and has yet to find a cowbird egg. He is inclined to think that even the cowbirds have learned that depositing an egg in a robin's nest is a fruitless endeavor.

Wood thrushes, a species that historically lived in the deep woods far from edges, have no defense against cowbirds. The fragmentation of the landscape--and the tremendous increase in cowbird numbers that this fragmentation has made possible--places a heavy cost on wood thrushes. In the five years of Whelan's study--from 1992 through 1996--he examined 198 wood-thrush nests. An average of 85 percent of them were parasitized.

Cowbirds typically remove one egg of the host species for every one they lay. When the host species is a small bird--such as one of the wood warblers--the young cowbirds usually take over the nest. The host young rarely have a chance to survive. The outcome is not quite so grim for birds the size of a wood thrush. They can raise one or more cowbirds along with their own young. Nonetheless, Whelan calculates that the parasitism costs the wood thrushes a third of their young. Over the five years the average wood-thrush nest produced only one young wood thrush each year. Since this species is a long-distance migrant that usually nests only once per summer, this level of reproduction is almost certainly not enough to keep the species alive.

Both robins and wood thrushes have to survive heavy nest predation as well--in Whelan's study more than half their nests were attacked. The major predators are raccoons and blue jays. They are both fond of small woodlands, so landscape fragmentation has probably made them a more serious problem than they used to be. We also have local evidence that raccoon populations go up in preserves that are surrounded by development, making things even worse.

The extent of predation seems to be related to the number of nests per unit area. Where there are more nests, predators seem to find it worth their while to search specifically for nests. In areas of lower nest density their depredations are limited to nests they come across by chance. Whelan has named this phenomenon "density dependent predation."

Biologists talk a great deal these days about the importance of "biodiversity." The term refers to the variety of life, whether the genetic variations in individuals within a population or the numbers of species that occupy particular ecosystems. Biodiversity is the thing we need to preserve in our nature preserves. To generalize broadly, diverse natural communities are healthy natural communities. The loss of some species from a community often leads to the loss of others. Ecosytems that are losing species are unhealthy. They are often on a downward spiral. A plant dies out, and the insects that feed on it go with it. The plants that depended on those insects as pollinators are the next to go.

Whelan's work has suggested another major dimension to biodiversity, a dimension he calls the "mulberry effect." The arboretum has a large collection of mulberry trees, and in 1995 they produced a bumper crop. When the berries began to ripen, the raccoons left the woods and headed for them. They stayed until the last berries ripened. Predation on robin nests dropped from 68 percent in 1994 to 46 percent in 1995. (In 1996 it was up to 83 percent.) In the year of the mulberries nest-predation rates in high-density areas--the places where there were enough birds to repay careful searching by predators--dropped 80 percent. The emergence of cicadas in 1990 had a similar effect. There were so many cicadas to eat--and they were so easy to catch--that predators concentrated on them and left the birds' eggs alone.

It could be that the availability of a variety of foods--the kind of variety that biodiversity provides--reduces the predation pressure on any single food source. It could also be that in a healthy natural system the rate of nesting success could vary from year to year depending on factors such as the availability of other food sources for predators. More mulberries could be as important as fewer raccoons to the maintenance of healthy bird populations.

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