Problem wildlife" has become one of the defining difficulties of suburban life, like crabgrass and boredom. Stories of raccoons in the attic, deer in the garden, and geese on the soccer field regularly appear in the media, and every suburbanite I meet can enrich the growing stock of anecdotes with stories of his own.
The problem of pushy animals is usually regarded as one of the prices we pay for sprawl. According to this view, we are moving into places where the animals already live. So the raccoon that used to den in a hollow tree that stood where our house now stands will seek out our chimney as an alternative.
This view is pessimistic about the future of the animals but optimistic about the solution to the problem. Animal-control companies and city agencies will carry off this generation of troublemakers, and the drastic loss of habitat will substantially reduce populations in the future. With smaller numbers of animals around, the problem will be reduced to a minor annoyance.
Arguing against this idea is the great abundance of deer and raccoons in and around older suburbs such as Glencoe, Wilmette, and other North Shore villages. An even stronger argument has now been supplied by a continuing research project conducted by Stan Gehrt, a research biologist at the Max McGraw Wildlife Foundation in Dundee.
Gehrt and his assistant, Suzie Hatten, a graduate student from the University of Missouri, have been monitoring raccoon populations at six sites in the Chicago area, and what their numbers show is that sprawl creates large increases in raccoon populations. Natural areas surrounded by farms have far fewer animals than natural areas surrounded by suburbs.
The six sites in Gehrt's study are divided into two groups of three based on size. The small sites are Beck's Lake, along the Des Plaines River near Wheeling, and two Kane County forest preserves. The large sites are the Ned Brown Preserve in Elk Grove Village, the Max McGraw Wildlife Foundation, and Glacial Park in McHenry County. Each set of sites represents a range of surroundings. The Ned Brown Preserve is next door to Woodfield Mall, and much of it is bordered by developments of single-family houses. Glacial Park is the most rural of the three large sites, with farms bordering it on all sides.
Based on two years of live trapping and radio tracking, Gehrt estimates that population density of raccoons at Ned Brown Preserve is 94 per square kilometer. At Glacial Park density is 62 per square kilometer. In other words, the rural site supports only two-thirds as many raccoons per unit area.
We don't yet know for sure why these numbers fall out the way they do, but food availability is a likely hypothesis. Raccoons, like human beings, will eat pretty much anything they can hold down. They are vegetarians at times, predators at other times, and scavengers whenever the opportunity arises. Their typical diet changes from season to season as different kinds of foods become available.
At Ned Brown garbage is readily available in the preserve during spring, summer, and fall. The preserve is one of the most heavily used in the Cook County system, with more than a million visitors every year. Spring, summer, and fall are also times when wild foods are most abundant. In rural areas summer provides ripening corn and vegetables in fields and gardens.
The critical difference in the suburban sites may be the existence of year-round food sources in the garbage cans and Dumpsters of homes and businesses. Combine that with the presence of the preserve as a refuge and you have a recipe for major raccoon abundance.
Gehrt and Hatten have outfitted a total of 93 raccoons with radio collars. Each animal has its own frequency, so the movements of individuals can be traced. Since raccoons are mainly nocturnal, Hatten spends many nights on roads in and around the preserves tracking their movements (one of the fundamental rules of American science is that the graduate students do the part of the job that involves staying up all night).
The radio tracking shows that animals move regularly between the preserves and the surrounding neighborhoods in search of food. However, thus far none of the animals from the preserves has made itself a major nuisance by trying to enter a building. They all return to the preserve to den up. Of course these are intelligent, adaptable animals, so things may change. Meanwhile, minor nuisance activity, such as tipping over garbage cans, continues.
If this were all there was to the story we might chuckle at the irony of suburbanites creating the problems that plague them and then forget about the whole thing. But there could be implications to high raccoon numbers beyond upended garbage cans. For example, raccoons are not the only omnivorous small mammals in the area. Opossums and skunks are also out there trying to make a living. During the fall of 1996 Gehrt and Hatten trapped 46 raccoons at Glacial Park. They also trapped 11 opossums and 17 skunks. A person who has livetrapped a skunk is rather like a man who has a wolf by the tail. My guess is that few skunks get outfitted with radio collars.
At the highly suburbanized Ned Brown Preserve the fall trapping season produced 71 raccoons and absolutely no skunks or opossums. At Beck Lake, the most suburbanized of the small sites, 31 raccoons were trapped along with just 9 opossums and no skunks. Gehrt suspects there is a connection, although he doesn't know what it is. It may be that raccoons outcompete skunks and opossums for food. It may be that raccoons eat skunks and opossums. Whatever the cause, large raccoon populations could lead to the loss of our opossums and skunks.
The other big worry is birds. During the nesting season raccoons are heavy predators on bird nests, eating both eggs and young. As agile climbers, raccoons can go after both ground nesters and tree nesters. A number of studies have shown that predation on nests is quite heavy in suburban areas where raccoon populations--and the populations of other nest predators--are high. In one study predation reached 100 percent; every single nest was raided. Gehrt intends to put out some artificial nests this spring to begin the job of measuring raccoon predation. We also need general studies of nesting success in this region like those carried out elsewhere in the state by Scott Robinson and Jeff Brawn of the Illinois Natural History Survey.
The costs of sprawl have been calculated in a variety of ways. We try to measure increased air pollution caused by longer commutes and record the constant demand for new water and sewer systems, roads, bridges, and other infrastructure. The loss of natural land has always been recognized as one of those costs, but most of our attention has been focused on the acreage converted from woods and fields to housing developments and shopping malls.
Gehrt's results suggest a more subtle effect. Altering the surroundings of the Ned Brown Forest Preserve can profoundly affect what happens in the preserve. When a preserve becomes an island in a sea of development the effects of development intrude on the preserved land. A woodland--even a fairly large woodland--may not be sufficient refuge for the species and communities native to it. It may take us decades to figure out all the harmful effects of sprawl.