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Big-city ecosystems feature large populations of a very small number of species. Natural ecosystems, outside of a few very difficult environments that offer few niches, tend to move in the opposite direction. The natural tendency reaches its apotheosis in the tropical forest, where rarity is the common condition of a bewildering number of species.

Big-city ecosystems do show some variety. In the Loop or on North Michigan Avenue, the most urban places in Chicago, you can see lots of birds, but almost all of them will be pigeons. This is the most intensified version of the "few species, many individuals" system. The downtown avifauna consists of pigeons, house sparrows, chimney swifts, and nighthawks nesting on flat roofs. Lately crows are showing up more downtown, but I think they are just feeding and not nesting.

The mammal list consists mainly of the exotic species: house mouse and brown rat, along with perhaps a few raccoons and, anywhere there are any trees at all, gray squirrels. It is unlikely that any reptiles or amphibians live on North Michigan Avenue.

A typical Chicago neighborhood--a neighborhood of single-family houses mixed with two- and three-flats, with maybe a somewhat larger apartment building on the corner--will have pigeons too, though more will feed there than nest there, along with house sparrows and starlings. The native songbirds will be robins, cardinals, and occasionally black-capped chickadees. Chimney swifts and nighthawks will also be around. If you get really lucky, you may get a pair of kestrels nesting on your block. Kestrels are small falcons that live on a mixed diet of mice, insects, and small birds. They are one of two American raptors that have adjusted to urban living. The other is that much larger falcon, the peregrine. Peregrines did nest on tall buildings in a few cities before DDT eliminated the species east of the Rockies. Now that they have been reintroduced, we have a few birds downtown and along the lakefront. Out in the neighborhoods kestrels are the closest things we have to top predators.

Raccoons and opossums can be added to the mammal list in the neighborhoods. When I first moved to my present neighborhood, I thought the raccoons were all coming to my backyard from the river, which provides a green corridor into the city. Before that I lived just west of Graceland Cemetery and thought the cemetery was a reservoir producing raccoons for the surrounding neighborhoods.

Yet research by the National Park Service in the parks in and around Washington, D.C., suggests that neighborhoods are more likely to be reservoirs than parks and cemeteries. A typical city neighborhood is a very attractive environment for raccoons, providing abundant food and good nesting sites. Survey the city north of, say, Irving Park Road and west of Clark Street, and you will find several times more raccoons per square mile than you would find in a prime virgin southern bottomland forest.

Brown rats and house mice are also around in the neighborhoods, and feral house cats become important ecological actors. You will find some populations of fox squirrels mixed with the more common gray squirrel. Maybe you can find an occasional garter snake, but no other reptiles and no amphibians at all unless there are refuges where a few animals can hang on, like the turtles in the ponds in the Lincoln Park Bird Sanctuary.

At the edge of the city, especially near forest preserves, you might see a deer from time to time, and once in a while a red fox might wander by. Skunks are more common, and if you are near a waterway of some kind you might see a beaver or a muskrat.

The difference between the city and suburban ecosystems is in most cases pretty small. Older suburbs with bigger trees and dense patches of shrubbery support more animals than new suburbs. Rich suburbs, with one-acre lots backed up against a wooded ravine, will support song sparrows, downy woodpeckers, goldfinches, northern orioles, and catbirds. Small houses on small lots with every square inch manicured will not. Garter snakes hang on, but other reptiles and amphibians will be scarce or absent.

And that is pretty much all there is to the vertebrate side of an urban ecosystem. There are more animals around than most city people notice, but the level of diversity is quite small. We could have as many as 150 species of birds nesting in Cook County, but only about 20 of those might be found in a city neighborhood or most suburban neighborhoods. A Cook County with nothing but neighborhoods, shopping malls, and industrial districts would have 20 to 30 species of nesting birds.

The mammal list also has some big holes in it. We have alien house mice and brown rats, but we lack all the native mice, voles, and wood rats. Small predators are gone too. When was the last time you heard of somebody calling animal control to get a long-tailed weasel out of his chimney?

The food webs that connect the creatures in city ecosystems are extremely short and simple. Garbage is the most important resource in the system. In the language of ecology, plants are producers in ecosystems, the only organisms capable of capturing solar energy and bringing it into the ecosystem. The producers provide the food for the consumers, the animals. We could trace human garbage back to some ultimate solar source: a Kansas wheat field, algae in the North Atlantic, an orchard in New Zealand. But regardless of where it comes from, humans move huge amounts of food into cities. They waste large amounts of it, and what is left over provides a large amount of energy to sustain these very large animal populations.

The food web consists mostly of connections with only a single link: garbage to raccoon, garbage to skunk, garbage to crow--and that's it. The multifarious paths that energy takes through natural ecosystems don't exist in city ecosystems. The delicate adjustments that these paths allow in natural ecosystems, the controls on the populations of all the species, can't happen. Garbage is constant, and its supply controls everything else.

The most common animals in urban settings are opportunistic, omnivorous hunter-scavengers. Rats, house mice, raccoons, opossums, crows, and gray squirrels all fit this description. Coyotes fit it too, and they seem to be moving into the Chicago area in a big way.

The secret to success as an urban animal is essentially the same as the secret to success for urban humanity: be smart, be adaptable, and be willing to eat anything. It's hard to imagine a long-tailed weasel eating kung pao chicken, but raccoons and opossums love it.

A few animals manage to hang on to lives that have little to do with humans. The eaters of flying insects: barn swallows, chimney swifts, nighthawks, and bats still eat flying insects. Robins live mostly the same sort of lives in city neighborhoods, suburbia, or wilderness.

When I started to write this, I didn't expect it to sound so gloomy. But as I thought about it, I realized that urban ecosystems really are one-dimensional parodies of the real thing. In a wild ecosystem raccoons would live on frogs, crayfish, birds' eggs, insects, wild fruits, and almost anything else that wouldn't eat them first. A great horned owl might eat a raccoon first, if it was young and unprotected. Crows sometimes kill and eat great horned owl nestlings, and owls return the attention by occasionally taking an adult crow for a late-evening snack. There are thousands of possible pathways for energy moving through the system to reach the great horned owl or the timber wolf at the top of the food web. In the city the raccoons eat garbage, and nothing eats them until they get run over and the crows pick their carcasses clean.

I was inspired to think about urban ecosystems by some stories I saw recently in the newspapers about how animal-control officers are being run ragged in the Chicago area by calls from home owners troubled by opossums, raccoons, deer, and assorted other wild things on their domesticated property. One suburbanite even complained that gray foxes were raising a family in a den in his backyard--right next to the swimming pool.

I don't have a swimming pool in my backyard, but I would dig one if I thought it would attract gray foxes. They would probably produce some temporary inconvenience, but wouldn't it be worth it?

Some observers of this increase in animal complaints believe the trespassing animals are refugees driven from their homes by development. If they are, then the complaints should drop in the next few years as these animals die off without issue.

Their sudden prominence is another reminder that our destruction of nature in heavily populated areas like Chicago has almost reached an end point. There are so many headlines about wetlands and other wildlands being threatened by development that we get dulled by the repetition into forgetting what is at stake. We read phrases like "important to wildlife" so often that it is hard to remember their serious meaning, especially when the wildland at issue is small. But conservationists have spent more than 20 years gaining protection for the 80-acre Wolf Road Prairie in Westchester. And a major effort is now under way to protect the Santa Fe Prairie, an equally tiny prairie fragment along the Des Plaines River. Without tiny pieces like these, some ecosystems would totally vanish. The issue, here and in many other places, is the continued existence of ecosystems of any kind other than our pale urban parody. And ecosystems are the places where evolution happens, the systems that produced the variety and complexity of life on earth.

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