Anyone who travels will quickly become familiar with what I think of as "the ethnic restaurant rule of city size and diversity." A city of a million people is likely to support a far greater diversity of restaurants than a city of one hundred thousand, perhaps including such rare varieties as Afghan or Tunisian restaurants. The city of one hundred thousand, meanwhile, will still be considerably more diverse than a town of one thousand, where restaurants might be restricted to a diner, a fast-food franchise or two, and a pizza joint. There isn't necessarily anything wrong with those places, but they're the culinary equivalents of starlings, pigeons, and raccoons — the common, cosmopolitan species on the restaurant scene.

Ecologists have known for some time that a similar pattern holds true for natural habitats. In the 1960s, biologists Robert MacArthur and Edward Wilson codified this notion with what they called the "island biogeography theory." By studying the collections of species that grew on oceanic islands of various sizes, they were able to generalize that smaller islands tend to support fewer species of plants and animals than large ones do. As an island's distance from others grows, it also is more likely to have fewer species, since fewer new species can colonize it from other islands.

Island Biogeography
Biologists readily apply the island biogeography theory to all kinds of places that aren't real islands but that do behave like them. A marsh or woodlot is an island if it's surrounded by fields; a prairie is an island if it's entirely surrounded by housing developments. Biologists have found that the same mathematical rules that govern real islands often hold true on these other sorts of islands. A 100-acre marsh is likely to support more species than a 10-acre one; a 10-acre marsh that's close enough to a 100-acre marsh that frogs or other animals can travel between the two is far more likely to maintain its populations than a 10-acre marsh totally isolated among shopping malls.

The leadplant flower moth survives on some small prairies — if leadplant is plentiful. Photo by Ron Panzer.

A truly isolated and small island can easily lose species through random events of predation, disease, or accident. When this happens, only the most mobile species, such as birds, are likely to be able to reestablish themselves. Others, such as reptiles or amphibians, will likely never make it back across acres of unsuitable habitat. Populations on an isolated island may also suffer from inbreeding as they are unable to exchange genes with others of their species.

The Chicago Wilderness area is a great place to eat, but as far as its nature preserves are concerned, it is quite obviously an archipelago of many different sorts of islands. It is, first of all, a naturally fragmented place in which a diverse topography of moraines, beaches, and wetlands caused marshes, prairies, and forests to grow in intermingled patches. Like much of the Midwest, it has also been drastically fragmented by development, transportation corridors, and other forms of human intervention. When the Illinois Natural Areas Inventory was conducted in the 1970s, survey crews found 1,289.9 acres of high-quality prairie in the six-county Chicago metropolitan area — a mere sliver of the more than 1.7 million acres estimated to exist here in 1820. About 80 percent of these remnants were ten acres in size or less. Other habitat types have become dramatically fragmented too. Chicago Wilderness, then, is a great case study in island biogeography — and a challenging place to apply some of its lessons.

Why Bigger Is Better
The central message of island biogeography theory is that bigger is better. For some species, the relationship between preserve size and survival is very clear. Many birds of forests and grasslands, for example, can't survive in small habitat patches. Wood thrushes and other songbirds might try to nest in woodlots of 30 acres or so, but if they do they are likely to lose their nests either to the predators that are common in fragmented landscapes, such as crows, jays, or raccoons, or to have their nests parasitized by brown-headed cowbirds, which lay their eggs in the nests of other species.

The northern harrier needs at least 500 acres of grassland to raise its young. If more grassland is in easy flying distance, so much the better. Photo by Alan G. Nelson, Root Resources.

One recent study by Purdue University biologist Peter Fauth found that 90 percent of wood thrush nests in scattered forest patches in northwest Indiana were parasitized by cowbirds and 58 percent were lost to predators. Adult thrushes in these patches, in other words, were not able to raise enough young to replace themselves. Over the years, such rates are not sustainable. Wood thrushes persist in that area only because they have bigger forest patches to nest in elsewhere, from which "surplus" thrushes can disperse into the smaller, unsuitable patches.

Researchers have found that a similar pattern holds true of grassland birds, which may avoid small prairie patches entirely or are more likely to suffer nest predation in them than in larger areas. Meadowlarks, for example, generally need fields of at least 20 acres, Henslow's sparrows grasslands of at least 80 acres, and upland sandpipers far larger areas still.

But the story changes when we talk about species such as red-headed woodpeckers, yellow-breasted chats, blue-winged warblers, and orchard orioles, all of which are species that prefer shrubby patches or savannas. Recent research by Jeffrey Brawn of the University of Illinois in the Palos Hills area and elsewhere has shown that a savanna patch of as little as two acres will attract some of these species and one of ten acres will support quite a few. He has not been able to identify any shrub- or savanna-dependent species that require larger areas than that.

"It looks like shrubland birds have a different level of sensitivity to fragmentation than forest and grassland birds," Brawn says. "This is good news for conservation because it means that small patches in a chronically fragmented landscape are still useful for them." And this, he says, is really good news because many shrubland and savanna species are declining on a national level more than forest species are.

Insects Disobey
Some insects appear also to disobey the recommendations of island biogeography. Ron Panzer of Northeastern Illinois University has been intensively studying insect populations on many prairie fragments in Chicago Wilderness since the late 1970s. When he began, he read that sites of less than 2,000 acres would not support many "conservative" insect species, those restricted to high-quality prairies. But he has found conservative species on each site he's looked at, and they have remained over the years.

There is a relationship between area and insect diversity, Panzer says, up to a point. Sites of five acres or less "are very much in need of expansion," Panzer says. "But 150-acre sites essentially are as rich in species as sites ten times larger, and sites over 100 acres in size have 'leaked' very few species. So we can approach the conservation of these insects with some real cautious optimism."

This newly protected preserve at Corron Farm, in Kane County, transitions from sedge meadow (foreground) through savanna to woodland. A variety of adjacent habitats provides viable preserves for animals that require different habitats at different life stages. Photo by Jack Shouba.

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Birds tend to look for a particular type of habitat structure more than particular plant species: a large forest or grassland, a savanna with scattered trees. Insects, on the other hand, are often tied to particular plant species, so they depend more heavily on a habitat's quality than its quantity. A small prairie preserve that has never been plowed and has experienced regular fires may, then, support many more rare plant and insect species than a much larger tract that has been heavily impacted by human uses.

The same holds true for reptiles and amphibians, most of which are not evenly spread across the landscape. Instead, particular species tend to require very particular microclimates, such as vernal pools, moist downed logs, or sandy tracts. As a result, says herpetologist Tom Anton, "we're finding that in some cases, the bigger the place, the more diverse the species; but in other places it's more a matter of microclimate and particular disturbance regimes, as well as of the footprint that we left in the past. There are some small preserves out there with a surprising number of species."

Conservation Design
Conservationists behind the Chicago Wilderness Biodiversity Recovery Plan are currently writing designs to provide for the long-term conservation of three threatened ecological community types in the region: grassland birds, savanna reptiles and amphibians, and oak woodlands (see sidebar). The idea is to establish general guidelines regarding how much acreage of appropriate habitat is necessary to ensure that these community types persist, how the preserve acreage should be distributed across the landscape, and how the habitats should be managed.

Many frogs need two habitats. This spring peeper needs temporary ponds in spring, and forests in summer. Photo by Mike MacDonald.

By analyzing species and ecosystem requirements, planners have come up with specific recommendations — a wish list. One such wish is for 7,500 acres of grasslands more than 500 acres in size each that will be managed at least in part for the nesting requirements of grassland birds. Karen Glennemeier of Audubon-Chicago Region, who is helping to write the plan, estimates that there are currently about 7,200 acres in such sites, though they are not necessarily of high quality. For that reason, she says, "mostly what we're talking about with prairies is getting the places we have into good shape."

For reptiles and amphibians, she says, the picture is more complex, largely because those animals often require a variety of habitats — such as wetlands for breeding and moist woodlands for adult life — and because their habitat requirements are generally not as well known as those of grassland birds. For that reason, planners would like to see preserve complexes with a connected assortment of habitat types. "If we have one big habitat complex in each natural subdivision, that assures diversity," Glennemeier says.

Dave Mauger, natural resources manager of the Forest Preserve District of Will County, tries to follow a similar strategy as the county increases the extent of its preserve system. He, too, doesn't like to think so much in terms of individual habitat types. Rather, what he would most like to see is preserved areas that maintain the transitions between habitats.

In order to do that, he says, Will County planners try to "build out from existing preserves. We try to increase their size and to provide buffers and remove gaps. We try to craft a plan that expands on existing high-quality areas."

Butting Heads with Politics
Of course, the ecological theory behind preserve design quickly butts heads with politics, funding issues, recreation, and many other factors when planners try to implement it. Will County still can buy up relatively large tracts of farmland for restoration, a luxury that is not possible in many of the more densely settled parts of the Chicago Wilderness area. In those places, conservation is to be practiced more through focused management than through land acquisition.

Grassland birds and forest birds both need large habitats. Shrubland birds, like the blue-winged warbler, can nest successfully in small sites. Photo by Rob Curtis, The Early Birder.

Some of that management can certainly be informed by island biogeography theory. For example, Jeffrey Brawn suggests that from the perspective of bird conservation it's probably better to manage a small, isolated preserve as a shrubland or savanna than as a grassland or forest, because only shrub or savanna birds are likely to nest successfully there. Large tracts of forest or grassland, on the other hand, should be kept intact, because forest and grassland birds absolutely need them.

When designing preserve patterns, planners also focus on corridors that allow animals to move back and forth between preserves, though some conservation biologists consider them overrated. Ron Panzer, for example, believes that a lot of corridors in the Chicago area serve mainly to allow such invasive species as purple loosestrife to move from one preserve to another. He cautions that corridors need to be carefully managed for the species they are intended to serve.

For the insects he studies on isolated prairie fragments, true habitat corridors are probably never going to be a possibility. For that reason, he proposes that insects will probably need to be translocated from site to site by hand to assure genetic exchange between populations. "We're going to need some artificial linkages between sites, or 'volunteer corridors,'" he says.

On the one hand, this seems a lot of work: people doing what nature used to do by itself. On the other, though, it's compelling to think of biodiversity in the Chicago Wilderness region as not limited to its established preserves, to think of the region's large preserves as core areas from which native plants and animals spread into backyards, and to think of the citizens of Chicago Wilderness transporting rare plants or insects from preserve to preserve to assure their long-term survival. In the long run, it is this interconnection between places managed for a diversity of natural and human communities that will make people full members of the region's natural community.

Preserve Recommendations from the Chicago Wilderness Conservation Design The plan for amphibians and reptiles calls for at least one 800-acre habitat complex in each of five different natural subdivisions of the Chicago Wilderness region: Grand Prairie, Western and Kettle Morainal, Lake Plain, and Gary Lake Plain. For grassland birds, planners want to see tracts of at least 500 acres without trees or shrubs. There should be at least 2,500 acres each of three prairie types: dry, medium, and wet. The entire region should also have at least five prairie complexes of at least 4,000 acres, and a total of at least 27,000 acres of grassland. For various oak woodland types, there should be from two to ten 400-acre sites each. Plus, there should be at least one site for each subtype that is at least 800 acres. The region should have at least 51,000 combined acres of dry, medium, and wet woodland. You can find more information in the complete text of the Chicago Wilderness Biodiversity Recovery Plan.