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A century ago introducing new birds to North America was something of a fad. People who did this sort of thing called themselves acclimatizers. They formed societies such as New York's American Acclimatization Society to pool their resources so they could buy more birds--mostly in Europe--and import them to North America. We owe our starlings to the New York group. A similar society in Cincinnati imported more than 70 species of European songbirds during the last three decades of the 19th century. None of those birds survived.

Acclimatizers saw themselves as public-spirited lovers of nature. Their goal was to enrich the lives of their fellow citizens by exposing them to true natural wonders. If nature was good, then more nature was even better. And the best way to get it was to import it. Imports were also to make up for the shortcomings in North America's supply of uplifting natural stuff. Nightingales, for example, had been inspiring poets for millennia. How could we hope to produce an American Keats if we had no nightingales to awaken the muses of the poetically inclined?

The acclimatizers' project has survived through the 20th century mainly among state fish and game agencies. Charged as they are to provide hunters with something to shoot at and anglers with something to catch, and faced as they often are with massive habitat changes that have driven native species to near extinction, they ransack the world in search of pheasants, partridges, francolins, and salmonids that can survive in local circumstances at least long enough for people to shoot or hook them. Lately opponents of the Endangered Species Act have adopted a sort of twisted version of the acclimatizers' vision. The acclimatizers thought we could safely add appealing species to the North American biosphere. The anti-ESA people think we can safely remove them. They are both wrong.

The rest of us have supposedly gotten too aware of the glories of American nature and too sophisticated in the concepts of ecology to be attracted to either the acclimatizers or the casual slaughterers. Or so I thought. Lately I have been taking part in discussions about preserving and restoring the natural wonders of the Chicago area--don't laugh; we have them in abundance--and some of what I have heard suggests that the spirit of acclimatization is still very much alive.

The root of the problem seems to be confusion over the concept of biodiversity combined with a failure to recognize the reality of biological communities. Biodiversity has become a buzzword among those of us who are into the protection of nature. Indeed some people I know can scarcely string together four sentences without using the word at least once. "Biodiversity" may not be the most felicitous coinage in history, but it describes something quite real and quite important: the glorious profusion of nature, the clear evidence that God is not an accountant. Or to put it more plainly, the clear tendency of life to evolve in the direction of endless variety.

Given half a chance, life will diversify. The famous finches of the Galapagos, the birds that according to legend made Darwin slap his forehead and exclaim "It's evolution, stupid!" presumably started out as a small flock of one species. Set loose on an empty archipelago, they diversified into all sorts of wonderful forms. And somewhere back in the Paleozoic, a proto-arthropod thought to itself, "Six legs, three body segments, exoskeleton--hey, it works for me," and the unequaled diversity of the insects was born.

It is diversity that gives ecosystems their resiliency, their ability to survive life's blows. Even that most midwestern of ecosystems, the settled, conservative tall-grass prairie, had to have its get-rich-quick weedy plants to begin the job of repairing the damage left by a herd of bison stomping and wallowing its way across the land.

Five times during the history of life, catastrophes have cut down species in wholesale lots, breaking down ecosystems and destroying their resiliency. After each catastrophe life has come back, returned to its exuberantly diverse state. We are living through a sixth catastrophe now, but if we can avoid destroying everything, in ten million years or so life on earth could be as good as new. If in the meantime we have gotten out of the way.

The potential for biodiversity exists in the genes of every living thing. Doubtless, if we had designed genes, they would be as flawless as possible. They would pass along exactly the information they had received, and life would still consist of a few bacteria living in the shallow waters of the sea. Fortunately, real genes screw up constantly, producing innovation and providing the raw material for evolution. But the winnowing that separates good mutations from bad is provided by the environment. And the most important aspect of the environment is the living community of plants and animals.

The community modifies the physical environment in countless ways. It is shadier, wetter, and cooler under a canopy of trees, and most prairie plants cannot grow there. Spruce and fir trees create acidic soils, and only plants adapted to those conditions can thrive in a spruce-fir forest. When sea otters were nearly extirpated from the kelp beds of the Pacific coast, the kelp beds themselves disappeared--along with the myriad crustaceans, mollusks, small fish, and algae that lived in the beds. Sea urchins, whose principal food is kelp and whose principal predator is the sea otter, had enjoyed a population explosion in the absence of otters. Now that sea otters have been reintroduced, the kelp beds are coming back. Midwestern oak woods have a rich herbaceous understory that provides fuel for frequent fires that keep the woods open and sunny, providing perfect conditions for reproduction of both the herbs and the oaks. Insert an exotic species like the tall shrub called common buckthorn, and its dense shade prevents the growth of the herbs, stopping the fires, and ultimately destroying the oak woods.

I could fill all four sections of this paper with similar examples. Biologists have argued for years about the precise ontological status of communities. They are not quite organisms. If they were we could predict with certainty what species lived in every example of a particular community type. But they are also not random assortments of individuals who all just happen to live in the same kind of place. If they were, the removal of sea otters wouldn't destroy the kelp beds and the addition of buckthorn could not destroy the oak woods.

Most people can recognize that a family or a human community is an entity that is somehow more than the sum of the individuals who make it up. That's why, for instance, Mexican Indians celebrate the Day of the Dead. The community includes people who are no longer living.

That said, we need to be clear on the fundamental differences between human communities and human diversity and biodiversity. The human diversity we see around us in Chicago's neighborhoods is about the differences between different populations of the same species, and those differences are a matter of culture, not genes. We learn that diversity, learn who we are to consider like us and who we are to think of as different. Biodiversity is about genetic differences between large numbers of different species. Human diversity shifts with history. The immigrants from eastern and southern Europe who seemed so exotic to native-born Americans a century ago are now just white folks. The starlings imported by the American Acclimatization Society are still starlings, and still very much aliens. It is important to insist on the difference between bio- and human diversity, because otherwise defenders of the integrity of natural communities start sounding like xenophobic racists.

We can get an idea of the effect of exotic species on biodiversity by looking back just three million years to the moment when the drifting continents of North and South America became joined at the isthmus of Panama. With a land bridge in place animals began to move north and south into new territories. In the short term, biodiversity increased. But in the long term, the new competitors drove whole groups to extinction. South America before the juncture was home to saber-toothed marsupial cats, toxodont rhinoceroids, long-trunked pyrotheres, and flightless, carnivorous birds ten feet tall. None of these wonders survived the invasion of North American forms.

The flood of humanity into every corner of the natural world has made ark building one of the most urgent tasks of our time. To be successful ark builders we must recognize that our arks have to be designed to hold communities, not individual species. The lone species is like a human in a refugee camp, a creature without context, robbed of family, work, associates, and everything else that makes it possible for life to go on.

The Karner blue is a small butterfly native to oak and pine savannas in the northeastern United States. Its caterpillars live exclusively on wild lupines. The Karner blue is on the federal endangered list, so the U.S. Fish and Wildlife Service and other agencies are working to save it. They might be able to keep it alive in captivity in a cultivated garden of lupines. But in that constricted context it would be a curiosity, a zoo exhibit.

So work on protecting the species emphasizes protecting the savanna communities it is part of. If we can sustain the ecological processes--such as fire--that sustained the savanna, and if we can prevent the invasion of exotics that kill the lupines with their shade, we can save the Karner blue as a participant in its native community. And we can save the other insects that pollinate the lupines, the birds that eat the Karner blue larvae, the mice that eat lupine seeds, and all the rest of the rich community of the oak and pine barrens. If we do our thinking in terms of community rather than individual species we can hope for the survival of nature.

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