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What are muskrats worth? This is a complicated question. We can easily calculate their value as furs. About ten million muskrats are trapped every year in North America. Pelts bring an average of between three and four dollars each, so we could declare the value of muskrats to be between $30 million and $40 million a year.

Of course the pelts are only the beginning of the process. Individual pelts combined into coats are worth much more than raw furs. In their finished form, muskrat furs support manufacturers, wholesalers, retailers, and cleaners, whose combined contribution to the GNP is doubtless well over $100 million a year.

Live muskrats also have some value as experiences for humans. It is exciting to stand at the edge of a marsh or to paddle a canoe through shallow water and spot the head of a swimming muskrat just breaking the surface. Bureaucrats calculating the costs and benefits of major development projects try to turn that experience into dollars and cents. They survey citizens, asking them how much they would pay to see a muskrat, or, as a sort of semiterrorist alternative, how much they would pay to avoid the extinction of muskrats.

But any direct value to humans is only a small part of the total worth of muskrats. What about their ecological value? Muskrats are major actors in marsh ecosystems. They are food for bald eagles and minks. They eat huge amounts of cattails and other emergent vegetation, which helps prevent these plants from covering the entire marsh and produces areas of open water for mallards and blue-winged teal. They also build their low-domed houses of cattails in the water far from shore, providing protected nesting sites for black terns.

And muskrats make a more general ecological contribution, one they share with every species of plant and animal no matter how rare: they contribute to the biological diversity of their native ecosystem. Their presence puts competitive pressure on plants, on other plant-eating animals, and on the predators that hunt them. For example, many plants have evolved chemical defenses against herbivores. Muskrats might selectively feed on plants without defenses while leaving those with chemical weapons alone. In time these feeding habits could create whole populations with effective chemical protection. Other plant eaters might also be pressured into more selective feeding in order to escape competition with the gnawing incisors of muskrats.

The individual species in ecosystems with high levels of diversity are crowded ecologically. They constantly face competition, dangerous predators, and elusive prey. Crowding fuels the processes of evolution by promoting specialization that can lead to speciation as plants and animals seek secure niches. A high level of biological diversity creates an upward spiral as diversity produces even more diversity.

Do muskrats have some further value beyond their use to humans and beyond their various roles as constituents of an ecosystem? Is there some essence of muskratitude that gives them an intrinsic value, the sort of value we accord to human beings?

Questions like these are the focus of Why Preserve Natural Variety? by Bryan G. Norton (Princeton University Press). Norton is a professor of philosophy at Georgia Tech, and his book is a philosophical inquiry, not a scientific tract. It is a critique of various rationales for the protection of natural systems, with emphasis on the protection of rare and endangered species, and it offers Norton's thoughts on why the protection of nature is necessary. His arguments are complex, but his goal is to provide a basis for making public policy on the protection of nature.

He begins by distinguishing anthropocentric and nonanthropocentric arguments. Should we endeavor to save endangered species because their preservation will benefit us in some way, or should we save them because they have a value of their own completely separate from us and our wishes?

The benefits we derive from nature have to be looked at broadly. We can save natural things because we think they are pretty or because we can make fur coats from their hides. We can save them because experiencing wild places and wild creatures can transform us, change the way we think about our lives and our place in the world.

We can save them because natural diversity is essential to the functioning of ecosystems, and our lives--however much technology may insulate us from nature-- depend on the functioning of natural systems. Diversity promotes diversity and simplicity promotes even more simplicity. When a species is removed from an ecosystem its absence ripples through the entire system.

We have some excellent local examples of how this works. With the native large predators removed from our forest preserves, deer populations boom. The hungry deer denude the forest floor, and so the absence of timber wolves eventually eliminates trilliums and other woodland wildflowers from the system. When the wildflowers go, the insects they support go too. The absence of the insects, combined with the physical changes in the forest-floor environment created by large deer populations, may wipe out wood thrushes, veeries, and ovenbirds, all insect eaters that feed on the forest floor. Plants eaten while they are young and green never have a chance to set seed. Seed eaters such as chipmunks may suffer, and with them owls and weasels, predators that feed on the seed eaters.

Ecosystems are so complex that we have no way of predicting how the removal of a single species will affect the rest of the system. It could cause a total collapse. The continuing loss of species poses what Norton calls a zero-infinity risk, the same sort of problem we face with nuclear reactors. Reactors may operate with perfect safety for years, but if one does melt down, the effects will be catastrophic. So we may take species from a system with little apparent effect, until we take one too many and disaster follows.

The effects of species removal may not be apparent for a long time. Norton points out that the mass extinction that ended the reign of the dinosaurs may have begun with a single event, the impact of an asteroid on the earth. But it took 40,000 years, the blink of an eye in geological terms but a very long time in human terms, for the effects of the collision to work their way through living systems.

An ecological view of life and our place in it, Norton writes, recognizes that humans are evolved animals and that evolution, the process that created us, "works within almost unbelievably complex and interrelated organic systems." Our awareness of that complexity should lead us to approach nature with humility and to seek harmony with it. "It is good, in this view," Norton writes, "to do things in a way that mimics nature's patterns; it is good to promote the natural processes that . . . produce greater diversity; it is good to introduce alterations slowly enough to allow nature to react."

Of course, some believe that we should protect other living things because they have the same intrinsic worth we do. They should be considered ethically as ends in themselves just as we are.

Norton sees two problems with this view. First, why are other creatures the same as us? Some animal-rights believers say it is because animals can feel pain and pleasure just as we do. This formulation leaves plants out of the ethical picture, even though they certainly can't be left out of ecosystems. Second, humans have rights as individuals. If other living things have the same rights, they must be treated as individuals and not as members of species. And then we cannot, for example, legitimately kill cowbirds to prevent them from parasitizing the endangered Kirtland's warbler, for cowbirds have as much right to live as Kirtland's warblers do. The animal-rights approach protects individual animals, but it provides no basis for protecting the ecosystems that sustain all life.

Ultimately, Norton's answer to our preservation problems is simple: We protect species by protecting habitats. Rather than compiling a list of endangered species and then working out protection plans on a species-by-species basis, we should seek to protect the full range of habitats within regions. In Illinois we would look to protect examples of the full range of prairie types, from the dry sandhill prairies to wet prairies. We would protect wetland types such as marshes and bottomland forests. We would protect oak savannas, and in the south, large blocks of forest.

Of course things are so bad in Illinois that we will have to create some of these places before we can protect them. But if we can, by preservation and restoration, protect adequate acreage of all these habitat types, our multitudinous endangered species will no longer be in danger.

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