When aliens attack

The case of Guam might seem to support the classical idea of an ecosystem as an interlocking mechanism finely honed by millennia of evolution, where each organism occupies a niche crucial to the whole, and where the addition (or subtraction) of a single species can wreak havoc up and down the system. One serpent arrived in Eden, and now the forest is silent. But dire case histories like the brown snake's are more the exception than the rule. As Burdick puts it, "Most successful invaders simply blend into the ecological woodwork."

San Francisco Bay, as he learns late in his impressive nine-year odyssey, supports more than 200 non-native species, making it the most-invaded marine ecosystem anywhere in the world. Only a handful of those new arrivals have caused conspicuous problems, and as yet there have been no mass extinctions, no ecological catastrophe. As in the Hawaiian example (and probably in the Catskills too) the net result has been more species living in the bay, rather than fewer. In other words, local biological diversity has actually increased, which -- again, according to classical ecology -- is supposed to be a good thing.

Yet surely something has been lost when the nature most of us see most of the time is a collage of native and non-native species, thrown together haphazardly in very recent historical time, even if it presents the appearance of relative stability and -- for want of a better word -- naturalness. For an environmental purist like writer Bill McKibben, such a landscape isn't nature at all, just a sort of human-centric golf-course fakery. Burdick takes a more nuanced and longer view, arguing that the central tragedy of the human condition is that we are both inside and outside nature, doomed to alter it and to observe ourselves altering it. In this he finds a kind of obscure hope.

How one thinks about the jumbled quality of nature on a planet constantly being circumnavigated by Homo sapiens and all its planes, ships and automobiles lies at the very heart of the still-young science of invasion ecology, which, as Burdick discovers, is only beginning to move away from anecdotal evidence and conjecture toward grappling with hard data. Generations of environmentalists, for example, have blamed the feral pigs of Hawaii (descended from 18th century English imports) for ravaging native plant life. Native Hawaiians, who conduct ritual pig hunts, argued that the porcine invaders did only cosmetic damage to the forest -- and they now appear to have been right.

Indeed, leading scientists in the field have grown skeptical of the idea that an ecosystem is a well-balanced, self-regulating and internally calibrated natural machine (and some now reject the word as well). Instead, they say, the only constant in nature is change. In one of his most breathtaking passages, Burdick elaborates on this "nonequilibrium model":

"Nature does not function precisely like clockwork, a tapestry, a cathedral, a pyramid, an airplane, or an international bank; metaphors are drawn from the world of human invention and knowledge, whereas nature is far larger than either of those things and has hardly begun to be understood ... Species disperse and invade, come and go, evolve and go extinct. Any organism can be an invader somewhere. Every ecosystem -- or whatever one calls it -- can be invaded by something. This is true even in the absence of humankind. An ecosystem is stable over time not because the list of species remains forever the same, but because it varies -- not in spite of disturbance, but because of it."

What is this, you may be wondering -- the Dick Cheney school of laissez-faire ecology? Are we supposed to accept that the human-enabled distribution of microorganisms, seeds, spores, bugs, fish, birds and insects from one place to another is just the natural process, speeded up a few thousand times? If the entire state of Hawaii becomes a tropical fantasyland plantation, and the zebra mussel chokes out the Mississippi River's native life forms, and the European green crab kills off the native shore crabs of the Pacific Northwest, well, that's life in the fast lane, right? (As one of Burdick's scientists has demonstrated, both mussel and crab almost certainly arrived in ballast water, the huge doses of seawater oceangoing cargo ships suck up in one location and then discharge in another.)

Not exactly. Burdick, like most of the ornithologists, entomologists, soil ecologists and marine biologists in his book, believes in trying to control the unlimited flow of ecological invasion, whenever and wherever that's possible. But he suggests that, first of all, we have to be realistic. One theme repeated throughout "Out of Eden" is that resources are often squandered on expensive and dubious attempts to save critically endangered species, when it would be more practical and cost-effective to protect as much wilderness habitat as possible and face the fact that some individual species won't survive.

Even more important, Burdick thinks that if we decide it's critical to save diverse kinds of ecosystems all over the world -- that we value the ecological distinctions between Hawaii and Australia, India and California -- we must recognize that we're doing it for our own reasons, not for any objective scientific reason to be found in nature itself. Nature, Burdick insists, is "not a reliable model for wilderness conservation." Instead, it is "heartless, mindless, raw, and insatiable; it is red in tooth and claw. However much we care about it or its more attractive artifacts, it does not care for us, or even for itself."

Next Page: Should we preserve biodiversity for the sake of aesthetics?

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