In her remarkable new book "The Rambunctious Garden," Emma Marris explores a paradox that is increasingly vexing the science of ecology, namely that the only way to have a pristine wilderness is to manage it intensively. Left unmanaged, a natural habitat will become dominated by certain species, often invasive aliens introduced by human beings. "A historically faithful ecosystem is necessarily a heavily managed ecosystem," she writes. "The ecosystems that look the most pristine are perhaps the least likely to be truly wild."
In the Netherlands, for example, cattle are being used to re-create a simulacrum of a Pleistocene woodland, because their aurochs ancestors would have been vital in keeping forest patchy. To keep African national parks from deforestation, elephant control is sometimes needed. To let aspen, willow and beaver return to Yellowstone, it was necessary to reintroduce the wolf, which reduced elk numbers. To preserve Mojave Desert tortoises, it is essential to control native ravens, whose numbers have been boosted by distant landfill sites.
Some ecosystems are enriched and made more productive by invasive species. In terms of "ecosystem services"-the provision of clean water, the absorption of carbon, the creation of soil, the prevention of erosion-Hawaiian forests dominated by alien tree species can perform better than the pristine habitats they replace. Though many invasive aliens are notorious for the harm they bring (pythons in Florida, cane toads in Australia, brown tree snakes in Guam), many others enhance the local nature scene.
Where I live, in the U.K., American gray squirrels are exterminating native red squirrels with the help of a parapox virus and a better ability to digest acorns. Aesthetically, this is a pity: The red is nicer to look at and part of local culture. But ecologically, one has to admit that the gray is better at filling the squirrel niche in our broadleaf woodland. Reds are really a pine-adapted species that had responded to a broadleaf vacancy after the most recent ice age.
Ms. Marris's book goes further, challenging the very idea of a balance of nature. In the first half of the 20th century, ecologists came to believe in equilibrium-that natural systems tended toward a steady state. So, for example, a bare patch of ground would be colonized by a succession of species-annual weeds, then grasses, then shrubs, then trees-until it reached its "climax" state. Conservation, therefore, was a matter of restoring this climax.
Academic ecologists have abandoned such a static way of thinking for something much more dynamic. For a start, they now appreciate that climate has always changed, and with it, ecology. Twenty thousand years ago the spot where I live was under a mile of ice. Then it was tundra, then birch forest, then pine forest, then alder, linden, elm and ash, then most recently oak, but beech was coming.
Which is its climax? We now know that oak seedlings rarely thrive under mature oaks (which rain caterpillars on them), so the oak climax was just a passing phase.
Yet even as academic ecologists have abandoned balance-of-nature thinking, it still dominates practical conservation management. Ms. Marris quotes the ecologist Daniel Botkin: "If you ask an ecologist if nature never changes, he will almost always say no. But if you ask that same ecologist to design a policy, it is almost always a balance-of-nature policy": preserve this rare species, maintain this habitat structure, freeze in time this ecological moment, return this degraded land to a particular state, whatever the weather and whatever the novel arrivals of exotic species. Just as in our management of the economy, we think of states, not processes.
So what's a good conservationist to do? Ms. Marris sets you free: "In a nutshell: Give up romantic notions of a stable Eden, be honest about goals and costs, keep land from mindless development and try just about everything."