Caroline Fraser’s book Rewilding the World is a call to retrofit more than a century of nature conservation in the United States and around the world. Why, at this late date, is it so important that we redesign the global conservation system? Conservationists are rightly proud of their collective accomplishment in bringing some 12 percent of the earth’s land under protection so that future generations may know and enjoy nature. Why should this success now be in question?
The answer lies in the fact that our zeal for conserving nature far outran the science of how to do it. The modern conservation movement dates to the founding of the World Wildlife Fund in Europe and the Nature Conservancy in the United States after World War II. Both organizations hired scientists to advise them, but the scientists found themselves having to invent programs and priorities out of thin air. Conservation did not have a solid scientific basis until a conference in San Diego in September 1977. Before that, scattered articles presented results of studies that could, by inference or extension, suggest conservation strategies, but as often happens in science, controversy erupted over the interpretation of the results, and when scientists disagree among themselves, everyone else stops listening. The San Diego conference was brilliantly conceived to bring the scientific community together in a consensus that would move the field forward.
Conservation biology had failed to develop earlier because it confronted a methodological impasse: the difficulty of studying the process of extinction of species. Indeed, one definition of conservation biology is that it is the science of why extinctions occur and how to prevent them. Until the 1970s, nearly all scientists who studied extinction were paleontologists who studied fossils. Extinctions are abundantly registered in the fossil record, but the great majority of ancient organisms appeared and disappeared without apparent cause. Exceptions occurred in rare global mass extinctions, of which there have been only five since the origin of multicellular life, one of which was the meteorite impact that ended the age of dinosaurs.
Between mass extinctions, which have occurred at intervals of roughly 100 million years, there were countless extinctions of individual species taking place in the “background.” But the rate was so slow, approximately one in a million species per year, that it didn’t appear relevant to a world in which native habitats were disappearing at an alarming rate and countless animals and plants were being exploited for commercial purposes. If humans were going to “manage” nature so as to prevent extinctions, an entirely new branch of science was needed to address the problem.
The need to retrofit the current conservation system arises out of science that developed during the latter half of the twentieth century. Fraser provides an introduction to this science as the rationale behind her call for “rewilding the world.” What, exactly, she means by “rewilding” will emerge from the following condensed account of the relevant science.
Even today, when extinctions are occurring with unprecedented frequency, they are extremely difficult to verify. The ivory-billed woodpecker is a prime example. The last substantiated photographs and sound recordings were made in the late 1940s, yet rumors about the bird’s existence and fervent claims of sightings continue to emerge from the Southeast. Is the ivory-billed woodpecker extinct? No one can say. Hence the extraordinary difficulty of studying extinction as a process.
As often happens in science, a solution to this impasse came from unexpected quarters. Two brilliant young biologists, Robert MacArthur, then of the University of Pennsylvania, and Edward O. Wilson of Harvard, published The Theory of Island Biogeography in 1967. The theory is based on a relatively simple finding: the number of species of birds, lizards, or other animals that occupy an island can be quite accurately predicted if one knows nothing more about the island than its area and its distance from a source of colonizing species. Nearby islands are more frequently colonized by new species than distant islands, and large islands support more species, on average, than smaller islands.
MacArthur and Wilson proposed that the number of species of a given type (birds, lizards, etc.) that occupies an island is maintained by a dynamic equilibrium between colonization of the island by new species and extinction of those already present. Colonization and extinction are thus represented as normal, ongoing processes that interact to regulate the number of species present at any time. Over long periods (decades, centuries, or millennia, depending on the size of the island), an island’s complement of species was posited to change while the total number of species remained more or less constant.
Soon after publication of this theory, biologists began to realize that it could be applied to conservation if one assumed that shrinking remnants of natural habitat, or parks surrounded by agricultural lands, were analogous to islands. There followed a rush to search for historical records listing which species had been found on a given island or in a particular suburban park decades or even a century earlier. Such efforts quickly yielded support for the theory by showing that, indeed, the species complements of some small islands and habitat fragments had changed over the time elapsed between surveys, whereas the faunas of large islands and habitat fragments appeared to remain constant.
Still, there were skeptics who maintained that appearances and disappearances of species from tiny islands and habitat patches were trivial because of the small numbers of individuals involved, and who asserted that there was no evidence that species went extinct in insular areas large enough to be of relevance for conservation. So long as these dissenting voices were contesting the validity of the MacArthur- Wilson theory, conservation planners and managers remained gun-shy and conservation practice continued as the opportunistic process it had always been.
Any lingering doubts about whether island biogeography theory was relevant to conservation were dispelled in 1987 by a paper published in the prestigious British journal Nature, a study that Fraser calls “a bombshell, the kind of logical observation that seems obvious only in retrospect.” Written by William Newmark while he was a graduate student at the University of Michigan, it presented startling evidence that numerous mammal species had disappeared from national parks in the western US and Canada. Newmark benefited from research conducted decades earlier by the US Biological Survey, a precursor of today’s Fish and Wildlife Service. The survey had systematically documented the mammal species present in each national park at the time it was established. Most of the parks Newmark studied dated to the early decades of the twentieth century and were between sixty and ninety-five years old at the time of his research. Using a variety of approaches, Newmark compiled contemporary data on the mammals occupying each park and compared the lists to those assembled earlier by the Biological Survey.
The contrasts between the two sets of lists stunned the entire conservation world. Bryce Canyon, Lassen Volcanic, and Zion, the three smallest parks, had each lost more than a third of the mammals larger than rats and chipmunks known to be present at the parks’ establishment. As expected from island biogeography theory, small parks had lost many species whereas large parks had lost few. The only parks in the study that retained all their species were Banff, Jasper, Yoho, and Kootenay, a back-to-back cluster of parks in the Canadian Rockies comprising an area roughly the size of Massachusetts. All US parks in the study lost species although the largest, Yellowstone, lost only one, the gray wolf, and that was owing to a campaign of extermination initiated by the US government. Still, populations of all the species Newmark documented as disappearing from US parks survive today in other locations.
Mount Rainier, Rocky Mountain, Yosemite, Grand Canyon—these are among the crown jewels of conservation in the United States and they were demonstrably failing to retain their biodiversity. Only a few years later, Newmark and another American biologist, Justin Brashares, produced similarly disturbing data for national parks in East and West Africa, respectively. It seemed as if parks could not save biodiversity, and ideological opponents of parks seized upon the results to make the claim that parks didn’t work, so why have them? The conclusion was a simplistic overreaction by people who did not comprehend the underlying science. Island biogeography predicted that a contraction in area, such as that experienced by a park after its surroundings have been converted to human uses, would lead to local extinctions, but it could not predict which species would disappear or why. Again, conservation science desperately needed a new theory, and fortunately, intimations of one soon appeared in the form of an ecological phenomenon known as “mesopredator release.”
As Fraser notes, the landmark paper that established mesopredator release as a driver of local extinctions was written by Michael Soulé, a professor at the University of California–Santa Cruz, and his graduate student Kevin Crooks. The two of them studied bird communities in twenty-eight canyons in San Diego County and found that some of the canyons resounded with the songs of a full complement of native birds, whereas others supported barely more than house sparrows, starlings, and park pigeons. The important difference between the two sets of canyons turned out to be the existence of habitat corridors that allowed coyotes to enter the canyons that supported vibrant native bird communities. In contrast, the ornithologically dead canyons were embedded in an urban/suburban landscape that offered no access to coyotes living outside the city.
Coyotes and birds? What was the connection? Coyotes don’t eat songbirds; they have nothing directly to do with them. The connection was cats, both domestic and feral. Coyotes can and do eat cats, and where coyotes are present, they impose a reign of terror on neighborhood cats, deterring them from entering canyons and hunting birds. So, indirectly, coyotes are good for birds. Where there are no coyotes, cats have nothing to fear and dedicate themselves to hunting, with strongly negative consequences for the bird community.
Cats are considered a “mesopredator,” one of a group of medium-sized predators that are normally held to low abundance by top predators, in this case coyotes. Other mesopredators are raccoons, foxes, and opossums. Where top predators like wolves, coyotes, and mountain lions are persecuted, their prey tend to increase, and can become up to ten times more abundant than where the top predators are present. Fox, feral cat, and raccoon populations have exploded all over the United States following eradication of top predators in the nineteenth (in the East) or early twentieth century (in the West) and, as Fraser aptly puts it, “ran wild in an orgy of predation.” Another consequence of predator elimination is “herbivore release,” familiar to many dwellers of suburbia as an overabundance of deer, beavers, and woodchucks.
Population explosions of mesopred- ators and herbivores have dire consequences for biodiversity. Mesopredators not only prey upon songbirds, but seek myriad other small prey as well, including frogs, lizards, snakes, salamanders, and small mammals. Many once-common species, Fraser observes, are now scarce or locally absent as a consequence. Overabundant herbivores have equally dire effects on vegetation, completely suppressing forest regeneration in the worst cases and eliminating many wildflowers.