Natural Acts

by David Quammen

  “For me,” David Jablonski says now, “that was a turning point.”

  But it’s not the starting point I asked about. By the time of the Flagstaff meeting, I remind him, the idea of convening biologists together with paleontologists for a discussion of mass extinction was almost obvious, as reflected by the fact that two such events occurred that year. When was the idea less obvious? When was it just a fresh, counterintuitive notion? Jablonski obliges me by pushing his memory a little harder—back, as it turns out, to his own work during graduate school.

  In the 1960s and early 1970s, concern about human-caused extinctions was neither widespread nor ecologically astute. Some writers warned about “vanishing wildlife” and “endangered species,” but generally the warnings were framed around individual species with popular appeal, such as the whooping crane, the tiger, the blue whale, the peregrine falcon. Back in 1958, the pioneering British ecologist Charles Elton had published a farsighted book about biological dislocations, The Ecology of Invasions by Animals and Plants; Rachel Carson in 1962, with Silent Spring, had alerted people to the widespread, pernicious effects of pesticides such as DDT; and David Ehrenfeld’s Biological Conservation appeared in 1970. But those three were untypical in their grasp of larger contexts. During the 1970s a new form of concern broke forth—call it wholesale concern—from the awareness that unnumbered millions of narrowly endemic (that is, unique and localized) species inhabit the tropical forests and that those forests were quickly being cut. The World Wildlife Fund and the Smithsonian Institution sponsored a symposium in 1974 on the subject of biological impoverishment; the chief scientist at WWF and the main organizer of that event was a young ecologist named Thomas E. Lovejoy, not long removed from his own doctoral work on Amazon birds. Another early voice belonged to Norman Myers, a Berkeley-trained biologist based in Nairobi. In 1976, Myers published a paper in Science recommending greater attention to the economic pressures that drive habitat destruction and the consequent loss of species; in passing, he also compared current extinctions with the rate during what he loosely called “the ‘great dying’ of the dinosaurs.” David Jablonski, then a graduate student struggling to do his dissertation and pay his bills, read Myers’s paper and tucked a copy into his files. The comparison to the Cretaceous extinction, an event about which he was knowledgeable, didn’t seem to him incongruous. Soon afterward, in early 1978, Jablonski was running out of cash and so “finagled the opportunity” to offer a seminar course, a special elective for undergraduates, through one of the Yale residential colleges. “I decided to teach it on extinction,” he says.

  Now suddenly energized by this recollection, Jablonski dodges among his paper-pile stalagmites to a cabinet and returns with a twenty-year-old file. He flips through it, mesmerized like an old athlete over a scrapbook from his improbable youth. The yellowing sheets tell us that his course ran in autumn 1978 as college seminar 130a, “Crises in the Evolution of Life.” Eleven weeks of class were devoted to paleontological fundamentals such as deep time, uniformitarian change, the tempo and mode of evolution, Darwin and Lamarck, Cuvier and Lyell, and then to signal episodes such as the Permian extinction, the Devonian extinction, the K-T event. Week twelve would connect paleontology with neontology. On that Tuesday evening, according to a typed outline saved in the old file, students would consider the past and future impact of Homo sapiens, concerning notably: “The diminution of global biotic diversity, and how (or if) it should be maintained. Climatic effects of human activities. Are we on the brink of a mass extinction? The past as the key to the present.” It was the first class that David Jablonski ever taught.

  Norman Myers’s early role in this matter was important from several angles. “He was the guy who really started the quantification of extinction,” Jablonski recalls. “Norman was a pretty lonely guy for a long time on that.” In 1979, Myers published The Sinking Ark, which explained the extinction problem for a popular audience, and in 1980 he produced a report for the National Academy of Sciences, drily titled Conversion of Tropical Moist Forests but full of eloquent data tracing the worldwide destruction of rainforest ecosystems. In the former book, he offered some rough numbers and offhand projections. Between the years 1600 and 1900, by his count, humanity had caused the extinction of about 75 known species, almost all of them mammals and birds. Between 1900 and 1979, humans had extinguished another 75 known species. Repeating what he had said in Science, Myers noted that this provisional tally—totaling 150 known species, all lost in less than four centuries—was well above the rate of known losses during the Cretaceous extinction. But more worrisome was the inferable rate of unrecorded extinctions, recent and now impending, among tropical plants and animals still unidentified by science. He guessed that 25,000 plant species presently stood jeopardized, and maybe hundreds of thousands of insects. “By the late 1980s we could be facing a situation where one species becomes extinct each hour. By the time human communities establish ecologically sound life-styles, the fallout of species could total several million.” Rereading those sentences now, I’m struck by the reckless optimism of his assumption that human communities eventually will establish “ecologically sound life-styles.” But back in 1981, when I first encountered Myers’s book, his predictions seemed shocking and gloomy.

  A year after The Sinking Ark appeared, Tom Lovejoy of WWF offered his own cautionary guesstimate in a section of the Global 2000 report to outgoing president Jimmy Carter. Based on current projections of forest loss and a plausible relationship between forest area and endemism, Lovejoy suggested that 15 to 20 percent of all species—amounting to millions—might be lost by the end of the twentieth century. In the course of his discussion, Lovejoy also coined a new phrase, “biological diversity,” which seems obvious in retrospect but hadn’t yet been in use for denoting the aggregate of what was at stake. The portmanteau version, “biodiversity,” would be buckled together a few years later. Among field biologists, a sense of focused concern was taking hold.

  These early tries at quantification proved consequential for two reasons. First, Myers and Lovejoy helped galvanize public concern over the seemingly abstract matter of how many species may be lost as humanity claims an ever larger share of Earth’s landscape and resources. Second, the Myers and Lovejoy warnings became targets for a handful of critics, who used the inexactitude of those numbers to cast doubt on the reality of the whole problem. Most conspicuous among the naysayers was Julian Simon, an economist at the University of Maryland, who argued bullishly that human population growth and human resourcefulness would solve all problems worth solving, of which a decline in diversity of tropical insects wasn’t one.

  In a 1986 issue of New Scientist, Simon rebutted Norman Myers, based on his own construal of select data, to the effect that there was “no obvious recent downward trend in world forests—no obvious ‘losses’ at all, and certainly no ‘near catastrophic’ loss.” He later coauthored an op-ed piece in the New York Times under the headline “Facts, Not Species, Are Periled.” Again he went after Myers, asserting a “complete absence of evidence for the claim that the extinction of species is going up rapidly—or even going up at all.” Simon’s worst disservice to logic in that statement and others was the denial that inferential evidence of wholesale extinction counts for anything. Of inferential evidence there was an abundance—for example, from the Centinela Ridge in a cloud-forest zone of western Ecuador, where in 1978 the botanist Alwyn Gentry and a colleague found thirty-eight species of narrowly endemic plants, including several with mysterious black leaves. Before Gentry could get back, Centinela Ridge had been completely deforested, the native plants replaced by cacao and other crops. As for inferential evidence generally, we might do well to remember what it contributes to our conviction that approximately 105,000 Japanese civilians died in the atomic bombing of Hiroshima. The city’s population fell abruptly on August 6, 1945, but there was no one-by-one identification of 105,000 bodies.

  Nowadays a few younger writers have taken Simon’s line
, pooh-poohing the concern over extinction. As for Simon himself, who died in 1998, perhaps the truest sentence he left behind was “We must also try to get more reliable information about the number of species that might be lost with various changes in the forests.” No one could argue.

  But it isn’t easy to get such information. Field biologists tend to avoid investing their precious research time in doomed tracts of forest. Beyond that, our culture offers little institutional support for the study of narrowly endemic species in order to register their existence before their habitats are destroyed. Despite these obstacles, recent efforts to quantify rates of extinction have supplanted the old warnings. These new estimates use satellite imaging and improved on-the-ground data about deforestation, records of the many human-caused extinctions on islands, and a branch of ecological theory called island biogeography, which connects documented island cases with the mainland problem of forest fragmentation. These efforts differ in particulars, reflecting how much uncertainty is still involved, but their varied tones form a chorus of consensus. I’ll mention three of the most credible.

  W. V. Reid, of the World Resources Institute, in 1992 gathered numbers on the average annual deforestation in each of sixty-three tropical countries during the 1980s, and from them he charted three different scenarios (low, middle, high) of presumable forest loss by the year 2040. He chose a standard mathematical model of the relationship between decreasing habitat area and decreasing species diversity, made conservative assumptions about the crucial constant, and ran his various deforestation estimates through the model. Reid’s calculations suggest that by the year 2040, between 17 and 35 percent of tropical forest species will be extinct or doomed to extinction. At either the high or the low end of this range, it would amount to a bad loss, though not as bad as the K-T event. Then again, 2040 won’t mark the end of human pressures on biological diversity or landscape.

  Robert M. May, an ecologist at Oxford, coauthored a similar effort in 1995. May and his colleagues noted the five causal factors that account for most extinctions: habitat destruction, habitat fragmentation, overkill, invasive species, and secondary effects cascading through an ecosystem from other extinctions. Each of those five is more intricate than it sounds. For instance, habitat fragmentation dooms species by consigning them to small parcels of habitat left insularized in an ocean of human impact and by then subjecting them to the same jeopardies (small population size, acted upon by environmental fluctuation, catastrophe, inbreeding, bad luck, and cascading effects) that make island species especially vulnerable to extinction. May’s team concluded that most extant bird and mammal species can expect average life spans of between two hundred and four hundred years. That’s equivalent to saying that about a third of one percent will go extinct each year until some unimaginable end point is reached. “Much of the diversity we inherited,” May and his coauthors wrote, “will be gone before humanity sorts itself out.”

  The most recent estimate comes from Stuart L. Pimm and Thomas M. Brooks, ecologists at the University of Tennessee. Using a combination of published data on bird species lost from forest fragments and field data they gathered themselves, Pimm and Brooks concluded that 50 percent of the world’s forest bird species will be doomed to extinction by deforestation occurring over the next half-century. And birds won’t be the sole victims. “How many species will be lost if current trends continue?” the two scientists asked. “Somewhere between one third and two thirds of all species—easily making this event as large as the previous five mass extinctions the planet has experienced.”

  Jablonski, who started down this line of thought in 1978, offers me a reminder about the conceptual machinery behind such estimates. “All mathematical models,” he says cheerily, “are wrong. They are approximations. And the question is: Are they usefully wrong, or are they meaninglessly wrong?” Models projecting present and future species loss are useful, he suggests, if they help people realize that Homo sapiens is perturbing Earth’s biosphere to a degree it hasn’t often been perturbed before. In other words, that this is a drastic experiment in biological drawdown we’re engaged in, not a continuation of routine.

  Behind the projections of species loss lurk a number of critical but hard-to-plot variables, among which two are especially weighty: continuing landscape conversion and the growth of human population.

  Landscape conversion can mean many things: draining wetlands to build roads and airports, turning tallgrass prairies under the plow, fencing savanna and overgrazing it with domestic stock, cutting second-growth forest in Vermont and consigning the land to ski resorts or vacation suburbs, slash-and-burn clearing of Madagascar’s rainforest to grow rice on wet hillsides, industrial logging in Borneo to meet Japanese plywood demands. The ecologist John Terborgh and a colleague, Carel P. van Schaik, have described a four-stage process of landscape conversion that they call the landuse cascade. The successive stages are: 1) wildlands, encompassing native floral and faunal communities altered little or not at all by human impact; 2) extensively used areas, such as natural grasslands lightly grazed, savanna kept open for prey animals by infrequent human-set fires, or forests sparsely worked by slash-and-burn farmers at low density; 3) intensively used areas, meaning crop fields, plantations, village commons, travel corridors, urban and industrial zones; and finally 4) degraded land, formerly useful but now abused beyond value to anybody. Madagascar, again, would be a good place to see all four stages, especially the terminal one. Along a thin road that leads inland from a town called Mahajanga, on the west coast, you can gaze out over a vista of degraded land—chalky red hills and gullies, bare of forest, burned too often by graziers wanting a short-term burst of pasturage, sparsely covered in dry grass and scrubby fan palms, eroded starkly, draining red mud into the Betsiboka River, supporting almost no human presence. Another showcase of degraded land—attributable to fuel-wood gathering, overgrazing, population density, and decades of apartheid—is the Ciskei homeland in South Africa. Or you might look at overirrigated crop fields left ruinously salinized in the Central Valley of California.

  Among all forms of landscape conversion, pushing tropical forest from the wildlands category to the intensively used category has the greatest impact on biological diversity. You can see it in western India, where a spectacular deciduous ecosystem known as the Gir Forest (home to the last surviving population of the Asiatic lion, Panthera leo persica) is yielding along its ragged edges to new mango orchards, peanut fields, and lime quarries for cement. You can see it in the central Amazon, where big tracts of rainforest have been felled and burned, in a largely futile attempt (encouraged by misguided government incentives, now revoked) to pasture cattle on sun-hardened clay. According to the United Nations Food and Agriculture Organization, the rate of deforestation in tropical countries has increased (contrary to Julian Simon’s claim) since the 1970s, when Norman Myers made his estimates. During the 1980s, as the FAO reported in 1993, that rate reached 15.4 hectares (a hectare being the metric equivalent of 2.5 acres) annually. South America was losing 6.2 million hectares of forest a year. Southeast Asia was losing less in sheer area but more proportionally: 1.6 percent of its forests yearly. In terms of cumulative loss, as reported by other observers, the Atlantic coast forest of Brazil is at least 95 percent gone. The Philippines, once nearly covered with rainforest, has lost 92 percent. Costa Rica has continued to lose forest, despite that country’s famous concern for its biological resources. The richest old-growth lowland forests in West Africa, India, the Greater Antilles, Madagascar, and elsewhere have been reduced to less than a tenth of their original areas. By the middle of the twenty-first century, if those trends continue, tropical forest will exist virtually nowhere outside of protected areas—that is, national parks, wildlife refuges, and other official reserves.

  How many protected areas will there be? The present worldwide total is about 9,800, encompassing 6.3 percent of the planet’s land area. Will those parks and reserves retain their full biological diversity? No. Species w
ith large territorial needs will be unable to maintain viable population levels within small reserves, and as those species die away, their absence will affect others. The disappearance of big predators, for instance, can release limits on medium-sized predators and scavengers, whose overabundance can drive still other species (such as ground-nesting birds) to extinction. This has already happened in some habitat fragments, such as Panama’s Barro Colorado Island, and been well documented in the literature of island biogeography. The lesson of fragmented habitats is Yeatsian: Things fall apart.