Natural Acts

by David Quammen

  Human population growth will make a bad situation worse by putting ever more pressure on all available land.

  Population growth rates have declined in many countries within the past several decades, it’s true. But world population is still increasing, and even if average fertility suddenly, magically dropped to 2.0 children per female, population would continue to increase (on the momentum of birthrate exceeding death rate among a generally younger and healthier populace) for some time. The annual increase is now 80 million people, with most of that increment coming in less developed countries. The latest long-range projections from the Population Division of the United Nations, released in early 1998, are slightly down from previous long-term projections in 1992 but still point toward a problematic future. According to the UN’s middle estimate (and most probable? that’s hard to know) among seven fertility scenarios, human population will rise from the present 5.9 billion to 9.4 billion by the year 2050, then to 10.8 billion by 2150, before leveling off there at the end of the twenty-second century. If it happens that way, about 9.7 billion people will inhabit the countries included within Africa, Latin America, the Caribbean, and Asia. The total population of those countries—most of which are in the low latitudes, many of which are less developed, and which together encompass a large portion of Earth’s remaining tropical forest—will be more than twice what it is today. Those 9.7 billion people, crowded together in hot places, forming the ocean within which tropical nature reserves are insularized, will constitute 90 percent of humanity. Anyone interested in the future of biological diversity needs to think about the pressures these people will face, and the pressures they will exert in return.

  We also need to remember that the impact of Homo sapiens on the biosphere can’t be measured simply in population figures. As the population expert Paul Harrison pointed out in his book The Third Revolution, that impact is a product of three variables: population size, consumption level, and technology. Although population growth is highest in less-developed countries, consumption levels are generally far higher in the developed world (for instance, the average American consumes about ten times as much energy as the average Chilean, and about a hundred times as much as the average Angolan) and also higher among the affluent minority in any country than among the rural poor. High consumption exacerbates the impact of a given population, whereas technological developments may either exacerbate it further (think of the automobile, the air conditioner, the chainsaw) or mitigate it (as when a technological innovation improves efficiency for an established function). All three variables play a role in every case, but a directional change in one form of human impact—on air pollution from fossil-fuel burning, say, or fish harvest from the seas—can be mainly attributable to a change in one variable, with only minor influence from the other two. Sulfur dioxide emissions from developed countries fell dramatically during the 1970s and ’80s, owing to technological improvements in papermaking and other industrial processes; those emissions would have fallen still further if not for increased population (accounting for 25 percent of the upward vector) and increased consumption (accounting for 75 percent). Deforestation, in contrast, is a directional change that has been mostly attributable to population growth.

  According to Harrison’s calculations, population growth accounted for 79 percent of the deforestation in less developed countries between 1973 and 1988. Some experts would argue with those calculations, no doubt, and insist on redirecting our concern toward the role that distant consumers, wood-products buyers among slow-growing but affluent populations of the developed nations, play in driving the destruction of Borneo’s dipterocarp forests or the hardwoods of West Africa. Still, Harrison’s figures point toward an undeniable reality: More total people will need more total land. By his estimate, the minimum land necessary for food growing and other human needs (such as water supply and waste dumping) amounts to one fifth of a hectare per person. Given the UN’s projected increase of 4.9 billion souls before the human population finally levels off, that comes to another billion hectares of human-claimed landscape, a billion hectares less forest—even without allowing for any further deforestation by the current human population or for any further loss of agricultural land to degradation. A billion hectares—in other words, 10 million square kilometers—is, by a conservative estimate, well more than half the remaining forest area in Africa, Latin America, and Asia. This raises the vision of a very exigent human population pressing snugly around whatever patches of natural landscape remain.

  Add to that vision the extra, incendiary aggravation of poverty. According to a recent World Bank estimate, about 30 percent of the total population of less developed countries lives in poverty. Alan Durning, in his 1992 book How Much Is Enough? The Consumer Society and the Fate of the Earth, puts it in a broader perspective when he says that the world’s human population is divided among three “ecological classes”: the consumers, the middle-income, and the poor. His consumer class includes those 1.1 billion fortunate people whose annual income per family member is more than $7,500. At the other extreme, the world’s poor also number about 1.1 billion people—all from households with less than $700 annually per family member. “They are mostly rural Africans, Indians, and other South Asians,” Durning writes. “They eat almost exclusively grains, root crops, beans, and other legumes, and they drink mostly unclean water. They live in huts and shanties, they travel by foot, and most of their possessions are constructed of stone, wood, and other substances available from the local environment.” He calls them the “absolute poor.” It’s only reasonable to assume that another billion people will be added to that class, mostly in what are now the less-developed countries, before population growth stabilizes. How will those additional billion, deprived of education and other advantages, interact with the tropical landscape? Not likely by entering information-intensive jobs in the service sector of the new global economy. Julian Simon argued that human ingenuity—and, by extension, human population itself—is “the ultimate resource” for solving Earth’s problems, transcending Earth’s limits, and turning scarcity into abundance. But if all the bright ideas generated by a human population of 5.9 billion haven’t yet relieved the desperate needfulness of the 1.1 billion absolute poor, why should we expect that human ingenuity will do any better for roughly 2 billion poor in the future?

  Other writers besides Durning have warned about this deepening class rift. Tom Athanasiou, in Divided Planet: The Ecology of Rich and Poor, sees population growth only exacerbating the division, and notes that governments often promote destructive schemes of transmigration and rainforest colonization as safety valves for the pressures of land hunger and discontent. A young Canadian policy analyst named Thomas Homer-Dixon, the author of several calm-voiced but frightening articles on the linkage between what he terms “environmental scarcity” and global sociopolitical instability, reports that the amount of cropland available per person is falling in the less developed countries because of population growth and because millions of hectares “are being lost each year to a combination of problems, including encroachment by cities, erosion, depletion of nutrients, acidification, compacting and salinization and waterlogging from overirrigation.” In the cropland pinch and other forms of environmental scarcity, Homer-Dixon foresees potential for “a widening gap” of two sorts—between demands on the state and its ability to deliver, and more basically between rich and poor. In conversation with the journalist Robert D. Kaplan, as quoted in Kaplan’s book The Ends of the Earth, Homer-Dixon said it more vividly: “Think of a stretch limo in the potholed streets of New York City, where homeless beggars live. Inside the limo are the air-conditioned post-industrial regions of North America, Europe, the emerging Pacific Rim, and a few other isolated places, with their trade summitry and computer information highways. Outside is the rest of mankind, going in a completely different direction.” That direction, necessarily, will be toward ever more desperate exploitation of landscape. Kaplan himself commented: “We are entering
a bifurcated world.”

  H. G. Wells foretold that bifurcation a century ago in his novel The Time Machine. Wells’s time traveler, bouncing forward from Victorian London to the year A.D. 802,701, found a divided planet too, upon which the human race had split into two very different forms: the groveling, dangerous Morlocks, who lived underground, and the epicene Eloi, who enjoyed lives of languid comfort on the surface. The only quaint thing about Wells’s futurology, from where we sit now, is that he imagined it would be necessary to travel so far.

  As for Homer-Dixon’s vehicle: When you think of that stretch limo on those potholed urban streets, don’t assume there will be room inside for tropical forests. Even Noah’s ark managed to rescue only paired animals, not large parcels of habitat. The jeopardy of the ecological fragments that we presently cherish as parks, refuges, and reserves is already severe, due to both internal and external forces: internal, because insularity leads to ecological unraveling; and external, because those areas are still under siege by needy and covetous people. Projected forward into a future of 10.8 billion humans, of which perhaps 2 billion are starving at the periphery of those areas, while another 2 billion are living in a fool’s paradise maintained by unremitting exploitation of whatever resources remain, that jeopardy increases to the point of impossibility. In addition, any form of climate change in the midterm future, whether caused by greenhouse gases or by the natural flip-flop of climatic forces, is liable to change habitat conditions within a given protected area beyond the tolerance range for many species. If such creatures can’t migrate beyond the park or reserve boundaries in order to chase their habitat needs, they may be “protected” from guns and chainsaws within their little island, but they’ll still die.

  We shouldn’t take comfort in assuming that at least Yellowstone National Park will still harbor grizzly bears in the year 2150, that at least Royal Chitwan in Nepal will still harbor tigers, that at least Serengeti in Tanzania and Gir in India will still harbor lions. Those predator populations, and other species down the cascade, are likely to disappear. “Wildness” will be a word applicable only to urban turmoil. Lions, tigers, and bears will exist in zoos, period. Nature won’t come to an end, but it will look very different.

  The most obvious differences will be those I’ve already mentioned: tropical forests and other terrestrial ecosystems will be dramatically reduced in area, and the fragmented remnants will stand tiny and isolated. Because of those two factors, plus the cascading secondary effects, plus an additional dire factor I’ll mention in a moment, much of Earth’s biological diversity will be gone. How much? That’s impossible to predict confidently, but the careful guesses of Robert May, Stuart Pimm, and other biologists suggest losses reaching half to two thirds of all species. In the oceans, deepwater fish and shellfish populations will be drastically depleted by over-harvesting, if not to the point of extinction, then at least enough to cause more cascading consequences. Coral reefs and other shallow-water ecosystems will be badly stressed, if not devastated, by erosion and chemical runoff from the land. The additional dire factor is invasive species, the fifth of the five factors contributing to our current experiment in mass extinction.

  That factor, even more than habitat destruction and fragmentation, is a symptom of modernity. Maybe you haven’t heard much about invasive species, but in coming years you will. Daniel Simberloff, the same ecologist who gave that sobering paper that Jablonski remembers from 1983, takes it so seriously that he recently committed himself to founding an institute on invasive biology at the University of Tennessee, and Interior Secretary Bruce Babbitt sounded the alarm in April 1998 in a speech to a weed-management symposium in Denver. The spectacle of a cabinet secretary denouncing an alien plant called purple loosestrife struck some observers as droll, but it wasn’t as silly as it seemed. Forty years ago, Charles Elton warned in The Ecology of Invasions by Animals and Plants that “we are living in a period of the world’s history when the mingling of thousands of kinds of organisms from different parts of the world is setting up terrific dislocations in nature.” Elton’s word “dislocations” was nicely chosen to ring with a double meaning: Species are being moved from one location to another, and as a result ecosystems are being thrown into disorder.

  The problem dates back to when people began using ingenious new modes of conveyance (the horse, the camel, the canoe) to travel quickly across mountains, deserts, and oceans, taking with them rats, lice, disease microbes, burrs, dogs, pigs, goats, cats, cows, and other forms of parasitic, commensal, or domesticated creature. One immediate result of those travels was a wave of island-bird extinctions, claiming more than a thousand species, which followed oceangoing canoes across the Pacific and elsewhere. Having evolved in insular ecosystems free of predators, many of those species were flightless, unequipped to defend themselves or their eggs against ravenous mammals. Raphus cucullatus, a giant cousin of the pigeon lineage, endemic to Mauritius in the Indian Ocean and better known as the dodo, was only the most easily caricatured representative of this much larger pattern. Dutch sailors killed and ate dodos during the seventeenth century, but probably what guaranteed the extinction of Raphus cucullatus is that the European ships put ashore rats, pigs, and Macaca fascicularis, an opportunistic species of Asian monkey. Although commonly known as the crab-eating macaque, M. fascicularis will eat almost anything. The monkeys are still pestilential on Mauritius, hungry and daring and always ready to grab what they can, including raw eggs. But the dodo hasn’t been seen since 1662.

  The European age of discovery and conquest was also the great age of biogeography—that is, the study of what creatures live where, a branch of biology practiced by attentive travelers such as Carl Linnaeus, Alexander von Humboldt, Charles Darwin, and Alfred Russel Wallace. Darwin and Wallace even made biogeography the basis of their discovery that species, rather than being created and plopped onto Earth by divine magic, evolve in particular locales by the process of natural selection. Ironically, the same trend of far-flung human travel that gave biogeographers their data also began to muddle and nullify those data, by transplanting the most ready and roguish species to new places and thereby delivering misery unto death for many other species. Rats and cats went everywhere, causing havoc in what for millions of years had been sheltered, less competitive ecosystems. The Asiatic chestnut blight and the European starling came to America; the American muskrat and the Chinese mitten crab got to Europe. Sometimes these human-mediated transfers were unintentional, sometimes merely shortsighted. Nostalgic sportsmen in New Zealand imported British red deer; European brown trout and coastal rainbows were planted in disregard of the native cutthroat trout of Rocky Mountain rivers. Prickly-pear cactus, rabbits, and cane toads were inadvisedly welcomed to Australia. Goats went wild in the Galápagos. The bacteria that cause bubonic plague journeyed from China to Europe by way of fleas, rats, Mongolian horsemen, and sailing ships, and eventually traveled also to California. The Atlantic sea lamprey found its own way up into Lake Erie, but only after the Welland Canal gave it a bypass around Niagara Falls. Unintentional or otherwise, all these transfers had unforeseen consequences, which in many cases included the extinction of less competitive, less opportunistic native species. The rosy wolfsnail, a small creature introduced onto Oahu for the purpose of controlling a larger and more obviously noxious species of snail, which was itself invasive, proved to be medicine worse than the disease; it became a fearsome predator upon native snails, of which twenty species are now gone. The Nile perch, a big predatory fish introduced into Lake Victoria in 1962 because it promised good eating, seems to have exterminated at least eighty species of smaller cichlid fishes that were native to the lake’s Mwanza Gulf.

  The problem is vastly amplified by modern shipping and air transport, which are quick and capacious enough to allow many more kinds of organism to get themselves transplanted into zones of habitat they never could have reached on their own. The brown tree snake, having hitchhiked aboard military planes from the New Guinea region near
the end of World War II, has eaten most of the native forest birds of Guam. The same virus that causes monkeypox among Congolese villagers traveled to Wisconsin by way of certain African rodents, which were imported for the exotic wildlife trade; the virus then crossed into captive American prairie dogs and from them into people who thought prairie dogs would make nifty pets. SARS rode from Hong Kong to Toronto as the respiratory distress of one airline passenger. Ebola will next appear who knows where. Apart from the frightening epidemiological possibilities, agricultural damages are the most conspicuous form of impact. One study, by the congressional Office of Technology Assessment, reports that in the United States, 4,500 nonnative species have established free-living populations, of which about 15 percent cause severe harm; looking at just seventy-nine of those species, the OTA documented $97 billion in damages. The lost value in Hawaiian snail species or cichlid diversity is harder to measure. But another report, from the UN Environmental Program, declares that almost 20 percent of the world’s endangered vertebrates suffer from pressures (competition, predation, habitat transformation) created by exotic interlopers. Michael Soulé, a biologist much respected for his work on landscape conversion and extinction, has said that invasive species may soon surpass habitat loss and fragmentation as the major cause of “ecological disintegration.” Having exterminated Guam’s avifauna, the brown tree snake has lately been spotted in Hawaii.

  Is there a larger pattern to these invasions? What do fire ants, zebra mussels, Asian gypsy moths, tamarisk trees, maleleuca trees, kudzu, Mediterranean fruit flies, boll weevils, and water hyacinths have in common with crab-eating macaques or Nile perch? Answer: They are weedy species, in the sense that animals as well as plants can be weedy. What that implies is a constellation of characteristics: They reproduce quickly, disperse widely when given a chance, tolerate a fairly broad range of habitat conditions, take hold in strange places, succeed especially well in disturbed ecosystems, and resist eradication once they’re established. They are scrappers, generalists, opportunists. They tend to thrive in human-dominated terrain because in crucial ways they resemble Homo sapiens: aggressive, versatile, prolific, and ready to travel. The city pigeon, a cosmopolitan creature derived from wild ancestry as a Eurasian rock dove (Columba livia) by way of centuries of pigeon fanciers, whose coop-bred birds occasionally went AWOL, is a weed. So are those species that, benefiting from human impacts upon landscape, have increased grossly in abundance or expanded their geographical scope without having to cross an ocean by plane or by boat—for instance, the coyote in New York, the raccoon in Montana, the whitetail deer in northern Wisconsin or western Connecticut. The brown-headed cowbird, also weedy, has enlarged its range from the eastern United States into the agricultural Midwest at the expense of migratory songbirds. In gardening usage, the word “weed” may be utterly subjective, indicating any plant you don’t happen to like, but in ecological usage it has these firmer meanings. Biologists frequently talk of weedy species, referring to animals as well as plants.