摘录

Chapter One Evolution in Reverse Some 240 million years ago, well before the reign of the dinosaurs, all of the Earth's major landmasses were locked into a single continent. A monstrous plaque of rock called Pangaea sat alone amidst the waters of an even more monstrous planetary ocean. Eventually, Pangaea fragmented. At geologic pace, its shards sailed out over the blank blue immensity to create, for the human moment, the present continental configuration.     The macro structure of the planet might seem to be the one aspect of the world that people cannot change. And yet the currents of human movement are beginning to alter the ancient evolutionary function of the planetary surface. As vessels for the natural communities that evolved within them, the Earth's pockets and humps, its wet and dry places, are losing their integrity--their separateness. At a frenetic and ever-increasing pace, the global economy is merging the world's ecosystems, smearing them into each other.     We are in the throes of a vast and little-noticed biotic upheaval. As ecological entities, the continents are coming together again; the seas are spilling into each other. And this biotic turmoil is reaching levels of disturbance that no actual meeting of rock or water could possibly achieve. Modern commerce is wrapping the world's natural systems in a web of connections that is far more comprehensive than anything that could have existed on the ancient super-continent. A kind of hyper-Pangaea is emerging.     The physical roughness of the Earth--its structural variety--has tended to hold its living communities in place. The barriers that surround any particular ecosystem help set the terms of life within it. They tie a particular assemblage of plants and animals together, and they tend to exclude predators, competitors, and diseases that evolved elsewhere. Islands provide the extreme case. In their isolation, many island creatures have evolved into forms found nowhere else--the giant tortoises of the Galápagos, for example, or the colorful "picture-winged" fruit flies of Hawaii.     The planet is scored by thousands of more subtle barriers too. A "rain shadow" downwind from a mountain ridge may be too dry for forest; an ocean current may isolate two distinctive coral reefs. Even the lives of highly mobile creatures are likely to be governed by barriers of one sort or another. The salmon that hatch in the rivers of western North America may swim together in the ocean, yet each strain returns to its own river to breed, thereby preserving its distinctive genetic identity. And so from one horizon to the next, a subtle matrix of barriers has allowed the communities of life to work out evolutionary answers to a particular spot of land, a stream, or a set of ocean currents. Natural barriers are the instruments of evolution.     Today these barriers are losing their ecological reality, as more and more organisms are moved around them. A western Atlantic jellyfish, for example, is pumped out of a ship's ballast tank and into the Black Sea, where it wrecks the fisheries. Escaped garden plants strangle North American wetlands and rare island forests. Plantations of Australian eucalyptus trees displace native forests throughout the developing world--and sometimes native forest peoples as well. The farming of commercial shrimp species obliterates coastal fisheries and the local economies that depend on them. Range-devouring weeds sprout from contaminated crop seed; virus-laden mosquitoes emerge from shipping containers. In these and hundreds of other ways, the silent uproar of biotic mixing is damaging both worlds that people inhabit: the natural and the social.     Some degree of movement through the Earth's barriers has always been a part of life, of course--no natural community is hermetically sealed. A shift in the prevailing sea breeze might bring a colony of bats to an island; an increase in rainfall might allow forest out onto a prairie. But the artificial Pangaea that we are creating differs fundamentally from such natural range extensions in three respects: • In the frequency of movement. Under natural conditions, the arrival of a new organism--an "exotic species"--was in most areas a rare event. Today, it can happen any time a ship comes into port or an airplane lands. In places where the current rate of arrival has been estimated, it generally appears to be thousands of times faster than the previous natural rate. • In the pervasiveness of movement. In the past, a major ecological change sometimes allowed a mixing of one biota (the set of plants and animals native to an area) with another. One of the most dramatic episodes involved Beringia, the ancient land bridge that once linked Siberia to Alaska. Over the course of many millennia, Beringia admitted numerous Eurasian species (including people) into the New World. Today, intense biotic mixing has moved from being an occasional regional event to a chronic global phenomenon. • In the fact that "impossible migration" is now not only possible, but common. Under natural conditions, the planet's physical structure imposes formidable barriers to certain types of movement. Bounded by 6,000 kilometers of salt water, for example, or 1,000 kilometers of desert, many organisms would live out their evolutionary lives without crossing to terra incognita on the other side. Today, such crossings are routine. Water hyacinth, an aquatic weed that is suffocating East Africa's Lake Victoria, comes from South America; the disease that is killing off the crayfish in European streams comes from the crayfish that live in North American streams; melaleuca, a tree that is invading the Florida Everglades, is from northern Australia.     The effective collapse of the world's ecological barriers is a phenomenon, so far as we know, without precedent in the entire history of life. (The real Pangaea, of course, would have had plenty of very durable barriers.) During the past several centuries, and today at an ever-increasing rate, the Earth's natural communities are being disrupted by exotic species--organisms that have crossed those barriers to take up residence in ecosystems where they did not evolve. Bioinvasion, the spread of exotics, is fast becoming one of the greatest threats to the Earth's biological diversity.     As a global threat of extinction, bioinvasion may already rank just behind "habitat loss"--a much more general category that can be taken to include almost any kind of physical disruption. For certain types of organisms, exotics are clearly the principal threat: during the past century in the United States, for example, exotics have been a factor in 68 percent of fish extinctions. And increasingly, these two forms of ecological decay appear to be merging into a single syndrome. As more and more habitat is burned or bulldozed away, the remnant natural areas grow ever more vulnerable to invasion. The wilds of the new millennium are melting into degraded landscapes infested by exotic weeds, weakened by exotic pathogens, chewed over by exotic browsing mammals. (See Chapter 5.)     Although the process often ends in extinction, current extinction statistics do not come close to capturing the full dimensions of the problem. That is because exotics frequently suppress large numbers of native species without pushing them completely over the brink. Take the melaleuca, the tree that is invading the Florida Everglades. Undisturbed Florida wet prairie typically contains 60-80 native plant species, while an area covered by a melaleuca thicket usually contains only 3-4, if that. Successful invasions often cause "functional extinctions" like this. The native species may still exist, but over much of the terrain they are growing at densities too low to perform their former ecological role--for instance, as food plants for native animals.     Bioinvasion is perhaps the only category of environmental degradation that can corrode virtually every level of biological organization. On a broad landscape level, exotics like the melaleuca can replace entire communities of native plants and animals. At the other end of the spectrum, interbreeding between an exotic and a native relative can unleash a "genetic invasion" that undermines a native gene pool. In western North America, for example, mass releases of hatchery-bred salmon have undermined some wild salmon stocks. In such cases, the native in effect becomes exotic.     The cultural effects of exotics can be as profound as the biological ones. Human pathogens, for instance, travel as readily as crop pests or weeds, and entire branches of humanity have fallen away as a result. The diseases brought into the Americas by European colonists precipitated one of the greatest cultural crises in history, and the spasms of that crisis reach right into the present. In the century following the conquistadors' arrival, as many as two thirds of the western hemisphere's native inhabitants--perhaps 30 million people--may have succumbed to smallpox, malaria, and various other Old World diseases--diseases to which they had almost no resistance. To a considerable degree, the Europeans inadvertently "created" the wilderness they then went on to explore. Today, miners and settlers continue to spread these pathogens to the native peoples of the Amazon basin, with disastrous effect. Since the mid-1980s, for example, about a quarter of the Yanomami people have succumbed to exotic diseases.     The exploding volume of migration and travel is now pulling most of humanity into a single microbial system, and no society may really be prepared for the results. Epidemic cholera has recently returned to the Americas, yellow fever may be poised to invade Asia, and we have barely begun to identify the malign synergies produced by overlapping epidemics.     Nor are the social effects of bioinvasion limited to disease. Exotics ruin our crops and suppress our fisheries. They cause declines in forest and rangeland productivity. Aggressive exotic water weeds and shellfish are fouling dams, power plant intake pipes, and irrigation canals. Some exotic plants are increasing the rate and intensity of brushfires; others are dropping water tables. In these and many other ways, exotics are costing societies all over the world billions of dollars every year.     All of this damage, both natural and cultural, results from a process that is profoundly "counterintuitive." Conceptually, the problem of invasion comes down to this: why should adding a species to an area end up reducing that area's biological diversity? The exotic dandelion on your lawn, for example, is presumably just one more species in the local plant community--its only known victims are people who value uniform grass. And most exotics never even make it to dandelion status. Most do not succeed in establishing themselves in their new ranges--they just die out. Even those that do establish themselves will not necessarily have a detectable ecological effect.     But the paradox disappears once you look from the individual exotic to the process as a whole. The proportion of exotics that cause serious trouble is difficult to estimate, but a very rough rule of thumb, sometimes called the "tens rule," is that 10 percent of exotics introduced into an area will succeed in establishing breeding populations, and 10 percent of those will go on to launch a major invasion. When that happens, the exotic has graduated from dandelion to melaleuca status. It has escaped the predators, diseases, and other factors that kept it in check in its native range, and has found nothing comparable in its new range. It is facing organisms that did not evolve in its presence and that may not be adapted to competing with it or escaping from it. This scenario keeps replaying itself all over the world; the result is usually lots of exotic and a lot less of everything else.     Since the global economy is continually showering exotics over the Earth's surface, there is little consolation in the fact that 90 percent of these impacts are "duds" and only 1 percent of them really detonate. The bombardment is continual, and so are the detonations.     Many invaders seem to owe their explosive ecological power to a complex of traits known as "weediness." Weedy invaders mature quickly, multiply prolifically, spread easily, and often do especially well in disturbed conditions. Animal "weeds" tend to be highly adaptable in their diets. The ubiquitous rats and house sparrows, the zebra mussels invading North American waterways, the water hyacinth and the melaleuca--all these organisms are weeds.     As the weeds spread, displacing more and more local diversity, the world becomes a steadily more homogenized place. The same weeds begin to crowd every rangeland; water hyacinth smothers warm-climate waters the world over. Goats gnaw the shrubbery to stubble on island after island. In all of these places, as the local creatures disappear, the ecosystem they formed a part of tends to weaken. An artificially simplified community, like a machine that is missing a lot of its parts, is more likely to break down. A fire, say, or an outbreak of disease that might have had little effect in a healthy, complex community may seriously disturb a simplified, sick one. And that disturbance will lay the area open to yet more invasion. This is the cycle of degradation that is coming to characterize our era. * * * Bioinvasion is now a profound and global challenge to our economic system, to our technical conservation skills, and to our ethics--our ability to recognize a "right to existence" in other living things. Yet policy responses to the threat have generally been weak and uncoordinated. Only the worst invaders get serious attention, and even then there is rarely any systematic inquiry into the social and economic processes that launched the invasion in the first place.     To some degree, this lack of response can be explained by the enigmatic qualities of the problem itself. Despite 40 years of study, ecologists have not been able to discover natural "rules" that govern the processes of invasion and that have any real predictive value. Bioinvasion is a deeply unsatisfying policy topic. It is messy, frustrating, depressing, and unpredictable: it does not lend itself to neat solutions. Consider, very generally, what we don't know.     We don't know which organisms will become successful invaders. No common characteristic has been detected. It is true that many of the worst invaders are highly adaptable "generalists"--weeds, in other words. But there are "specialist" invaders too. Some invaders have huge home ranges; some have very small ones. Some have close relatives that are also dangerous, while the closest relatives of others do not seem to be invasive at all. And some very aggressive invaders may actually be retreating in their home ranges. The melaleuca, one of South Florida's nastiest pest plants, is being crowded in its native northern Australia by catclaw mimosa, a spiny South American shrub--and by pond-apple, a native of the Florida Everglades.     We don't know where invasions will occur. True, disturbed ecosystems are generally more vulnerable to exotics than intact ones. One of the reasons that Eurasian cheat grass now dominates 25 million hectares of western North America is almost certainly that ranchers allowed their cattle to overgraze the native grasses. But as with the "weediness" rule, there are all kinds of exceptions. In surviving tracts of undisturbed Hawaiian rainforest, for example, the dominant insects are now frequently exotic. In the Great Lakes, water quality improvements have probably helped the sea lamprey, a predatory exotic fish, since lamprey larvae are fairly sensitive to pollution.     We don't know when an invasion will occur. Many exotics probably find their way into a new range several times before they succeed in establishing themselves. Even then, an exotic may spend decades as an innocuous good citizen in its new home before some subtle adaptation or some shift in the ecological dynamic triggers an explosive invasion. This "incubation period" is so common in plant invasions that it has a kind of reverse predictive value: there are almost certainly many more exotic plants out there than anyone has noticed. According to one expert on weed invasions in the United States, exotic weeds usually have to be in the country for 30 years or to have spread to more than 4,000 hectares before they are even discovered. (See Figure 1-1 for a graph of a typical plant invasion.)     We don't know what an invasion will do. Because invasive exotics can do a great deal more than simply displace native species, they have a considerable capacity for surprise. Take, for example, the case of the tiny opposum shrimp in Montana's Flathead River system. Wildlife officials introduced the shrimp around 1970 to increase the forage base for the kokanee salmon, another introduced species. But salmon tend to feed near the surface and the shrimp only rose to the surface at night, when the salmon could not see them. So the salmon could not eat the shrimp, but the shrimp ate all the plankton that the salmon fry depended on. The salmon population crashed, then the bears, birds of prey, and other creatures that had come to depend on the salmon disappeared. A tiny shrimp had starved eagles out of the sky.     Exotics, by and large, seem to make up their own rules. We invite them to play but they name the game. What will this creature do if it lands in that spot? About all we can say with assurance is this: if it's causing trouble somewhere, then you don't want it anywhere else. Bioinvasion may be the least predictable of all the major forms of environmental disruption.     It may also be the hardest to fix. In general, the key to dealing with environmental problems--habitat destruction, say, or pollution--is to stop the offending activity. Granted, that is usually no mean feat. But if it can be achieved, natural processes, with thoughtful management, will then heal the system. Time, however, does not heal invasions. An intense invasion may "peak out" and subside after exhausting most of the local resources, but that does not mean the exotic will go away. It may rebound when its food supply recovers, or it may spread elsewhere. So while an oil spill that occurred 20 years ago is probably not a pressing concern today, there are hundreds of invasions that began more than a century ago and that are desperately urgent problems right now. This "biological pollution" is smart pollution. It adapts, it looks for ways to survive, and instead of diminishing over time, it usually entrenches itself.     Beyond the tortured topics of invasion ecology, there is another reason why exotics have attracted so little in the way of policy response: they are so common, such a standard part of our environment, that their presence is not usually suggestive of dysfunction. The tendency to spread exotics is a deeply embedded and nearly universal aspect of culture. For thousands of years, people all over the world have deployed exotics for both necessity and pleasure--to feed themselves, to mold the landscapes in which they live, to stock their gardens, rivers, and forests. Accidental releases seem to be a cultural constant as well. Humanity has always been a wandering species and we have offered the globe to those creatures that profit from our presence--our diseases and parasites, our crop pests, our mosquitoes and fleas.     Nearly everywhere, invasion has been, in varying degrees and in different ways, a standard feature of the human past. But as with other forms of environmental degradation, there is a big difference between regional or low-level pressure and what happens when the process gains intensity and goes global. Biotic mixing on a global level began in earnest five centuries ago, as the Age of Discovery dawned. It is reaching its logical extreme today, in the emergence of a global economy. At its current level, it is no more sustainable than are current levels of deforestation or atmospheric carbon emissions. Bioinvasion has become another way of measuring the unsustainability of the contemporary economic order.     At its current rate, bioinvasion may not be "culturally sustainable" either. The march of the weeds is robbing the landscapes in which we live of their "naturalness"--of their power to reflect something other than our own mismanagement. Invasion threatens us with a kind of culturally deadening solipsism, in which it becomes harder and harder to experience nature as distinct from ourselves. Humanity is not meant to be a patient in a sickroom, with nothing to contemplate but our own diseases. We came into being out-of-doors; our social and psychological welfare may be linked in ways we cannot fathom to the welfare of nature as a whole. We may need the "otherness" of nature just as much as we need clean water and air.     But human memory is so short compared with the scale on which nature operates. Who can remember where each weed originally came from? And it is tempting to speed the process of forgetting, by rechristening any entrenched exotic as a native. Capitulating to the invader may often seem the most realistic course of action. No doubt, nature eventually will transform many of these organisms. The scattered populations of many exotics may ultimately go their separate evolutionary ways and become distinct species, each within its own native range. (Some plant diseases may already be doing this, but that is not good news: each new version of such a pathogen is another potential invader.)     For most types of organisms, however, the process will not occur on a time scale that can matter much to those of us alive today. In the meantime, blurring the distinction between native and exotic is a tactical mistake because it invites people to view every invader as just a native in the making. Given the current levels of biotic mixing, that is a little like dismissing AIDS with the notion that one day that virus too may evolve into something more benign.     Besides, capitulation is not necessary. Despite the formidable ecological and social difficulties of countering invasions, we already have the tools necessary for rapid progress. Our principal challenge now is not so much technical as cultural. Our contemporary global reach is no longer compatible with an invasion mentality--an attitude toward nature that accepts invasion as inevitable or even desirable. The key to discarding that mentality is a kind of historical consciousness, an awareness of how the social machinery of invasion was built in the first place. Our current problems have been a long time in the making. To deal with them effectively, we will need to understand not just present ecological conditions, but the history of our "invasion cultures."     Contemporary ecology and the cultural past: all the following chapters deal with these two topics, each from a different perspective. Part I is concerned primarily with invasion as an ecological process. Chapters 2, 3, and 4 survey three very broad ecosystem categories, along with the primary human enterprise in each--prairies and agriculture, forests and forestry, the waters and fisheries. Chapter 5, "Islands," explores the type of landscape that has suffered the most from invasion, and presents it as a model for the problem as a whole. Part II looks at invasion primarily as a cultural process. It begins with two short cultural histories of invasions, first the intentional ones (chapter 6), and then the accidents (chapter 7). Chapter 8 sketches out the economic implications of the current situation and considers the global economy itself as a homogenizing force. In Part III, the book's final chapter reviews the resources at our disposal--legal, political, ecological, and personal--for treating the planet's invasion disease. Copyright © 1998 Worldwatch Institute. All rights reserved.