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Abstract
Any child who watches nature shows on television knows that humans are altering biological diversity at an astonishing rate. It is the depressing fashion in the science community to say that we're teetering on the brink of a mass extinction event, the sixth in Earth's history.
| The Red List of Threatened Species has over 11,000 entries. |
Few experts who have studied the situation deny that something serious is going on. The 2000 IUCN Red List of Threatened Species, the reigning authority on such things, currently shows 11,046 described species threatened with extinction and another 816 already extinct or extinct in the wild. Of the threatened species, 5,435 are animals and 5,611 are plants. The Red List also shows the sketchiness of our knowledge about what lives and dies in other taxa. The document states, "Other than mammals and birds only a small or extremely small proportion of the total number of species in any group have been assessed for threatened status." Other experts, such as David S. Woodruff of the Division of Biology at the University of California at San Diego, offer much higher figures - 3,000 to 30,000 species extinctions a year, with 10 to 20 times that many expected to become extinct during the remainder of this century [1].
Biologists know (though the nature shows rarely acknowledge) that extinction is part of living. Often a plant's or animal's being in the wrong place at the wrong time can lead to its extermination. This "background extinction," as well as the catastrophic mass deaths of species as in a meteor strike, actually promotes diversity by leaving ecological niches in which newer species can evolve. The current mass extinction event, though, is different in two very important ways: It is the first one that is the product of deliberate human activity, and it is taking place at a much faster rate than "normal" extinction.
| This mass extinction is occurring on "a catastrophically short timescale." |
Michael J. Novacek of the American Museum of Natural History and Elsa E. Cleland of the Global Ecology Field Lab at Stanford University, participating in a National Academy of Sciences colloquium on the biotic crisis and evolution, wrote that the current imperilment of biodiversity is occurring on "a catastrophically short timescale." Extinction rates that are thousands of times greater than background, they say, generate a "not unrealistic" projection of 30 percent extermination of all species by the middle of the present century [2].
The Causes
The causes of this sixth extinction are not very controversial among scientists. They include habitat destruction and fragmentation, invasive and introduced species, pollution, over-harvesting, and shifts in the environment brought on by climate change and other factors.
| Major contributors are habitat loss and invasive species. |
Habitat destruction usually heads the list of causes. Rain forests and other forests fall to agriculture, ranching, logging, and population growth. In the process, the forests' indigenous species, now without a supporting environment, may perish. Coming up fast as a major cause is the threat of invasive species (also known as exotics, nonindigenous species, and introduced species). Stuart Pimm, a conservation biologist at Columbia University's Center for Environmental Research and Conservation and a long-time student of extinction and its causes, thinks the big problem may be both habitat loss and invasives, working in tandem. "Maybe the pathway of causality is that habitat destruction makes it possible for introduced species to come in," he said in a recent interview, "and the introduced species then do further harm. There is simply no doubt that introduced species are a massive problem."
Hawaii, where Pimm has done research (along with the Everglades, another corner of biologist heaven), offers the perfect example of the connection between invasive species and extinction. In a sense, everything that lives on the volcanic mid-Pacific archipelago is an exotic. It got there by swimming, floating, flying (on its own wings or in the cargo hold of an airliner), or in the gut of a bird. Once there, many plants and animals that colonized the Hawaiian islands have flourished, at least for a while. By one count, Hawaii has been home to more than 1,000 flowering plants, 5,000 insects, and 750 land snails, none found elsewhere on Earth. But many of those species have become extinct, the victims of other species that crowded them out. An estimated 20 to 70 new invaders arrive each year.
| Hawaiians do battle with Miconia, the "purple plague." |
There was ample evidence of all this a few miles from the campus of the University of Hawaii at Hilo, on the Big Island, during last summer's annual meeting of the Society for Conservation Biology. Miconia calvescens, a lovely tree with large leaves of rich green and purple, arrived from South America in 1960 as an ornamental. Now the plant is known as "the green cancer" and "purple plague" and is listed as a state noxious weed. It has elbowed out indigenous plants, interfered with agriculture, and grown like mad. Bands of dedicated Hawaiians from the Maui Invasive Species Committee and Operation Miconia slog through dense, wet forest, attacking Miconia with machetes and pulling seedlings from the ground. It seems a losing battle, but the volunteers and government scientists are proud of the progress they've made.
But Miconia is attractive, and homeowners like it around their homes. Therein lies one of the big problems of invasive species. Many are deliberately imported for ornamental or agricultural purposes, and plants that a governmental agency considers noxious may be available by mail order from nursery catalogs. Also, we rely on many imported species for our livelihoods. Neither pineapple nor sugarcane is indigenous to Hawaii, but both have played important roles in the economy. On the other hand, destruction of forest habitat to clear land for both crops has been implicated in local extinctions.
| Hawaii has a long list of truly unwanted species. |
There's no such ambivalence with many other invaders. The islands' long list of truly unwanted species includes the brown tree snake, Bioga irregularis, which has caused bird extinctions on Guam (and invaded houses, bitten infants, and eaten pets and chickens), feral pigs and cats (both intentionally introduced), three rat species, the Indian mongoose (imported to control the rats and now out of control itself, as are the rats), 18 parrot species, bullfrogs (imported for human food), mosquitoes and dozens of other insects, and more than 70 species of snails. The islands are covered with plant invaders, many of them innocently imported by homeowners and nurseries, some deliberately introduced by government scientists to "control" other invaders.
A Mishmash of Agencies
One big problem with invasives is that, while science is building ever-greater dossiers on their role in extinction, the world lacks a clearly defined way of dealing with them. A few nations, including New Zealand and Australia, have strict rules about what enters the country, but in the United States, a mishmash of agencies dilutes authority to the point that not much gets done.
| The OTA criticized U.S. policy on exotic species. |
In 1993, the U.S. Office of Technology Assessment (OTA, which itself would be made extinct by a Republican Congress in 1995) published a detailed study that referred to the government's approach as a "largely uncoordinated patchwork of laws." Phyllis N. Windle, who was the project director for the OTA effort, was asked in a recent interview how she would amend that statement now.
"I wouldn't," she replied, noting that 21 federal agencies had, and still have, a piece of the invasive species turf. Most of them respond to particular interest groups, such as agriculture, while few seem concerned about threats to noncommercial organisms. (The U.S. Geological Survey is a a prominent exception.) Windle is now a senior scientist at the Union of Concerned Scientists, where she directs a new project on reforming invasive species policy.
| "I do try to remain hopeful." |
"I do try to remain hopeful," she said. "The number of people involved in this issue has just skyrocketed in the last 10 years. It's hard to keep up with the literature now. When we started at OTA, there was no problem. We could read everything, and we were on top of it, and now it's hard to stay on top of everything. There are a lot of enthusiastic people, especially at the state level. There are people out there actually doing the work and getting rid of these things. So I do think that there's great hope. We just have to be better at it."
Like many of her colleagues, Windle hopes the new National Invasive Species Council, formed by a presidential executive order in 1993, will live up to its mandate to "provide national leadership regarding invasive species." Last January, the council issued its "management plan, first edition," a document that talks the talk. Whether it also walks the walk depends, in great part, on whether the 21 federal agencies are willing to sacrifice turf in the name of holding off extinction.
Hot Spots
| Protecting "hot spots" may be the best conservation strategy. |
In the meantime, the issue of extinction dominates most discussions of conservation biology. A large contingent places its hopes on the idea of identifying and protecting "hot spots" of biodiversity throughout the world. Oxford scholar and independent scientist Norman Myers, working with the nongovernmental organization Conservation International, has identified 25 such places. They contain 44 percent of all vascular plant species and 35 percent of all species in the vertebrate groups of mammals, birds, reptiles, and amphibians, but they take up less than 2 percent of Earth's surface.
Many of these cauldrons of biological diversity are in the tropics, but they are definitely not pristine tropical paradises. Novacek and Cleland have pointed out that "they are already in a marked state of degradation," with primary vegetation missing in many. Little of the officially designated "hot spot" area is really protected, and even that part that does have protection is vulnerable, like the rest of nature, to such other agents of extermination as climate change, pollution, and invasive species.
| "Hot spots constitute a static view of diversity." |
Those who argue for "hot spot" protection occasionally have to fend off criticism not only from those who would like to use such areas for mining and logging, but from some of their fellows as well. Michael L. Rosenzweig of the Department of Ecology and Evolutionary Biology at the University of Arizona, who took part in the National Academy of Sciences colloquium, asked, "What is the apparent significance of hot spots of diversity? They tantalize us with the promise that we can minimize the amount of reservation needed to prevent a mass extinction." Diversity, he said, "is the outcome of dynamical processes, whereas hot spots constitute a static view of diversity." Rosenzweig advocates, instead, what he calls "reconciliation ecology," which acknowledges that people and their needs are part of the picture and "develops management techniques that allow humans to share their geographical range with wild species."
Some observers of the extinction crisis minimize its importance because, historically, there has been a "recovery" of diversity after an extinction event. New species will evolve to take the place of the dearly departed.
| It took 10 million years to regain diversity. |
Stuart Pimm has a chilling reply to that. "The argument is true," he said, "that, 'Oh, well, life will recover on Earth,' but you have to ask the follow-up question, which is, 'How long?' And we have a very precise answer for that. It comes from when the dinosaurs became extinct. How long did it take to recover the variety of life on Earth?" He spread the words out for emphasis: "Ten . . . million . . . years. So the variety of life on Earth that we're going to lose now isn't something that my kids or my grandkids are going to see recover. It's something that's not going to happen for a hundred times the time it's taken for tool-using humanity to be around. People say, 'Oh, well, other species will come along to take their place.' That may be true, but 10 million years is a hell of a long time to find out."
Fred Powledge is a freelance writer who specializes in issues of biodiversity.
Susan Wolsborn is Web designer of HMS Beagle.
Habitat Loss: Ecological, Evolutionary and Genetic Consequences - a report on a conference held in Helsinki, Finland, September 7-12, 1999. From Trends in Ecology & Evolution, 2000, 15:4:132-134. Full text available from BioMedNet.
Species Loss after Habitat Fragmentation - examines species-area relationships relative to predictions of species loss. From Trends in Ecology & Evolution, 2000, 15:10:396. Full text available from BioMedNet.
The Future of Evolution - highlights from a recent National Academy of Sciences colloquium held in Irvine, California, March 16-19, 2000. From Trends in Ecology & Evolution, 2000, 15:8:307-308. Full text available from BioMedNet.
Eradication Revisited: Dealing With Exotic Species - a review of recent eradication programs. From Trends in Ecology & Evolution, 2000, 15:8:316-320. Full text available from BioMedNet.
Mass Extinction Underway, Majority of Biologists Say - an April 21, 1998 article from the Washington Post. Includes an excellent collection of links.
The Sixth Extinction - online excerpt from the book by Richard Leakey and Roger Lewin.
The Sixth Extinction - an article from the February 29, 1999 issue of National Geographic. The issue also includes Biodiversity: Taking Stock of Life and The Variety of Life.
Confirming the Global Extinction Crisis and Species: Unprecedented Extinction Rate, and It's Increasing - two news articles from the World Conservation Union.
Hawaiian Ecosystems at Risk Project - offers general information about harmful alien species in Hawaii, as well as species-specific information including distribution maps and special reports.
Invasivespecies.gov - a gateway to information relating to non-native plant and animal species present in U.S. ecosystems. Includes a nonindigenous species database. From the National Biological Information Infrastructure.
Harmful Non-Indigenous Species in the United States - provides an excellent introduction to the impacts of harmful non-indigenous species.
Evolution Website - an extensive resource featuring the Extinction Files. From BBC Online.
Evolution and Extinction - a good collection of links on the topics.
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