Life on the Edge.

Will a mass extinction usher in a world of weeds and pests?

About 250 million years ago, life went into shock. Set in motion by forces of geology and climate, a great wave of extinction wiped out well over 70 percent of all species on Earth.

The lifeforms that repopulated the planet were tough. Hardy filter feeding species carpeted the marine realm. Fungi and shrubs covered the land. An army of mammal-like reptiles dominated vertebrate life. The bleak landscape, what some paleontologists call a dead zone, persisted for well over 3 million years.

The biological cataclysm that ended the Permian geological period is the greatest of five mass extinctions recognized by paleontologists (SN: 2/1/97, p. 74). Each extinction eliminated well over 50 percent of the species existent at the time, and after each event except one, life's diversity lay dormant for as long as 9 million years.

Once an esoteric pursuit, studies of mass extinction have recently been infused with new relevance. That's because the pessimistic viewpoint of a handful of biologists in the early 1980s has entered the mainstream. The world is heading toward a sixth great mass extinction, many scientists now say.

The exact cause of most past mass extinctions, including the end-Permian, is still up for debate. Likely culprits include bizarre oceanic chemistry, volcanic eruptions, climate shifts, and asteroids slamming into Earth. If life on Earth teeters over the brink again, scientists argue, it will have but one cause: human activity. People tear up habitat, destabilize ecosystems, and make way for invasive species. These activities by some estimates already result in the extinction of three species a day.

Each month, major biology journals come up with a new evaluation of what they call the "biodiversity crisis." Studies evaluate recent extinctions' breadth, impact, and relevance to people. Often they include new proposals for preventing further species destruction.

Many ecologists and paleontologists predict that extinctions could reach particularly ruinous proportions within as few as hundreds or thousands of years from now. "What we do or don't do in the next few decades will have influence for the next millions of years," says Norman Myers of the University of Oxford in England, who assembled a symposium in Irvine, Calif., on the topic last year.

There's a yet longer-term question: If a mass extinction happens, what will the world look like in thousands or millions of years? Far-future scenarios emerge from an interdisciplinary mix of paleontologists, ecologists, conservation scientists, and evolutionary biologists.

Although scientists admit that they're basing their speculations on uncertain data, most of the scientists agree that a mass extinction will result in a bleak world. Scarce in large mammals, it will be overrun with species that multiply in the face of adversity--mostly organisms that we consider weeds and pests. It's the grasses, insects, and rodents that will endure. If people are still around, then it's possible that a full-scale recovery will never occur.

Predictions about the distant future are hampered by ecology's imprecise knowledge of the present, paleontology's shadowy assessment of the past, and uncertainty about developments in the next few centuries. No one even knows how many plants, animals, and other living things exist currently. About 1.75 million species have been documented, but estimates of the total range from 5 million to 100 million.

Scientists also disagree on the rate of current extinctions. Some projections rely on the Red List, a tally of threatened species produced by an international consortium of academic, government, and environmental groups. The Red List classifies a species as threatened if, according to selected criteria, it has a high probability of extinction in the next century.

If species listed as threatened indeed go extinct by 2100, the world will see a great loss of biodiversity. For instance, Georgina Mace of the Zoological Society of London and her colleagues concluded from Red List data that 15 to 20 percent of today's species of birds, reptiles, fish, and amphibians could be eliminated.

Documented cases of extinction are fewer. Since 1500, 318 vertebrates, or about 0.8 percent of the total described, have disappeared, according to the Red List. Among plants, 380, or 0.1 percent of the estimated total, have officially gone extinct.

"That may not seem like a lot," admits paleontologist John Alroy of the University of California, Santa Barbara, but he notes that undocumented extinctions are undoubtedly much higher. He adds, "There is no reason to think that these rates will let up very soon."

Ecologists often estimate extinction rates by crunching the numbers for the density of species in a habitat, the size of the habitat, and its rate of loss. These estimates raise concern. For example, tropical rainforests, dubbed "evolutionary powerhouses" by paleontologist David Jablonski of the University of Chicago contain about 70 percent of the world's species. A variety of estimates put tropical-rainforest destruction at 0.5 to 2 percent per year.

Bjorn Lomborg, a statistician at Arhus University in Denmark, says that many evaluations of tropical rainforest devastation are overblown. Although he accepts a United Nations estimate that the world loses 0.5 percent of such land per year, he anticipates that in the future, the destruction will ebb. Standards of living will increase substantially in tropical countries, according to predictions of the U.N. Intergovernmental Panel on Climate Change. The result will be greater conservation awareness, argues Lomborg.

In a 1995 analysis, Stuart Pimm, now at Columbia University, and his colleagues calculated the current extinction rate using estimates of habitat destruction rate. They reported that the current extinction rate is 100 to 1,000 times the "natural background rates" ascertained from paleontologists' analyses of long dramafree periods in the fossil record.

However, paleontologist David M. Raup of the University of Chicago challenges the usefulness of balancing the current extinction rate against one based on the fossil record. Says Raup, "There are very serious problems with that comparison." He argues that it's likely that Pimm vastly underestimated the rates of extinction in the fossil record and thus is unduly pessimistic about the future.

Human activity in the coming decades and centuries may determine the extent of diversity loss. Some developments are likely to ramp up the extinction rates, while others may dampen them.

Only about 5 percent of the planet's surface lies in protected reserves, notes Pimm. If human activity destroys or greatly modifies 90 to 95 percent of the land outside reserves, only half the planet's species will survive, he predicts. That level of habitat destruction may occur as soon as 2100, he says.

Many of the scientists who say that Earth is facing a new mass extinction also hold that it's not inevitable. As Jablonski puts it, "We are trying to turn the Titanic."

Many scientists, including Jablonski, venture that conservation decisions would optimally take into account projected shifts in the world's climate. For example, preserves on the sides of mountains may enable plants and animals to move to higher altitudes as valleys heat up.

Pimm notes also that effective ecological choices must recognize "what is economically practical, politically feasible, locally acceptable."

While scientists may agree that current analyses of worldwide extinctions are tenuous, most hold that the overall trend is clear. "Are we causing a mass extinction?" asks Alroy. "The answer is yes."

Indeed, a 1998 survey commissioned by New York's American Museum of Natural History found that among 400 biologists, 70 percent agreed that animal and plant life is in "the midst of a mass extinction."

If a mass extinction does occur, Earth may be in for a bleak period that lasts for a very long time. Of the prolonged periods of low biological diversity that followed four of the five documented mass extinctions, the shortest was the 200,000-year period after the end-Cretaceous dinosaur death blow, and the longest was the 3-million-to-9-million-year span after the end-Permian extinction.

Close interactions among species may underlie these bleak intervals. Examples of tight relationships abound in the ecological literature. For example, in the 1700s and 1800s, fur traders purged the Pacific Ocean of many of its sea otters. That resulted in a ripple effect far beyond the livelihood of the hunters. The mainstay of the sea otter's diet, the sea urchin, experienced a population boom. As a result, the urchins' grazing of seabeds caused declines in kelp forests, which serve as a primary breeding ground for fish and many other marine creatures.

The importance of close interactions shows up in mathematical models of events leading up to a mass extinction. Douglas H. Erwin at the Smithsonian Institution in Washington, D.C., and his colleagues will publish one such model in an upcoming PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY OF LONDON. It shows extinction kicking into overdrive when interdependent relationships begin to unravel. "You get to a,point where you wipe out a certain number of species and it crystallizes," he says. "A whole lot of species disappear."

The same model shows that the time to rebuild such interactions explains, in part, the long interval of low biodiversity after a mass extinction.

Species that survive a mass extinction must have a wide geographic range and the capability to adapt to shifts in their environment, says Erwin. If the events that precipitate an extinction persist--a long-lasting climate shift, for instance--the species that survive the initial impact are culled even further, leaving only what ecologists call generalists, species that can take advantage of a broad range of conditions.

Erwin has investigated what types of creatures persisted after the past mass extinctions. After the end-Permian, for example, one hardy, 4-meter-tall animal dominated the vertebrates. This single genus of synapsid, a mammal-like reptile, populated much of Earth for 5 million years.

Catherine Badgley, a paleontologist at the University of Michigan in Ann Arbor, offers up one scenario for the aftermath of a mass extinction. The only creatures that survive will be those that successfully weather fragmented forests, rips in food webs and ecosystems, and climate shifts, she predicts. Those survivors will be, she says, "mostly the kinds of organisms we call pests."

In line with previous extinctions, the largest animals won't make it. Says Jablonski, "It will be a good time to be a cockroach."

Such forecasts have their roots not only in paleontology, but in observations of the present. An analysis led by Julie L. Lockwood of the University of California, Santa Cruz has evaluated studies of diversity after local ecosystem disturbances such as deforestation and urbanization. In each instance, altering habitat resulted in a reduction in diversity.

Large birds and mammals are already taking the brunt of current extinctions. A recent analysis by Badgley based on the Red List concluded that mammals over 200 kg are most vulnerable. About 80 percent of these species are categorized as threatened or have recently gone extinct. Threatened mammals include gorillas, tigers, manatees, and some species of rhinoceros. Badgley found similar statistics for birds. The larger the bird, the greater the chance that it's on the verge of extinction.

After past mass extinctions and their low-diversity aftermaths, evolution burst into action (see box "On the Rebound"). Over millions of years, new species filled the gaps left by the old. But, say many ecologists, such recovery is not inevitable.

Amidst all the uncertainty, there's at least one thing that scientists agree on. If the human species survives and continues to put pressure on the environment, life has a slim chance for a full rebound. Says Michael L. Rosenzweig of the University of Arizona in Tucson, "There will be no period of rediversification because all the elements are in place for a permanent reduction to biodiversity."

Jablonski notes, "Attempts to predict evolutionary behavior after mass extinction events can operate in broad generalities, and always with the caveat, expect the unexpected."

RELATED ARTICLE: On the Rebound

Extinction creates opportunities. The fossil record shows that after a prolonged period, new life explodes into the ecological vacuum left after a mass extinction. Evolution is almost whimsical in the life forms it ratchets out from the survivors.

"Mass extinctions remove successful incumbents," says paleontologist David Jablonski of the University of Chicago. The last mass extinction, about 65 million years ago, is famous for decimating the dinosaurs. Their death blow, an asteroid that slammed into Mexico's Yucatan Peninsula, paved the way for new incumbents.

At first, those were mostly large birds--many of which towered above today's ostrich by several feet. But they ruled for only about 5 million years--until mammals burst upon the scene. One theory has it, says Douglas Erwin at the Smithsonian Institution in Washington, D.C., that the big birds bit the dust because the mammals got clever and large enough to crack open the gallon-size eggs.

Small mammals that survived the dinosaur's demise formed the template for a gigantic wave of mammalian evolution. In the age of the dinosaurs, mammalian fossils show enlarged eye sockets, adapted to night vision, says mammalian paleontologist Catherine Badgley of the University of Michigan in Ann Arbor. These skulking night creatures, none larger than an opossum, gave rise to modern mammals, whether human, hippo, or house rat. These include novel and bizarre forms, some of which went extinct about 11,000 years ago--wombats the size of hippos, 200-pound beavers, sloths that stood 9 feet tall on their hind legs, and other plus-size versions of some of today's animals, she says.

If extinction wipes out large chunks of life again, what will form the templates for the next outburst of diversity? Of the mammals, Badgley suspects that the survivors will consist of "lots of different kinds of rodents, bats, shrews, and other insectivores." Will these beasts sprout new innovations, morph into giants, and recreate another great mammalian age? Badgley only answers, "Any of those groups of organisms could evolve a very different form."

Although there's much uncertainty in paleontologists' estimates of how long it takes for the planet to rediversify after a mass extinction, the period would be vast in terms of the time-scales that people normally consider. After the era of lowest biodiversity and its postextinction frenzy, the rates at which new species emerge remain high for 5 to 10 million years, estimates Erwin.

Will people witness such events? Erwin hazards that humans may just begin to see it happen. Erwin puts the species' collective life-span at 5 to 10 million years--in the range of most other animal species--and people have been around between 100,000 and 2 million years already.

Some scientists predict a sooner demise, but others see people as longer-term contenders. As Badgley puts it, humans are a "weedy species." People thrive in many different habitats, consume a variety of different resources, and have reasonably sustained populations under a variety of conditions.

"I can't imagine anything short of the winding down of the sun that would cause people to go extinct," says Badgley.

The most adaptable species on Earth is likely to stick around for the next 100 million years, says ecologist Michael L. Rosenzweig of the University of Arizona in Tucson, "and that species is us."

And the rodents? Rodents will thrive right along with people. Paleontologist John Alroy of the University of California, Santa Barbara says, "If you are standing in Hawaii in thousands of years, you may see 10 species of the black rat." --C.S.