Is the biosphere today on the verge of anything like the mass extinctions of the geological past? Could some equivalent of meteorite impacts or dramatic climate change be underway, as humankind's rapid destruction of natural habitats forces animals and plants out of existence?
Increasingly, researchers are doing the numbers, and saying, yes, if present trends continue, a mass extinction is very likely underway. The evidence is pieced together from details drawn from all over the world, but it adds up to a disturbing picture. This time, unlike the past, it's not a chance asteroid collision, nor a chain of climatic circumstances alone that's at fault. Instead, it is chiefly the activities of an ever-growing human population, in concert with long-term environmental change.
The background level of extinction known from the fossil record is about one species per million species per year, or between 10 and 100 species per year (counting all organisms such as insects, bacteria, and fungi, not just the large vertebrates we are most familiar with). In contrast, estimates based on the rate at which the area of tropical forests is being reduced, and their large numbers of specialized species, are that we may now be losing 27,000 species per year to extinction from those habitats alone.
The typical rate of extinction differs for different groups of organisms. Mammals, for instance, have an average species "lifespan" from origination to extinction of about 1 million years, although some species persist for as long as 10 million years. There are about 5,000 known mammalian species alive at present. Given the average species lifespan for mammals, the background extinction rate for this group would be approximately one species lost every 200 years. Of course, this is an average rate -- the actual pattern of mammalian extinctions is likely to be somewhat uneven. Some centuries might see more than one mammalian extinction, and conversely, sometimes several centuries might pass without the loss of any mammal species. Yet the past 400 years have seen 89 mammalian extinctions, almost 45 times the predicted rate, and another 169 mammal species are listed as critically endangered.
Therein lies the concern biologists have for many of today's species. While the number of actual documented extinctions may not seem that high, they know that many more species are "living dead" -- populations so critically small that they have little hope of survival. Other species are among the living dead because of their interrelationships -- for example, the loss of a pollinator can doom the plant it pollinates, and a prey species can take its predator with it into extinction. By some estimates, as much as 30 percent of the world's animals and plants could be on a path to extinction within 100 years. These losses are likely to be unevenly distributed, as some geographic areas and some groups of organisms are more vulnerable to extinction than others. Tropical rainforest species are at especially high risk, as are top carnivores, species with small geographic ranges, and marine reef species.
Humanity's main impact on the extinction rate is landscape modification, an impact greatly increased by the burgeoning human population. Now standing at 5.7 billion and growing at a rate of 1.6 percent per year, the population of the world will double in 43 years if growth continues at this pace. By draining wetlands, plowing prairies, logging forests, paving, and building, we are altering the landscape on an unprecedented scale. Some organisms do well under the conditions we've created: They tend to cope well with change, tolerate a broad range of habitats, disperse widely, and reproduce rapidly, and they can quickly crowd out more specialized local species. City pigeons, zebra mussels, rats, and kudzu and tamarisk trees -- these are examples of what biologists call "weedy" species, both animals and plants. Many weedy species will probably survive, and even thrive, in the face of the current mass extinction. But thousands of others, many never known to science, are likely to perish.
And what is the fate of our own species likely to be, if we really are in the midst of a sixth mass extinction? One possibility is that as diversity and abundance wither, the species causing it all -- Homo sapiens, the most dominant species in history -- could also be on the road to oblivion. But another possibility is that Homo sapiens, which has proved to be a very effective weedy species itself, will persist. That's the view of paleobiologist David Jablonski, who sees us as one of the survivors, "sort of picking through the rubble" of a world that has lost much of its biodiversity -- and much of its comfort. For along with that species richness, the ecosystem is likely to loose much of its ability to provide many of the valuable services that we take for granted, from cleaning and recirculating air and water, to pollinating crops and providing a source for new pharmaceuticals. And while the fossil record tells us that biodiversity has always recovered, it also tells us that the recovery will be unbearably slow in human terms -- 5 to 10 million years after the mass extinctions of the past. That's more than 200,000 generations of humankind before levels of biodiversity comparable to those we inherited might be restored.