Unit 9: Biodiversity Decline // Section 6: A Sixth Mass Extinction?
Species have appeared and disappeared continually throughout Earth's history, with extinctions occurring on average at a rate of 0.1 to 1 species per million species-years. At several points, however, this rate has risen sharply, producing five "mass extinction" events (Fig. 6). The most famous of these events, the Cretaceous-Tertiary (K-T) extinction 65 million years ago, is thought to have been caused at least in part by a giant asteroid that struck Earth off the coast of Mexico, causing tidal waves and climate-altering dust clouds. Dinosaurs, along with two-thirds of the other species on Earth, were killed off by the K-T extinction.
Figure 6. Earth's five mass extinctions
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Source: © University of California Museum of Paleontology's Understanding Evolution/evolution.berkeley.edu.
Several lines of evidence suggest that Earth is experiencing a sixth mass extinction today. Estimates of extinction rates are imprecise for many reasons. In particular, they are extrapolated from a few well-known groups with relatively few species, such as birds and large mammals, to other groups for which there is little data—for example, fungi. However, there is wide agreement that current rates of extinction are at least several hundred times greater than historical background levels. Most scientific studies estimate that in the near term, extinction rates could rise by three to four orders of magnitude above past averages (footnote 15).
In addition to tracking extinction rates, scientists can look at population decline and habitat loss trends to estimate how quickly Earth's biodiversity levels are changing. To date, about 50 percent of the planet's natural habitats have been cleared for human use, and another 0.5 to 1.5 percent of nature is lost each year. Ongoing and current mass extinctions have been documented for many groups of organisms, including marine and freshwater fish, amphibians, and European farmland birds and macrofungi.
The current mass extinction is different from past events in several ways. First, it is happening much more quickly: each of the "Big Five" played out over thousands of years, but the current mass extinction is likely to be concentrated within 200 years. By the end of the 21st century, we may have lost two-thirds of the species on Earth (footnote 16). Second, past mass extinctions are thought to have been caused by natural phenomena such as the shifting of continents, comet or meteoroid impacts, or climate change independent of human influence, or some combination of these factors. In contrast, as we will see below, humans are causing the current mass extinction.
Despite concerns about a sixth mass extinction, new species are identified each year. For example, on a joint expedition to China and Nepal in early 2006, scientists from Conservation International and Disney's Animal Kingdom found new species that included a wingless grasshopper, a subspecies of vole, up to three new species of frogs, eight new species of insects, and ten new species of ants. These discoveries are evidence that we still know very little about Earth's biodiversity. In addition, new techniques for analyzing organisms' molecular structures have led scientists to reclassify some groups once viewed as single species into multiple species.
Sometimes it can be hard to determine the exact status of a rare species. In 2004, scientists from Cornell University and other institutions reported that they had seen and videotaped an ivory-billed woodpecker in Arkansas (Fig. 7). Ivory-bills had been presumed extinct since the 1930s, so this sighting caused great excitement but also spurred debate over whether the bird that was caught for a few seconds on film was in fact a more common type of woodpecker. Other researchers subsequently reported more than a dozen sightings and sound recordings of ivory-billed woodpeckers in Florida, but debate about whether ivory-bills still exist was ongoing as of late 2006. As the ivory-bill controversy shows, there is no definitive standard of proof for existence of a rare species, save perhaps a conclusive DNA sample—which may be impossible to get.
Figure 7. Watercolor painting of Ivory-billed Woodpeckers by John James Audubon
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Source: Courtesy National Audubon Society, Inc., 700 Broadway, New York, NY 10003, USA.