16.6: We are in the midst of a mass extinction.

Sometimes you can be so close to something that it’s difficult to really see it. That may be the situation that humans are in right now when it comes to the global loss of biodiversity. But we are becoming increasingly aware that we are in the midst of a mass extinction. In a recent survey by the American Museum of Natural History, 70% of biologists indicated that they believed this is true and that steps must be taken by governments and individuals to stop this massive loss of species.

Data on current rates of extinction in every well-studied group of plants and animals support the hypothesis that a mass extinction is under way. Among the mammals, more than 10% of all species are currently endangered, under imminent threat of extinction, and approximately 21% are endangered or threatened, characterized as vulnerable to extinction. Among birds, 4% of all species are endangered and approximately 13% are endangered or threatened. Close to 50% are in decline. Almost a third of all amphibian species are endangered or threatened. Among fishes, mollusks, insects, fungi, and plants, too few species have been evaluated to make an accurate estimate of the proportion of all species that are endangered or threatened. But for the species that have been evaluated, the situation is similar.

Historically, evidence from the fossil record suggests that background extinction rates are about one extinction per million species per year. Currently, as the numbers above suggest, extinction rates may be 1,000 times (or more) greater than this. It is difficult to know the exact magnitude of the problem because, as we saw in Chapter 10, biologists don’t have much of an idea about how many species there are on earth—the estimates range from 5 million to 100 million.

Unlike the last mass extinction event, in which the dinosaurs and most other species were wiped out in the wake of an asteroid’s collision with earth, this current mass extinction seems to be the result of the activity of one species—humans. Ironically, it is the unprecedented success of humans that is responsible for this situation. The resource needs of so many people have inevitably interfered with the ability of other species to coexist with us.

The chief reason for the loss and impending loss of so many species is habitat loss and habitat degradation. Particularly harmful is habitat loss in earth’s tropical rain forests, where biodiversity is greatest. In the past 25 years, half of the world’s tropical rain forests have been destroyed, usually by burning to make way for agricultural use of the land or by logging (FIGURE 16-12). Urban development, too, is responsible for destruction of rain forests, as the growing human populations continue to expand. Intensive agriculture, livestock grazing, and the development of urban centers have led to the destruction and fragmentation of habitats worldwide.

The loss of biodiversity-supporting habitat is not restricted to the tropics. In the Pacific Northwest, as well, logging of forests has occurred at an unsustainable rate, leading to significant reductions in a diverse range of populations. Beyond habitat loss, another factor that reduces biodiversity is overexploitation of species. Such overexploitation includes the killing of animals for food, pelts, tusks or other body parts, and medicinal products. Bonobos (Pan paniscus), for example, a type of chimpanzee and our closest living relatives, are almost never seen in the wild. As a consequence of habitat loss and their exploitation as a source of food in parts of Central Africa, they have been driven to near extinction (FIGURE 16-13).

Additionally, the introduction of exotic species into habitats where they would not naturally be found is also having a significantly adverse effect on biodiversity. We explore this in Section 16-8. The consequences of the loss of biodiversity, beyond the values of biodiversity discussed earlier, are largely unknown, but most likely include a serious reduction in the capacity of the environment to provide clean air and water and to recover from environmental and human-induced disasters.

Later in this chapter, we discuss the strategies by which the current high rate of biodiversity loss can be reduced.