12-1 Species Extinction

What Are Three Types of Species Extinction? Local, Ecological, and Biological

Biologists distinguish among three levels of species extinction:

  1. Local extinction: It occurs when a species is no longer found in an area it once inhabited but is still found elsewhere in the world. Most local extinctions involve losses of one or more populations of a species.

  2. Ecological extinction: It occurs when so few members of a species are left that it can no longer play its ecological roles in the biological communities where it is found.

  3. Biological extinction: When a species is no longer found anywhere on the Earth. Biological extinction is forever.

What Are Endangered and Threatened Species? Ecological Smoke Alarm

Biologists classify species heading toward biological extinction as either endangered or threatened.

  • An endangered species has so few individual survivors that the species could soon become extinct over all or most of its natural range.

  • A threatened species is still abundant in its natural range but, because of declining numbers, is likely to become endangered in the near future.

As biodiversity expert Edward O. Wilson puts it, “the first animal species to go are the big, the slow, the tasty, and those with valuable parts such as tusks and skins.”

How Do Biologists Estimate Extinction Rates? Peering into a Cloudy Looking Glass

Biologists trying to catalogue extinctions have three problems:

  1. The extinction of a species typically takes such a long time that it is not easy to document.

  2. We have identified only about 1.4–1.8 million of the world’s estimated 5–100 million species.

  3. We know little about most of the species we have identified.

One approach is to study past records documenting the rate at which mammals and birds have become extinct since we came on the scene, and compare this with the fossil records of such extinctions prior to our arrival.

Another way that biologists project future extinction rates is to observe how the number of species present increases with the size of an area. This species–area relationship suggests that on average, a 90% loss of habitat causes the extinction of about 50% of the species living in that habitat.

How Are Human Activities Affecting Extinction Rates? Taking Out More Species

In due time all species become extinct, but there is considerable evidence that we are hastening the final exit for a growing number of species. Using the methods just described, biologists conservatively estimate that the current rate of extinction is at least 1 000 to 10 000 times the rate before we arrived.

Most biologists consider extinction rates of 0.1–1% to be conservative estimates for several reasons:

  1. Both the rate of species loss and the extent of biodiversity loss are likely to increase during the next 50–100 years because of the projected exponential growth of the world’s human population and per capita resource use. In other words, the size of our already large ecological footprint is likely to increase.

  2. Current and projected extinction rates are much higher than the global average in parts of the world that are endangered centres of biodiversity.

  3. We are eliminating, degrading, and simplifying many biologically diverse environments—such as tropical forests, tropical coral reefs, wetlands, and estuaries—that serve as potential colonization sites for the emergence of new species. In other words, we are also creating a speciation crisis.