16.5–16.6: Extinction reduces biodiversity

The Bengal tiger has been hunted, captured, and poisoned to near extinction.

Imagine that you are on a deserted island. Alone. It would be hard and it would be lonely. The hope of rescue someday, however, would probably make things bearable. “Martha,” a passenger pigeon living in the Cincinnati zoo in the early 1900s, found herself in a situation of this sort—minus the hope. Passenger pigeons in the wild had been completely wiped out by a combination of factors, including hunting of the birds for meat, loss of their habitat due to deforestation in North America, and possibly disease. And so, except for the small flock that Martha was part of, the species was gone. One by one, those birds died as well, and when the second-to-last passenger pigeon died in 1912, Martha was all alone. She lived for two more years, with no possibility for ever reproducing, and died on September 1, 1914 (FIGURE 16-9).

Figure 16.9: The only way to see one today. Shown here is a stuffed passenger pigeon. Inset: the aftermath of a pigeon-hunting expedition; in a single day, as many as 50,000 birds could be killed.

On average, species persist for about 10 million years, but there is huge variation. Some may last for hundreds of millions of years, but for many others the number of years is much lower than the average. An extinction occurs when all individuals in a species have died. And whether you look at biodiversity from a utilitarian, aesthetic, or symbolic perspective, extinction is a tragic loss.

As we discussed in Chapter 10, extinctions can be divided into two general categories: mass extinctions and background extinctions (FIGURE 16-10), which are distinguished by the numbers of species affected. A mass extinction occurs when a large proportion of species on earth are lost in a short period of time (typically less than two million years, and in some cases much less)—such as when an asteroid struck earth 65 million years ago and about 75% of all species were wiped out. These extinctions are above and beyond the normal rate of extinctions that occur in any given period of time, referred to as the background extinction rate.

Figure 16.10: Mass and background extinctions.

Both background and mass extinctions result in the same outcome for the species involved, but the causes tend to differ. In catastrophic mass extinction events, such as an asteroid impact, for example, the particular features of a species’ biology—its biochemistry, physiology, behavior—don’t necessarily play a role, in which case it’s more like really bad luck. Background extinctions, by contrast, tend to be a consequence of one or more aspects of a species’ biology.

For any species, there is always a risk of extinction. But this risk can be larger or smaller, depending on certain features. Here we look at three important aspects of a species’ biology that can influence its extinction risk (FIGURE 16-11).

Figure 16.11: Range, population size, and habitat tolerance influence risk of extinction.

1. Geographic range: extensive versus restricted. Species restricted in their range—including those limited to small bodies of water and those confined to islands—are more vulnerable than species with extensive ranges. The Tasmanian devil is a marsupial carnivore about the size of a dog. Although these animals once thrived in Australia, they now are confined to the island of Tasmania, smaller than the state of Maine. Unable to expand their range, Tasmanian devils are more vulnerable to extinction than if their range was not limited. The house mouse, on the other hand, is found nearly everywhere in the world and so has a much lower risk of extinction.

2. Local population size: large versus small. Tigers and California condors, along with Welwitschia, a slow-growing, long-lived plant in southwest Africa, are examples of species that—as a consequence of their small population sizes—are at increased risk of extinction. With a small population size, a species is more susceptible to extinction due to the variety of factors that can kill individuals, including fire, diseases, habitat destruction, and predation. Put simply, the more individuals that are alive, the more likely it is that some of them will survive such events. In addition, having more individuals to breed with typically leads to greater genetic variation. When a population is small, inbreeding is increased, which generally reduces the fertility and longevity of individuals.

3. Habitat tolerance: broad versus narrow. “Habitat tolerance” describes the breadth of habitats in which a species can survive. Some plant species, for example, can tolerate large swings in water availability or soil pH or temperature. Others are limited to very narrow habitat ranges. The now-extinct passenger pigeons could only build nests in a specific type of forest and needed large numbers of individuals, breeding communally, in order to breed successfully. As forests were cut down and as the birds were hunted, the size of their flocks diminished. Their narrow habitat tolerance then made them extremely vulnerable to extinction. In general, because species with narrow habitat tolerance cannot adapt in the face of habitat degradation and loss, they are at greater risk than species with broader habitat tolerance.

One species that is enormously successful, thanks in part to its extensive geographic range, large local population sizes, and broad habitat tolerance, is our own. Humans are growing and expanding their range at a staggering rate, consuming resources in an unprecedentedly voracious fashion. By any measure, humans are one of the most successful species in earth’s history. This success, unfortunately, is having an increasingly negative effect on the species with which we share the planet, causing extinctions at a significantly higher rate than ever before. We next explore in more detail the conflict between humans’ success as a species and the survival of other species.