The physical and biological components of Earth’s environment are constantly changing. As we saw in Chapter 23, new species are constantly being generated. At the same time, some species decline to the point that they can no longer maintain viable populations and go extinct. Both speciation and extinction have been occurring throughout the history of life. Why do species go extinct? Short-term or long-term environmental fluctuations can make growth or reproduction difficult. As new species arise, they may outcompete existing species for food, space, or other resources. If a prey species goes extinct, predators that depend on the prey may decline or disappear as well.

As population sizes decrease, the probability of extinction accelerates. At any population size, there is variation in reproductive success, and a particular individual may fail to reproduce successfully. When population sizes are very small, such variation has a bigger influence. Low reproduction by a few individuals in a small population is likely to lead to loss of the population. The smaller the population, the larger the role that chance plays. Variation in reproductive rates is less likely to lead to extinction in large populations, where chance effects play a much smaller role.

Smaller populations can also have density-dependent disadvantages compared with larger populations—a phenomenon known as the Allee effect. Allee effects can occur, for example, from the inability of a small group to defend itself against predators, or from a decline in the ability of individuals to find mates. Small populations can also decline from the negative effects of inbreeding depression and genetic drift (see Chapter 21). These processes can result in loss of genetic variation, uncovering of deleterious alleles, and loss of beneficial alleles.

An experiment on sea palms (a species of kelp) illustrates the relative influence of various factors on extinction probability as it relates to population size (Figure 24.10). Biologists experimentally established isolated populations of sea palms on intertidal rocky shores and examined their survivorship through time. At small population sizes, random demographic factors were most likely to lead to local extinction, whereas environmental fluctuations were much more likely to cause the extinction of larger populations.

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The five mass extinction events shown in Figure 24.2 are notable because they suddenly increased the extinction rate and affected a large percentage of living organisms at the same time. In each case, rapid and extreme environmental changes caused many species to go extinct simultaneously.