Chapter 10. Chapter 10 Graphic Content

The blue line hovers around a rate of 10% of families becoming extinct at any time during the period recorded on the graph.

The data points for background extinction rates never get much higher than 11% or 12% of all families becoming extinct at that point. In each of the mass extinctions shown on the graph, the rate is significantly higher—usually 20% or more, except for the most recent event, which is about 15%, but still significantly higher than the typical background rate.

There are several reasons why the graph may record data on the extinction of families rather than of species. First, because the events documented in the figure occurred millions or even hundreds of millions of years ago, it is difficult to estimate the total number of species present on earth at that time. (It is nearly impossible to do this even today, when we can, at least in theory, count all of the species.) Second, it is difficult to identify whether multiple fossils belong to the same or different species. But because all of the organisms within a family share basic similarities, it is possible to identify fossils as members of the same family with much more certainty. Third, due to the limited number of fossils found, researchers are much more likely to find at least one representative from each family that has existed across time than they are to find at least one representative of each species. This enables a more accurate identification of when all species within a family became extinct (because after that point, no fossils are found of any members of the family). Still, even if no fossils of a family are found after a certain point in time, we can’t be sure whether extinction occurred or whether the environmental situation just changed so that members of that family were less likely to be fossilized.

Several factors influence whether a species or a family becomes extinct. These include competition with other species, the range over which the species can grow, and other aspects of the species’ environment. For this reason, conditions leading to extinction won’t necessarily occur at exactly the same rate. There will be variation over time, influenced by climate, population sizes of competitors and predators, and other factors, all leading to ups and down.

As described in Section 8.18, fossils are left only by certain types of organisms, and under a very limited range of environmental conditions. For this reason, the magnitude of mass extinctions noted on this graph may overestimate or underestimate the rate of extinctions of the many groups of organisms that did not leave fossils. This figure is an accurate measure of overall extinction rates only if our (unstated) assumption is true: that the extinction rates of the groups of organisms not leaving fossils were proportional to the extinction rates of the groups that did leave fossils.

If the current rate begins to rise above typical background rates of extinction, we might be in the midst of another mass extinction. This is important in that it may be an indicator that human activities are causing a higher than usual rate of extinctions. If this is the case, it may signal a need to reduce the human impact on extinction rates so as to reduce the likelihood of large (and maybe catastrophic, for us) changes in the species composition of earth.