We've already seen that extinction is non-random. Some taxonomic groups are more likely to go extinct than others. What may be a little surprising is that the ways in which species become endangered is also non-random. Although threatened taxa typically show declines in range size, numbers of populations, and overall abundance, the relative importance of range losses and population deletions differs depending on which broad taxonomic group you examine .
As Table 1 shows, range contractions and population deletions have played a greater role in invertebrates than in vertebrates. Plants show less range contraction than vertebrates, and vertebrates show the fewest population losses. At a finer taxonomic scale, we see the same pattern: All of the 51 listed clams show a range reduction but only 5 of the 12 listed crustaceans do. Ten of the 15 listed amphibians show population deletions, but only 7 of the 14 listed reptiles do.