Extinctions

Some 1.7 million species have been identified, but these probably comprise less than 15% of the total number of species thought to exist on Earth (Hammond, 1995). Of the major taxonomic groups, only in the case of plants and vertebrates have more than 80% of all the species been described and it is only for these groups that any direct assessment of rates of extinction can be calculated on a percentage basis. One of the few comparisons between regional extinction rates in birds, plants and insects – in this case, butterflies in Britain – suggests that extinction rates calculated for birds and plants may well be equally indicative of trends in insect populations (Thomas et al., 2004). Although the comparisons made are for a small area in global terms, and the populations and drivers of change involved may not be representative of those in the wider world, the results still lend some support to the view that groups of organisms for which data are sparse may be just as threatened by extinction as those studied more comprehensively.

Figure 11.1 shows estimates of the percentage of species of birds, mammals, fish and plants regarded as currently under threat of extinction (Pimm et al., 1995). McCann (2000) states that one third of the plant and animal species in the United States are at risk of extinction.