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50 year periods

Figure 14.4 Rate of extinction. This graph illustrates the number of species of mammals and birds known to have become extinct since 1600. Note that the rate of extinction in both groups has been generally increasing, with the most dramatic increase occurring within the last 150 years.

A few searches for species of concern give a hint of the recent extinction rate. In peninsular Malaysia, a 4-year search for 266 known species of freshwater fish turned up only 122. In Africa's Lake Victoria, 200 of 300 native fish species have not been seen for years. On the Hawaiian island of Oahu, half of 41 native tree snail species were not found; and in the Tennessee River, 44 of the 68 shallow-water mussel species are missing. However, few of the missing species in any of these searches is officially considered extinct.

The most complete records of documented extinction occur in groups of highly visible organisms, primarily birds and mammals. Since a.d. 1600, 83 mammal species out of an approximate 4,500 identified have become extinct, while 113 of approximately 9,000 known bird species have disappeared. The known extinctions of mammals and birds correspond to a rate of 0.005% per year spread out over the 400 years of these records. Compared to the background rate of extinctions calculated from the fossil record, the current rate of extinction is 50 times higher. If we examine the past 400 years more closely, we see that the extinction rate has actually risen since the start of this historical record (Figure 14.4)—to about 0.01% per year, making the current rate 100 times higher than the calculated background rate.

In addition, there are reasons to expect that the current elevated rate of extinction will continue into the future. The World Conservation Union (known by its French acronym, IUCN), a highly respected global organization made up of and funded by states, government agencies, and nongovernmental organizations from over 140 countries, collects and coordinates data on threats to biodiversity. According to the IUCN's most recent assessment, 11% of all plants, 12% of all bird species, and 24% of all mammal species (the three best-studied groups of organisms) are in danger of extinction, and human activities on the planet pose the greatest threat to most of these species.

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