The loss of the heath hen and other extirpations caused by humans (see Figure 58.3) remind us of the finality inherent in species extinctions. However, over the scope of Earth’s history extinction has been a constant theme; most of the species that have lived on Earth are extinct today. Extinctions have occurred throughout Earth’s history at what is referred to as a “background” rate as changes in environmental conditions have favored some species and negatively affected others. But the rate of extinctions taking place today rivals those of the five great mass extinction events, which were the result of cataclysmic natural disturbances rather than human activities (see Table 24.1 and Figure 24.2). One estimate, from renowned evolutionary biologist Edward O. Wilson suggests that Earth is losing some 30,000 species per year, putting us in the midst of a sixth mass extinction event.
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This mass extinction event has been ongoing for some time. For example, research suggests that when humans first arrived in North America from Siberia about 14,000 years ago, they encountered a diverse and spectacular group of large mammals, including saber-
We do not know how many species live on Earth today. Many species that are likely to become extinct in the near future have not been named and described by scientists. Insects provide a case in point: although more than 1 million species have been described (see Key Concept 31.4), estimates of the number of species yet to be discovered range from 2 million to more than 50 million. Even for larger organisms, our understanding of diversity is incomplete. For example, worldwide, an annual inventory of newly described species reported 19,232 species discovered in 2009 alone; this list included 9,738 insects, 2,184 plants, 1,360 fungi, 71 mammals, and 7 birds.
The distributions of most described species, particularly those that are small, reclusive, and rare, are poorly known. One tiny North American true bug, Corixidea major (so rare it has no common name), had been found in only one location near Clarksville, Tennessee, until entomologists using lights to collect insects at night discovered it in Virginia and Florida, extending the species’ known range by more than 1,000 kilometers.
It is difficult to determine whether a species is truly extinct. Rarely is the death of the last surviving member of a species recorded with certainty, as it was in the case of the last passenger pigeon (Ectopistes migratorius), a female named Martha, that died in the Cincinnati Zoo on September 1, 1914. The status of rare, reclusive species with poorly known life histories is much more difficult to determine, as has been the case with the pygmy tarsier (Tarsius pumilus) (see Figure 58.3A). This tiny primate, weighing less than 60 grams, was thought to have become extinct in its native cloud forests on the island of Sulawesi in Indonesia. In 2008—
We rarely know all of the interactions among species. The loss of one species—
Media Clip 58.1 New Species Found in the Twenty-
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