Much has been written lately about the catastrophic loss of biodiversity, (species extinction), which has been occurring during the twentieth century. Current extinction rates are estimated as between one thousand and ten thousand times the “natural” rate which existed before humans appeared (Pimentel, et al., 1997, among others). The rate may be even higher, since we know so little about how many species exist on Earth. Unlike past massive extinction events, this one can be traced directly to human activities, which replace natural ecosystems with fewer, species-poor systems, such as pastures, agricultural plots, and tree farms. Human responsibility in species extinction is not only recent (See above, Part I, Section D1). When the Polynesians migrated across the Pacific Ocean several thousand years ago, they initiated the disappearance of 2000 species of birds. When Europeans arrived in Hawaii, they extinguished 10% of the native plants, and more are going. Of the 135 bird species now present in Hawaii, only eleven still have populations large enough to ensure that they will survive (at least in the immediate future); twelve others are rare, and at least twelve more are endangered.
Not knowing how many species exist, by a factor of 10 or more (but, at conservative estimates, at least 3.4-10 million), we cannot estimate how many have already become extinct or are poised on the knife edge of destruction. Very roughly, we can say that 50% of the world’s (recent) species have become extinct or are threatened with extinction. It may be that as many as 10% of them have already disappeared. Kremen, et al., (2000) estimate that 14,000 to 40,000 species are lost annually from tropical forests alone. More than 20,000 species have been listed by organizations as “at risk of extinction” (and these are only conspicuous species). Some estimate that 25% of present species will have vanished by 2010, a rate of extinction comparable to that of the great extinctions of the Cretaceous period. This represents a rate one thousand times the normal one.
Over the past century, approximately 20 mammalian species have disappeared; to recover this number of species would take 200 centuries. Many groups of organisms have lost between 5% and 20% of their species as a consequence of human activities. And we are not at an end. Pimm and Raven (2000) estimate that 12% of all plants and 11% of all bird species, for example, are endangered and will probably become extinct within the medium range. Vitousek (1997) corroborates the 11% value for birds. As natural systems become increasingly fragmented, species will vanish with accelerating frequency.
The greatest loss of biodiversity is occurring in tropical rainforests and coral reefs, because they have such high species diversity and have been subjected to such heavy exploitation pressures. Rainforests have now been eliminated from 50% of the area on which they formerly existed. The coastal forest of Ecuador, the Atlantic forest of Brazil, the forests of Madagascar, many West African forests – all have been virtually destroyed in the past 40 years. If tropical forests continue to be cut at current rates, by the 22nd century all of the rainforests outside of reserves will have been cut or seriously disturbed. Because of this, tropical rainforests everywhere are heading for massive extinctions. The extinction problem is magnified in rainforests, not only because of their generally high levels of biodiversity, but also because many rainforest species are very specialized and have narrow ranges (a high degree of endemism). Therefore, removing even a relatively small portion of forest may destroy many species, and can multiply the extinction rate. The clearing of one mountain ridge-top in Peru led to the disappearance of 90 endemic plant species, for example. Of course, species have always become extinct, over time. Some species in the past have existed for only one or two million years; others have lasted more than ten million years. These extinctions have been due to changes in habitat, climate, and other alterations in environment to which the organisms could not adapt. However, during recent history the extinction rate has escalated greatly. Today, most of the loss of species is due to habitat destruction, abetted by overexploitation – hunting, fishing, and gathering.
Thus, the future for tropical forests is one of increasing fragmentation, extreme reduction in size, and extraordinary extinctions of flora and fauna. Very little of the remaining forest lies within the approximately 5000 reserves which have been set up by national or state governments (4% of land area in Africa, 2% in Latin America, and 6% in Asia). Even in most reserves, protection is only nominal. Often the governments in countries with large areas of tropical forest have no funds to expend upon conservation or protection of reserves and parks. Additionally even those whose job it is to protect reserves are vulnerable to bribery (often because of low salaries) or to importuning or threats from loggers, hunters and collectors. Those areas at most risk for biodiversity loss are in South Asia, The Philippines, the Caribbean, western Ecuador, tropical Andes and Madagascar.
“…the evolutionary impoverishment of the impending extinction spasm, plus the numbers of species involved and the telescoped time scale of the phenomenon, may result in the greatest single setback to life’s abundance and diversity since the first flickerings of life almost 4 billion years ago.” (Myers, 1988a, pp. 33-34)