Lester R. Brown
Chapter 3. Signs of Stress: The Biological Base: Species Disappearing
The archeological record shows five great extinctions since life began, each representing an evolutionary setback, a wholesale impoverishment of life on the earth. The last of these mass extinctions occurred some 65 million years ago, most likely when an asteroid collided with the earth, spewing vast amounts of dust and debris into the atmosphere. The resultant abrupt cooling obliterated the dinosaurs and at least one fifth of all other extant life forms.71
We are now in the early stage of the sixth great extinction. Unlike previous ones, which were caused by natural phenomena, this one is of human origin. For the first time in the earth's long history, one species has reached the point where it can eradicate much of life.
As various life forms disappear, they alter the earth's ecosystem, diminishing the services provided by nature, such as pollination, seed dispersal, insect control, and nutrient cycling. This loss of species is weakening the web of life, and if it continues it could tear huge gaps in its fabric, leading to irreversible and potentially unpredictable changes in the earth's ecosystem. Species of all kinds are threatened by habitat destruction, principally through the loss of tropical rainforests. As we burn off the Amazon rainforest, we are burning one of the great genetic storehouses, in effect one of the great libraries of genetic information. Our descendents may one day view the wholesale burning of this repository of genetic information much as we view the burning of the library in Alexandria in 48 BC.
Habitat alteration from rising temperatures, chemical pollution, or the introduction of exotic species can also decimate both plant and animal species. As human population grows, the number of species with which we share the planet shrinks. We cannot separate our fate from that of all life on the earth. If the rich diversity of life that we inherited is continually impoverished, eventually we will be as well.72
The share of birds, mammals, and fish that are vulnerable or in immediate danger of extinction is now measured in double digits: 12 percent of the world's nearly 10,000 bird species; 24 percent of the world's 4,763 mammal species; and an estimated 30 percent of all 25,000 fish species.73
When the World Conservation Union-IUCN released its newest Red List of Threatened Species in 2000, it showed an increase in the "critically endangered" in all categories. For example, the number of critically endangered primates rose from 13 in 1996 to 19 in 2000. The number of freshwater species of turtles in this category, many of them in strong demand in Asia for food and for medicinal uses, increased from 10 to 24. For birds overall, the number in the critically endangered category went from 168 in 1996 to 182 in 2000. Like many other trends of environmental decline, this one, too, is accelerating.74
Among mammals, the 600 known species of primates other than humans are most at risk. IUCN reports that nearly half of these species are threatened with extinction. Some 79 of the world's primate species live in Brazil, where habitat destruction poses a particular threat. Hunting, too, endangers many primate species. It is a threat principally in West and Central Africa, where the deteriorating food situation is creating a lively market for "bushmeat."75
The bonobos of West Africa, a smaller version of the chimpanzees of East Africa, may be our closest living relative both genetically and in terms of social behavior. But this is not saving them from the bushmeat trade or the destruction of their habitat by loggers. Concentrated in the dense forest of the Democratic Republic of the Congo, their numbers fell from an estimated 100,000 in 1980 to fewer than 10,000 by 1990. Today there are only 3,000 left. In less than one generation, 97 percent of the bonobos have disappeared.76
Birds, because of their visibility, are a useful indicator of the diversity of life. Of the 9,946 known bird species, roughly 70 percent are declining in number. Of these, an estimated 1,183 species are in imminent danger of extinction. Habitat loss and degradation affect 85 percent of all threatened bird species. For example, 61 bird species have become locally extinct with the extensive loss of lowland rainforest in Singapore. Some once-abundant species may have already dwindled to the point of no return. The great bustard, once widespread in Pakistan and surrounding countries, is being hunted to extinction. Ten of the world's 17 species of penguins are threatened or endangered, potential victims of global warming.77
The threat to fish may be the greatest of all, with nearly one third of all species—freshwater and saltwater—now facing possible extinction. Worldwide, the principal causes of this loss are habitat degradation in the form of pollution and the excessive extraction of water from rivers and other freshwater ecosystems. An estimated 37 percent of the fish species that inhabit the lakes and streams of North America are either extinct or in jeopardy. Ten North American freshwater fish species have disappeared during the last decade. In semiarid regions of Mexico, 68 percent of native and endemic fish species have disappeared. The situation may be even worse in Europe, where some 80 species of freshwater fish out of a total of 193 are threatened, endangered, or of special concern. Two thirds of the 94 fish species in South Africa need special protection to avoid extinction.78
Threatened species include both little known ones and those that are well known and highly valued. The harvest of the Caspian Sea sturgeon, for example, source of the world's most prized caviar, has fallen from 22,000 tons per year in the late 1970s to 1,100 tons in the late 1990s. Overfishing, much of it illegal, is responsible.79
Another indicator of the earth's environmental deterioration is the decline in various types of amphibians-frogs, toads, and salamanders. Widespread evidence that amphibian populations were disappearing initially surfaced at the first World Congress of Herpetology in Canterbury, England, in 1989. It was at this conference that scientists first realized that the seemingly isolated disappearances of amphibian populations were actually a worldwide phenomenon. Among the apparent contributing factors are the clearcutting of forests, the loss of wetlands, the introduction of alien species, changes in climate, increased ultraviolet radiation, acid rain, and pollution from both agriculture and industry. Spending their lives in both aquatic and terrestrial environments, amphibians are affected by changes in each, making them an unusually sensitive barometer of the earth's changing physical condition.80
The leatherback turtle, one of the most ancient animal species, and one that can reach a weight of 360 kilograms (800 pounds), is fast disappearing. Its numbers have dropped from 115,000 in 1982 to 34,500 in 1996. At the Playa Grande nesting colony on Costa Rica's west coast, the number of nesting females dropped from 1,367 in 1989 to 117 in 1999. James Spotila and colleagues, writing in Nature, warn that "if these turtles are to be saved, immediate action is needed to minimize mortality through fishing and to maximize hatchling production."81
One of the newer threats to species, and one that is commonly underestimated, is the introduction of alien species, which can alter local habitats and communities, driving native species to extinction. For example, non-native species are a key reason why 30 percent of the threatened bird species are on the IUCN Red List. For plants, alien species are implicated in 15 percent of all the listings. One consequence of globalization with its expanding international travel and commerce is that more and more species are being accidentally or intentionally brought into new areas where they have no natural predators.82
Efforts to save wildlife traditionally have centered on the creation of parks or wildlife reserves. Unfortunately, this approach may now be of limited value because of the nature of the principal threats to biological diversity. If we cannot stabilize population and climate, there is not an ecosystem on earth that we can save. To optimize resource use, this would argue for shifting some of the relatively abundant funds for parkland acquisition into efforts to stabilize population and climate.
The current species extinction rate is at least 1,000 times higher than the background rate, yet no one knows how many plant and animal species there are today, much less how many there were a half-century ago, when the explosion in human economic activity began. Current estimates range from 6 million species up to 20 million, with the best working estimates falling between 13 million and 14 million. We can measure losses where we have a complete inventory of species, as with birds, but with insects, where the species number in the millions, only a fraction of the species have been identified, described, and cataloged.83
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