Unit Eight. The Living Environment
38. Human Influences on the Living World
38.5. Loss of Biodiversity
Just as death is as necessary to a normal life cycle as reproduction, so extinction is as normal and necessary to a stable world ecosystem as species formation. Most species, probably all, go extinct eventually. More than 99% of species known to science (most from the fossil record) are now extinct. However, current rates of extinctions are alarmingly high. The extinction rate for birds and mammals was about one species every decade from 1600 to 1700, but it rose to one species every year during the period from 1850 to 1950, and four species per year between 1986 and 1990. It is this increase in the rate of extinction that is the heart of the biodiversity crisis.
Factors Responsible for Extinction
What factors are responsible for extinction? Studying a wide array of recorded extinctions, and many species currently threatened with extinction, biologists have identified three factors that seem to play a key role in many extinctions: habitat loss, species overexploitation, and introduced species (figure 38.6).
Figure 38.6. Factors responsible for animal extinction.
These data represent known extinctions of mammals in Australia, Asia, and the Americas. Some extinctions have more than one cause.
Habitat Loss. Habitat loss is the single most important cause of extinction. Given the tremendous amounts of ongoing destruction of all types of habitat, from rain forest to ocean floor, this should come as no surprise. Natural habitats may be adversely affected by human influences in four ways: (1) destruction, (2) pollution, (3) human disruption, and (4) habitat fragmentation (dividing up the habitat into small isolated areas). As you can see in figure 38.7, destruction j of rain forest habitat is occurring rapidly in Madagascar, which is endangering species.
Figure 38.7. Extinction and habitat destruction.
The rain forest covering the eastern coast of Madagascar, an island off the coast of East Africa, has been progressively destroyed as the island's human population has grown. Ninety percent of the original forest cover is now gone. Many species have become extinct, and many others are threatened, including 16 of Madagascar's 31 primate species.
Species Overexploitation. Species that are hunted or harvested by humans have historically been at grave risk of extinction, even when the species populations are initially very abundant. There are many examples in our recent history of overexploitation: passenger pigeons, bison, many species of whales, commercial fish such as Atlantic bluefin tuna, and mahogany trees in the West Indies are but a few.
Introduced Species. Occasionally, a new species will enter a habitat and colonize it, usually at the expense of native species. Colonization occurs rarely in nature, but humans have made this process more common, with devastating ecological consequences. The introduction of exotic species has wiped out or threatened many native populations. Species introductions occur in many ways, usually unintentionally. Plants and animals can be transported in nursery plants, as stowaways in boats, cars, and planes, and as beetle larvae within wood products. These species enter new environments where they have no native predators to keep their population sizes in check, and they crowd out native species.
Key Learning Outcome 38.5. Loss of biodiversity can be attributed to one of a few main causes, including habitat loss, overexploitation, and introduced species.
The Global Decline in Amphibians
Sometimes important things happen, right before our eyes, without anyone noticing. That thought occurred to David Bradford as he stood looking at a quiet lake high in the Sierra Nevada Mountains of California in the summer of 1988. Bradford, a biologist, had hiked all day to get to the lake, and when he got there his worst fears were confirmed. The lake was on a list of mountain lakes that Bradford had been visiting that summer in Sequoia-Kings Canyon National Parks while looking for a little frog with yellow legs. The frog's scientific name was Rana muscosa, and it had lived in the lakes of the parks for as long as anyone had kept records. But this silent summer evening, the little frog was gone. The last major census of the frog's populations within the parks had been taken in the mid-1970s, and R. muscosa had been everywhere, a common inhabitant of the many freshwater ponds and lakes within the parks. Now, for some reason Bradford did not understand, the frogs had disappeared from 98% of the ponds that had been their homes.
After Bradford reported this puzzling disappearance to other biologists, an alarming pattern soon became evident.
Throughout the world, local populations of amphibians (frogs, toads, and salamanders) were becoming extinct. Waves of extinction have swept through high-elevation amphibian populations in the western United States, and have also cut through the frog populations of Central America and coastal Australia.
Amphibians have been around for 350 million years, since long before the dinosaurs. Their sudden disappearance from so many of their natural homes sounded an alarm among biologists. What are we doing to our world? If amphibians cannot survive the world we are making, can we?
In 1998 the U.S. National Research Council brought scientists together from many disciplines in a serious attempt to address the problem. After years of intensive investigation, they have begun to sort out the reasons for the global decline in amphibians. Like many important questions in science, this one does not have a simple answer.
Five factors seem to be contributing in a major way to the worldwide amphibian decline: (1) habitat deterioration and destruction, particularly clear-cutting of forests, which drastically lowers the humidity (water in the air) that amphibians require; (2) the introduction of exotic species that outcompete local amphibian populations; (3) chemical pollutants that are toxic to amphibians; (4) fatal infections by pathogens; and (5) global warming, which is making some habitats unsuitable.
Infection by parasites appears to have played a particularly important role in the western United States and coastal Australia. Amphibian ecology expert James Collins of Arizona State University has reported one clear instance of infection leading to amphibian decline. When Collins examined populations of salamanders living on the Kaibab Plateau along the Grand Canyon rim, he found many sick salamanders. Their skin was covered with white pustules, and most infected ones died, their hearts and spleens collapsed. The infectious agent proved to be a virus common in fish called a ranavirus. Ranavirus isolated by Collins from one sick salamander would cause the disease in a healthy salamander, so there was no doubt that ranavirus was the culprit responsible for the salamander decline on the Kaibab Plateau.
Ranavirus outbreaks eliminate small populations, but in larger ones a few individuals survive infection, sloughing off their pustule-laden skin. These populations slowly recover.
A second kind of infection, very common in Australia but also seen in the United States, is having more widespread effects. Populations infected with this microbe, a kind of fungus called a chytrid (pronounced "kit-rid,” see chapter 18), do not recover. Usually a harmless soil fungus that decomposes plant material, this particular chytrid (with the Latin name of Batrachochytrium dendrobatidis) is far from harmless to amphibians. It dissolves and absorbs the keratinous mouthparts of amphibian larvae, killing them.
This killer chytrid appeared in Australia near Melbourne in the early 1980s. Now almost all Australia is affected. How did the disease spread so rapidly? Apparently it traveled by truck. Infected frogs moved all across Australia in wooden boxes with bunches of bananas. In one year, 5,000 frogs were collected from banana crates in one Melbourne market alone.
In other parts of the world, infection does not seem to play as important a role as acid precipitation, habitat loss, and introduction of exotic species. This complex pattern of cause and effect only serves to emphasize the take-home lesson: Worldwide amphibian decline has no one culprit. Instead, all five factors play important roles. It is their total impact that has shifted the worldwide balance toward extinction.
To reverse the trend toward extinction, we must work to lessen the impact of all of these factors. It is important that we not get discouraged at the size of the job, however. Any progress we make on any one factor will help shift the balance back toward survival. Extinction is only inevitable if we let it be.