Ecosystem Disruption

The human impacts on air, climate, land, and water described above are happening at the same time. These impacts contribute to ecosystem disruption, the destruction or substantial change in the functioning of natural ecosystems. Ecosystem disruption is evident as species—and sometimes entire communities— disappear.

Extinction

Organisms generally require specific habitats in order to thrive. For example, when whooping cranes are in their winter nesting grounds in Texas, they eat blue crabs. Along their migration routes, they eat various invertebrates in the marshes where they stop to rest. If people drain the marshes for farmland, the crane could experience extinction, the death of every member of the species. In fact, whooping cranes are in danger of extinction, or endangered. Worldwide, thousands of species have become endangered or extinct as a result of human activities.

One way that species become endangered is when their habitat is destroyed. Habitat destruction occurs as humans convert complex natural ecosystems into simplified systems that do not sustain as many species, such as farmland and urban areas. Since the development of agriculture, people have cut down more than half of the world's forests. Other examples of habitat destruction include damming rivers, draining swamps, surface mining, logging, and clearing land for buildings or roads.

In addition to habitat destruction, the main human causes of the endangerment of species include hunting or harvesting and the transfer of invasive species into new habitats. An increasing number of species are endangered, as Table 22-1 shows. Scientists estimate that perhaps 20 percent of Earth's plant and animal species may become extinct in the next 50 years, a situation called the biodiversity crisis. Living species have not been lost at such a high rate since the mass extinction of dinosaurs 65 million years ago. The mass extinction currently underway is unique in that humans are its primary cause.

Ecosystem Imbalances

No one can predict the results of the loss of millions of species. We do know that some species are critical to the functioning of ecosystems. For example, a species of sea otters, shown in Figure 22-8, once lived in great numbers along the Pacific coast. These otters feed heavily on sea urchins, whereas sea urchins feed on giant kelp (algae) that form the basis of an underwater "forest" community. When people hunted the otters to near-extinction, the sea urchins increased in number and overfed on the kelp. Entire kelp communities began to disappear. Later, biologists reintroduced and helped secure legal protections for the sea otters in some areas, and the kelp recovered. Species such as the sea otter that affect many other species in a community are called keystone species. Extinction of keystone species has serious and sometimes unknown effects on ecosystems.

Overusing resources can also unbalance ecosystems. For millennia, the waters off what is now Cape Cod supported huge populations of cod, flounder, haddock and other fish. Until the 1960s, people harvested the fish at about the same rate that the fish reproduced. When newer fishing methods brought in larger harvests, the fish populations decreased rapidly and have not yet recovered. Similar overuse of terrestrial resources has caused the formation of desertlike areas in Africa and the erosion of topsoils in the midwestern United States.

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