Unit 9: Biodiversity Decline // Section 9: Other Drivers of Biodiversity Loss
Humans are harvesting many plants and animals at faster rates than the populations can maintain themselves (harvesting includes the use of animals and plants for food as well as other applications like medicines, trophies, and clothing). The declining yield from world fisheries shows how over-harvesting threatens biodiversity on a large scale.
According to the Food and Agriculture Organization (FAO), about half of ocean fish stocks are being exploited close to their maximum sustainable limits (i.e., close to levels of harvesting that will not reduce future yields). About one-fourth of global stocks are either overexploited, depleted, or recovering from depletion, and the remaining fourth are underexploited or moderately exploited. In several Atlantic and Pacific fisheries, traditional stocks have been depleted and fishermen are targeting other, less-valuable species (footnote 20).
In addition to depleting human food supplies, overfishing disrupts marine ecosystems and affects other species in the food chain. "Overfishing removes the most important and abundant consumers in a natural ecosystem," says Jeremy Jackson of the Smithsonian Tropical Research Institute. "Fish eat fish, but fish also eat seaweed. If they're not there, the seaweed grows 10 times or 100 times faster than corals, it grows over corals, smothers them, and kills them."
Hunting may also threaten animal and bird species. When North America was settled by European explorers, millions of passenger pigeons wintered in southern forests and migrated north to summer nesting grounds around the Great Lakes, darkening the skies as they flew. But large-scale hunting and forest clearing in the 19th century decimated pigeon numbers. The last known individual died in the Cincinnati Zoo in 1914 (Fig. 12).
Figure 12. Passenger pigeons, from John James Audubon's Birds of America
See larger image
Source: Courtesy National Audubon Society, Inc., 700 Broadway, New York, NY 10003, USA.
Today, major hunting and fishing organizations such as Ducks Unlimited and the Izaak Walton League take a more balanced approach and devote significant resources to protecting and conserving wildlife habitat. However, in developing countries where hunting is less strictly regulated, poaching and illegal trafficking threaten many wild species. International trade in wildlife generates billions of dollars annually and has depleted many species that are prized for uses including trophies, jewelry, food, exotic clothing, ingredients for medicine, and other uses. Many of these applications have encouraged wasteful and inhumane harvesting practices, such as cutting fins off of live sharks for use in shark-fin soup and throwing the maimed sharks back into the ocean to drown.
Pollution reduces biodiversity by either changing organisms' biological functions or altering the environmental conditions that they need to survive. For example, pesticides can affect the health and reproductive patterns of many species. Bald eagles, which declined to near-extinction in the early 1960s, are a well-known example. The chlorinated hydrocarbon pesticide DDT, which was used heavily in the 1940s and 1950s, bioaccumulated in eagles' fatty tissue and caused them to lay eggs with thin shells that broke before hatching. In 1963 there were 417 breeding pairs of bald eagles in the Lower 48 states; a federal ban on DDT and other protective measures helped increase this to more than 6,400 pairs by 2000. However, other chemicals currently in use may also act as endocrine disruptors in species including turtles, amphibians, and some fish.
Pollution is an important threat to aquatic ecosystems, where it can affect many environmental parameters. Agricultural runoff carries excess nutrients that cause algal blooms and deplete dissolved oxygen levels, while siltation from logging and construction reduces available light. Mining generates toxic chemical wastes that can poison local water supplies.
Global climate change threatens biodiversity worldwide because it is modifying average temperatures and rainfall patterns, and thereby shifting climate zones. Ecologists have documented changes in the geographic ranges and breeding cycles of many species. However, some organisms that are highly adapted to specific conditions—such as the Arctic sea ice communities described above in Box 1—may not be mobile enough to find new habitats as their local climate conditions change. As a result, many scientists believe that climate change could increase current extinction rates. (For more details, see Unit 12, "Earth's Changing Climate.")