Natural resource consumption rates and human population size exert tremendous pressure on the world's plants and animals. Although direct use of wildlife is essential for human survival, overexploitation of resources (or using resources at an unsustainable rate) is a critical problem in conservation. Although habitat loss may be the greatest threat to most species, the overexploitation or nonsustainable use of wildlife is closely linked and plays an increasing role in the loss of biodiversity. Over-harvesting, nonsustainable use, and the illegal trade in some species are threatening not only their continued survival but also that of ecosystems and the livelihoods of communities and local economics that depend upon them.

There is no question that overexploitation has led to species extinctions in historic as well as modern times. Unsustainable hunting, fishing, logging, or gathering of wild populations leads to their commercial, ecological, or global extinction. Commercial extinction occurs when populations are too depleted or scattered to be harvested economically; ecological extinction indicates populations that may still be present in low numbers but no longer play important functional roles in the ecosystem. Global extinction signifies that no living individuals of the species remain anywhere in the world.

In theory some level of exploitation should be manageable. The difficulty is in determining what level is sustainable (in part because sustainability is an ambiguous term) and in keeping exploitation to that level or below. As with other aspects of conservation, short-term perspectives often call for higher rates of use than long-term perspectives.

Overexploitation can be divided into two major categories: direct and indirect exploitation. Direct exploitation ranges from commercial activities such as logging operations or trade in endangered species to subsistence hunting. Indirect exploitation encompasses the unintentional mortality of nontarget species such as fish or turtles killed as by-catch in fishery operations. Both endanger species around the world.

Direct Commercial Overexploitation Although not all commercial ventures lead to overuse of resources, commercial exploitation is a major cause of overexploited resources. Natural resources are generally communal and therefore vulnerable. With communal resources, the cost of overexploitation is borne by the whole community, not just the person using the resource, whereas the benefits go to the exploiter alone. It is in the best interest of individuals—in this case commercial venturers—to overexploit communal resources until there is nothing left; Hardin (1968) dubs this phenomenon the "Tragedy of the Commons." There are numerous examples of commercial overexploitation, and we will limit discussion to two very important ones: overexploitation of marine fisheries and wildlife trade.

Marine fisheries. The oceans were once considered a limitless resource. This philosophy, coupled with a policy of open access to the oceans, set the stage for overexploitation, as predicted by Hardin (ibid.). According to the Food and Agriculture Organization, the world's total capture fishery harvest reached 86 million tons in 1998, with marine catch accounting for 90.1 percent of the harvest. Among the major marine stocks exploited, 47 to 50 percent of species are considered fully exploited and are close to their maximum harvest; another 15 to 18 percent are overexploited; and 9 to 10 percent are depleted. Major fisheries have collapsed around the world, from the Peruvian anchovy fishery in the 1970s to the cod fishery in eastern North America in the 1990s. These collapses followed similar patterns. Initially these fisheries were so plentiful that they were judged impossible to overhar-vest. We systematically removed the largest, oldest fish from the populations. The largest fish are often the top predators, and removing them affects the prey species and other predators. The oldest fish generally have the highest reproductive capacity, and their loss led to declining populations. Over time, boats had to travel farther and fish longer to harvest the same catch. At the same time, the average size of the fish caught began to decline substantially. In 1963 the average swordfish caught off the East Coast of North America weighed 250 pounds (113.6 kg); by 1996 that had dropped to 90 pounds (40.9 kg).

As one species becomes overexploited, fishing pressure has simply shifted to other species—overharvested top predators are replaced with target species farther down the food web. Between 1950 and 1994, there has been a gradual shift in mean trophic level fished—from long-lived bottom fish that eat other fish, to lower trophic level invertebrates and open-water species that eat plankton. This shift—termed "fishing down marine food webs"—has been most noticeable in the Northern Hemisphere; while it initially leads to an increase in catch, it is followed by declines (Pauly et al., 1998). As we systematically remove the top predators and their prey from marine systems, we have put the oceans in a perilous state for recovery.

Recent research into historical and archaeological evidence has highlighted the toll of overexploitation on many marine systems. The resulting impoverished state of these marine systems leaves them more susceptible to major disturbances (for example, epidemic diseases, hurricanes, and climate change) and less productive for current and future human needs (Jackson et al., 2001). For instance, in Caribbean coral reefs, populations of predatory and large herbivorous fish were overfished during the seventeenth to twentieth centuries. The loss of these fish made those reefs more susceptible to other threats. An introduced disease killed off most of the sea urchins (Diadema antillarum) in 1983 and 1984, removing the other major herbivore in the reef system. With the loss of these herbivores, Caribbean corals perished under the overgrowth of macroalgae.

As in other cases of commercial overexploitation, technological advances have significantly contributed to overharvesting of marine fish. Engines, refrigeration, sonar, geopositional systems (GPS), and acoustic Doppler profilers have made it easier to locate, catch, and store fish, and to fish farther from shore and for longer periods of time. New fishing gear has allowed us to harvest faster and in areas that were once inaccessible. Long-lining has enabled fishermen to catch in three days the same amount of swordfish previously harvested in two weeks by harpooning. New trawling techniques—such as "rockhoppers" with wheels on the net that enable it to effectively trawl the ocean floor despite rocky terrain—afford access to areas previously out of reach.

Several regulations have been imposed to try to control the exploitation of fish, although with mixed success. In the 1970s, a 200-mile limit was imposed around the world's coastline to enable countries to regulate fishery harvest in their waters. Quotas on the number and size of fish caught, restrictions on fishing gear, and limitations on the number of boats allowed into a fishery have been used to help control harvest rates.

Aquaculture was considered a solution to already overfished oceans. Unfortunately, the species farmed are often carnivores and require wild-caught fish as food. It takes five pounds of wild fish to raise one pound of farmed salmon. So rather than reducing the harvest, aquaculture has placed a new burden on fisheries to supply fishmeal. Aquaculture can also pollute the environment with excess nutrients or antibiotics, or by introducing disease into wild populations, though new methods of raising fish to minimize some of these problems are being tested.

Wildlife trade. Trade in wildlife is pandemic, occurring in local, regional, and international settings. TRAFFIC, an international organization established by the World Wildlife Fund and the World Conservation Union, monitors the trade in wildlife and wildlife products. Based on declared import values, they estimate that the global wildlife trade is huge, with an annual turnover of billions of dollars and involving hundreds of millions of individual plants and animals. The Convention on the International Trade in Endangered Species of Wild Fauna and Flora (CITES) regulates international trade in some

30,000 species of plants and animals through a system of certificates and permits. Interpol estimates the illegal trade at $12 billion a year, second only to drugs. A large proportion of the world's wildlife trade is domestic and does not cross international borders, especially for products such as medicinal plants, timber, wild meat, and fisheries. The magnitude of the domestic trade for most wildlife species remains unknown.

Hunting for commercial bushmeat, prevalent across tropical Asia, Africa, and the neotropics, is a specific element of wildlife trade that has received increased attention of late (Robinson and Bennett, 2000). As humans colonize formerly remote regions, few places are immune to the effects of the bush-meat trade. In some communities harvesting of wild plants and animals fulfills a secondary role in the household economy, whereas in others these resources are irreplaceable. It is increasingly difficult to distinguish subsistence hunting from commercial hunting, traditional from modern, and sport from necessity. Each situation embraces its own nuances, making the search for sustainability—and particularly a formula for sustainability—complicated. The bushmeat trade has increased because of the replacement of traditional weapons with modern ones, new logging roads and other activities that increase access into formerly remote areas, and more permanent settlements along roadsides. Fundamentally, however, rising human populations and consumption rates drive this transition from subsistence to commercial hunting.

Hunting has a significant effect on wild populations and communities, lowering population densities and the average body size of individuals as well as decreasing the representation of large-bodied species in a community. Overhunting has led to the extinction of the passenger pigeon, flightless birds, tor toises and other island species, and the near extinction of the bison.

Indirect Overexploitation Nontarget species may inadvertently be exploited as commercial species are harvested. For instance, many fishing methods are not selective, catching other species besides the intended one. This indirect harvest—or "by-catch"—can be substantial. Total global by-catch is estimated at 16 to 40 million tons per year. Shrimp harvest has one of the highest by-catches of any fishery; for every pound of shrimp harvested, five pounds of by-catch is caught and wasted. Longlining for tuna and swordfish produces significant by-catch of sharks, sea turtles, and marlin. Nets positioned to catch schools of tuna have caught and drowned dolphins, prompting public outcry and consequent modifications of tuna fishing practices.

Fishing techniques that use cyanide or dynamite are nonselective and also result in substantial by-catch. Dynamiting on coral reefs not only kills the fish and invertebrates nearby but also destroys the physical structure where these species live and breed, causing long-term damage to the entire community.

Wildlife trade offers another example of indirect overexploitation, though in this instance individuals of the same species are in a sense indirectly exploited. Many animals die during capture and shipment, so traders must bolster the number they capture to ensure that an adequate supply reaches the market destination. Sadly, experts calculate that the mortality rate can reach up to 60 to 70 percent for some birds and reptiles and 80 to 90 percent for reef fish.

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