406 Extinction, Causes of social groups, need to move freely across the landscape to feed on widely scattered resources However, fences and other barriers may prevent the natural migration of animals, such as wildebeest or bison, forcing them to overgraze an unsuitable habitat, eventually leading to starvation and habitat degradation Barriers to dispersal can restrict the ability of widely scattered species to find mates, causing a loss of reproductive potential for many animal species Plants also may reduce seed production if butterflies and bees are less able to migrate among habitat fragments to pollinate flowers Finally, dividing a formerly large population into several smaller ones means that the new, smaller populations will be more vulnerable to genetic and other problems associated with small populations, increasing their risk of extinction Edge Effects Habitat fragmentation often changes the microenvironment at the fragment edge, resulting in increased light levels, higher daytime temperatures, higher wind speeds, and lower humidity Each of these edge effects can have a significant impact on the vitality and composition of the species in the fragment Species that are sensitive to humidity such as amphibians, many insects, and herbaceous plants, will be eliminated from the forest fragments Also, increased wind, lower humidity, and higher daytime temperatures make fires more likely in forest fragments Fires may spread into habitat fragments from nearby agricultural fields that are being burned regularly, as in sugarcane harvesting, or from the irregular activities of farmers practicing shifting cultivation In the process, many species will be eliminated Interspecific Interactions Habitat fragmentation also increases the vulnerability of the fragment to invasion by exotic and native pest species Omnivorous native animals, such as raccoons, skunks, and blue jays, and introduced animals, such as rats, may increase in population size along forest edges, where they can eat food found in both undisturbed and disturbed habitats These aggressive feeders may seek out the nests of interior forest birds, often preventing successful reproduction of many bird species hundreds of meters from the nearest forest edge Domestic cats can also be important predators in settled areas with fragmented landscapes Weedy plant species and exotic herbivores can eliminate native plant species along the edges of fragments, and disease can similarly spread into the interior of habitat fragments ground fires might not kill the mature trees, but the rich perennial wildflower community and insect fauna on the forest floor would gradually become impoverished Keeping too many cattle in grassland communities gradually changes the biological community, often eliminating many native species and favoring exotic species that can tolerate grazing Boat hulls, anchors, and divers’ flippers can crush fragile species on coral reefs Trawling destroys 15 million km2 of seafloor communities each year, 150 times greater than the area of forest cleared annually The subtlest form of environmental degradation is pollution, commonly caused by pesticides, sewage, fertilizer runoff from agricultural fields, industrial chemicals, and wastes, emissions from factories and cars, and sediment deposits from eroded hillsides The general effects of pollution on water quality, air quality, and even global climate are causes for great concern, not only because of the threats to biological diversity, but also because of their effects on human health Pesticides The dangers of pesticides were brought to world’s attention in 1962 by Rachel Carson’s influential book Silent Spring Carson described a process, now known as biomagnification, through which (dichlorodiphenyltrichloroethane DDT) and other organochlorine pesticides become concentrated as they ascend the food chain These pesticides, at that time widely used to kill agricultural pests and mosquito larvae were harming wildlife populations, especially birds that ate large amounts of insects, fish, or other animals exposed to DDT and its byproducts Birds with high levels of pesticides in their tissues, particularly raptors such as hawks and eagles, became weak and tended to lay eggs with abnormally thin shells that cracked during incubation As a result of failure to raise young and the outright death of many adults, populations of these birds showed dramatic declines throughout the world Recognition of this situation in the 1970s led many industrialized countries to ban the use of DDT and other chemically related pesticides The ban eventually allowed the partial recovery of many bird populations, most notably peregrine falcons (Falco peregrinus), ospreys (Pandion haliaetus), and bald eagles (Haliaeetus leucocephalus) Nevertheless, the continuing massive use of pesticides and DDT in other countries is still a cause for concern, not only for endangered animal species, but also for the potential long-term effects on people, particularly the workers who handle these chemicals in the field and the consumers of agricultural products treated with these chemicals These chemicals are widely dispersed in the air and water and are persistent in the environment even in countries where their use has been outlawed Habitat Degradation and Pollution Communities and species in a habitat can be profoundly affected by human activities, even if the habitat itself is not Biological communities can be damaged and species driven locally or globally extinct by external factors that not change the structure of dominant plants in the community, so that the damage is not immediately apparent For example, in temperate deciduous forests, frequent and uncontrolled Water Pollution Water pollution destroys important food sources and contaminates drinking water with chemicals that can cause immediate and long-term harm to human health Water pollution also often severely damages aquatic ecosystems Rivers, lakes, and oceans are used as open sewers for industrial and residential waste Pesticides, herbicides, oil products, heavy