Environmental Problems, Their Causes, and Sustainability 1

mostly on land and eat insects that can expose them topesticides. Their eggs have no protective shells to blockultraviolet (UV) radiation or pollution. As adults, theytake in water and air through their thin, permeableskins that can readily absorb pollutants from water, air,or soil.Since 1980, populations of hundreds of the world’sestimated 5,280 amphibian species have been vanishingor declining in almost every part of the world, evenin protected wildlife reserves and parks.No single cause has been identified to explain theamphibian declines. However, scientists have identifieda number of factors that can affect frogs and otheramphibians at various points in their life cycle.One factor is habitat loss and fragmentation, especiallybecause of the draining and filling of inlandwetlands, deforestation, and development. Another isprolonged drought, which dries up breeding pools sothat few tadpoles survive. Dehydration from lack ofwater can also weaken amphibians and make themmore susceptible to fatal viruses, bacteria, fungi, andparasites.Pollution can also play a role. Frog eggs, tadpoles,and adults are very sensitive to many pollutants, especiallypesticides. Exposure to such pollutants mayalso make them more vulnerable to bacterial, viral,and fungal diseases, and cause an array of sexualabnormalities.Increases in UV radiation caused by reductions instratospheric ozone—caused when certain chemicalswe make drift up into the stratosphere—can harmyoung embryos of amphibians found in shallowponds. Increased incidence of parasitism by a flatworm(trematode) may account for deformities found insome frog species but not the worldwide decline ofamphibians.Overhunting can be a factor, especially in Asia andFrance, where frog legs are a delicacy. Populations ofsome amphibians can also be reduced by immigrationor introduction of nonnative predators and competitors(such as fish) and disease organisms. A combination ofsuch factors probably is responsible for the decline ordisappearance of most amphibian species.So why should we care if various amphibianspecies become extinct? Scientists give three reasons.First, it suggests that environmental quality is deterioratingin parts of the world because amphibians aresensitive biological indicators of changes in environmentalconditions such as habitat loss and degradation,pollution, UV exposure, and climate change.Second, adult amphibians play important ecologicalroles in biological communities. For example, amphibianseat more insects (including mosquitoes) thando birds. In some habitats, extinction of certain amphibianspecies could lead to extinction of otherspecies, such as reptiles, birds, aquatic insects, fish,mammals, and other amphibians that feed on them ortheir larvae.Third, from a human perspective, amphibians representa genetic storehouse of pharmaceutical productswaiting to be discovered. Compounds in hundreds ofsecretions from amphibian skin have been isolated andsome are used as painkillers and antibiotics and intreating burns and heart disease.The plight of some amphibian indicator species isa warning signal. They do not need us, but we andother species need them.What Are Keystone Species? Major PlayersWho Help Keep Ecosystems Running SmoothlyKeystone species help determine the types andnumbers of various other species in a community.A keystone is the wedge-shaped stone placed at thetop of a stone archway. Remove this stone and the archcollapses. In some communities, certain species calledkeystone species apparently play a similar role. Theyhave a much larger effect on the types and abundancesof many other species in a community than their numberswould suggest.According to this hypothesis, keystone speciesplay critical ecological roles. One is pollination of floweringplant species by bees, hummingbirds, bats, andother species. In addition, top predator keystone speciesfeed on and help regulate the populations of otherspecies. Examples are the wolf, leopard, lion, alligator(Connections, at right), and great white shark.Have you thanked a dung beetle today? Maybe youshould, because these keystone species rapidly remove,bury, and recycle animal wastes or dung. Withoutthem we would be up to our eyeballs in such waste,and many plants would be starved for nutrients. Thesebeetles also churn and aerate the soil, making it moresuitable for plant life.Ecologist Robert Paine conducted a controlled experimentalong the rocky Pacific coast of the state ofWashington that demonstrated the role of the sea starPiaster orchaceus as a keystone species in an intertidalzone community (Figure 7-9, top, p. 134). He removedsea stars from one community but not froman adjacent community, which served as a controlgroup. In both communities he monitored the populationsof 18 other species. In the community fromwhich he removed the sea stars, all of the 18 speciesdisappeared except mussels. In the community wherethe sea stars remained, they ate the mussels and keptthem from multiplying and crowding out otherspecies.The loss of a keystone species can lead to populationcrashes and extinctions of other species in a com-148 CHAPTER 8 Community Ecology