Environmental Problems, Their Causes, and Sustainability 1

A third type of alteration is using, wasting, or destroyingan increasing percentage of the earth’s net primaryproductivity that supports all consumer species (includinghumans). This factor is the main reason we are crowdingout or eliminating the habitats and food suppliesof a growing number of other species.A fourth type of intervention has unintentionallystrengthened some populations of pest species and diseasecausingbacteria. This has occurred through overuseof pesticides and antibiotics that has speeded up naturalselection and caused genetic resistance to thesechemicals.A fifth effect has been to eliminate some predators.Some ranchers want to eliminate wolves, coyotes, eagles,and other predators that occasionally kill sheep.They also want to eradicate bison or prairie dogs thatcompete with their sheep or cattle for grass. A few biggamehunters push for elimination of predators thatprey on game species.Sixth, we have deliberately or accidentally introducednew or nonnative species into ecosystems. Most of thesespecies, such as food crops and domesticated livestock,are beneficial to us but a few are harmful to usand other species.Seventh, we have overharvested some renewable resources.Ranchers and nomadic herders sometimes allowlivestock to overgraze grasslands until erosionconverts these ecosystems to less productive semidesertsor deserts. Farmers sometimes deplete soil nutrientsby excessive crop growing. Some fish speciesare overharvested. Illegal hunting or poaching endangerswildlife species with economically valuable partssuch as elephant tusks, rhinoceros horns, and tigerskins. In some areas, fresh water is being pumped outof underground aquifers faster than it is replenished.Eighth, some human activities interfere with the normalchemical cycling and energy flows in ecosystems. Soilnutrients can erode from monoculture crop fields, treeplantations, construction sites, and other simplifiedecosystems and overload and disrupt other ecosystemssuch as lakes and coastal ecosystems. Chemicals suchas chlorofluorocarbons (CFCs) released into the atmospherecan increase the amount of harmful ultravioletenergy reaching the earth by reducing ozone levels inthe stratosphere. Emissions of carbon dioxide andother greenhouse gases—from burning fossil fuels andfrom clearing and burning forests and grasslands—cantrigger global climate change by altering energy flowthrough the troposphere.Ninth, while most natural systems are powered bysunlight, human-dominated ecosystems have become increasinglydependent on nonrenewable energy from fossilfuels. Fossil fuel systems typically produce pollution,add more of the greenhouse gas carbon dioxide to theatmosphere, and waste much more energy than theyneed to.To survive we must exploit and modify parts ofnature. However, we are beginning to understandthat any human intrusion into nature has multiple effects,most of them unintended and unpredictable(Figure 3-4, p. 38 and Connections, below).We face two major challenges. First, we need tomaintain a balance between simplified, human-alteredecosystems and the more complex natural ecosystemsEcological SurprisesMalaria once infected9 out of10 people in NorthBorneo, nowCONNECTIONS known as Sabah.In 1955, theWorld Health Organization (WHO)began spraying the island withdieldrin (a DDT relative) to killmalaria-carrying mosquitoes. Theprogram was so successful thatthe dreaded disease was nearlyeliminated.But unexpected things beganto happen. The dieldrin also killedother insects, including flies andcockroaches living in houses. Theislanders applauded. But thensmall insect-eating lizards thatalso lived in the houses died aftergorging themselves on dieldrincontaminatedinsects.Next, cats began dying afterfeeding on the lizards. Then, in theabsence of cats, rats flourished andoverran the villages. When the peoplebecame threatened by sylvaticplague carried by rat fleas, theWHO parachuted healthy cats ontothe island to help control the rats.Operation Cat Drop worked.But then the villagers’ roofs beganto fall in. The dieldrin hadkilled wasps and other insects thatfed on a type of caterpillar thateither avoided or was not affectedby the insecticide. With most ofits predators eliminated, the caterpillarpopulation exploded, munchingits way through its favoritefood: the leaves used in thatchedroofs.Ultimately, this episode endedhappily: both malaria and the unexpectedeffects of the sprayingprogram were brought under control.Nevertheless, this chain of unintendedand unforeseen eventsemphasizes the unpredictability ofinterfering with an ecosystem. It remindsus that when we intervene innature, we need to ask, “Now whatwill happen?”Critical ThinkingDo you believe the beneficial effectsof spraying pesticides on Sabah outweighedthe resulting unexpectedand harmful effects? Explain.http://biology.brookscole.com/miller14173