Environmental Problems, Their Causes, and Sustainability 1

HighHighHighRate of immigrationor extinctionImmigrationExtinctionRate of immigrationor extinctionImmigration(large island)Immigration(small island)Extinction(small island)Extinction(large island)Rate of immigrationor extinctionImmigration(near island)Immigration(far island)ExtinctionLowEquilibrium numberLowSmall islandLarge islandLowFar islandNear islandNumber of species on island(a) Immigration and extinction ratesNumber of species on island(b) Effect of island sizeNumber of species on island(c) Effect of distance from mainlandFigure 8-5 The species equilibrium model or theory of island biogeography, developed byRobert MacArthur and Edward O. Wilson. (a) The equilibrium number of species (blue triangle) onan island is determined by a balance between the immigration rate of new species and the extinctionrate of species already on the island. (b) With time, large islands have a larger equilibriumnumber of species than smaller islands because of higher immigration rates and lower extinctionrates on large islands. (c) Assuming equal extinction rates, an island near a mainland will have alarger equilibrium number of species than a more distant island because the immigration rate isgreater for a near island than for a more distant one.A second factor is an island’s distance from thenearest mainland (Figure 8-5c). According to the model,if we have two islands about equal in size and otherfactors, the island closer to a mainland source of immigrantspecies should have the higher immigrationrate and thus a higher species richness—assumingthat extinction rates on both islands are about thesame.In recent years, scientists have used the model tohelp protect wildlife in habitat islands such as nationalparks surrounded by a sea of developed and fragmentedland.8-2 TYPES OF SPECIESWhat Roles Do Various Species Play inCommunities? A Biological Play with ManyPartsCommunities can contain native, nonnative, indicator,keystone, and foundation species that play differentecological roles.Ecologists often use labels—such as native, nonnative,indicator, keystone, or foundation—to describe the majorecological roles or niches various species play in communities.Any given species may play more than oneof these five roles in a particular community.Native species are those that normally live andthrive in a particular community. Others that evolvedsomewhere else and then migrate into or are deliberatelyor accidentally introduced into a community arecalled nonnative species, invasive species, or alienspecies.Throughout the earth’s long history of life speciesin one part of the world have migrated from one communityto another. But moving into a new communityis not easy. Most organisms arriving in a newcommunity do not survive because they are not ableto find a suitable niche under their new environmentalconditions.Many people tend to think of nonnative or invasivespecies as villains. But most introduced and domesticatedspecies of crops and animals such as chickens,cattle, and fish from around the world and many wildgame species are beneficial to us. Indeed, most of thefood crops and domesticated animals we depend onare not native to the communities where we raise them.But sometimes a nonnative species can thrive andcrowd out native species—an example of unintendedand unexpected consequences. In 1957, for example,Brazil imported wild African bees to help increasehoney production. Instead, the bees displaced domestichoneybees and reduced the honey supply.Since then, these nonnative bee species, popularlyknown as “killer bees,” have moved northward intoCentral America and parts of the southwestern UnitedStates, such as Texas, Arizona, New Mexico, andCalifornia. They are still heading north but should bestopped eventually by cold winters in the centralUnited States unless they can adapt genetically to coldweather.146 CHAPTER 8 Community Ecology