Environmental Problems, Their Causes, and Sustainability 1

According to this hypothesis, mussel beds arehomes to hundreds of invertebrate species that dopoorly in the presence of mussel competitors such assea stars. When scientists measured the overall diversityof the species in a tide pool rather than just the 18species observed by Paine they found that the overalldiversity of species was greater when the keystone seastar species was absent. Its absence allowed the numberof mussel species and the species they interact withto expand. From this point of view, the mussels shouldbe viewed as a foundation species that expanded speciesrichness.8-3 SPECIES INTERACTIONS:COMPETITION AND PREDATIONHow Do Species Interact? Ways to Getan EdgeCompetition, predation, parasitism, mutualism,and commensalism are ways in which speciescan interact and increase their ability tosurvive.When different species in a community have activitiesor resource needs in common, they may interact withone another. Members of these species may be harmed,helped, or unaffected by the interaction. Ecologistsidentify five basic types of interactions between species:interspecific competition, predation, parasitism, mutualism,and commensalism.The most common interaction between species iscompetition for shared or scarce resources such as spaceand food. Ecologists call such competition betweenspecies interspecific competition.When this occurs, parts of the fundamental nichesof the competing species overlap (Figure 5-7, p. 94).With significant overlap, one of the competing speciesmust migrate, if possible, to another area, shift its feedinghabits or behavior through natural selection andevolution, suffer a sharp population decline, or becomeextinct in that area.Humans are in competition with many otherspecies for space, food, and other resources. As weconvert more and more of the earth’s land and aquaticresources and net primary productivity to our uses wedeprive many other species of resources they need tosurvive.Over a time scale long enough for evolution to occur,some species competing for the same resources evolveadaptations that reduce or avoid competition. Oneway this happens is through resource partitioning. Itoccurs when species competing for similar scarce resourcesevolve more specialized traits that allow themto use shared resources at different times, in differentways, or in different places.Through evolution, the fairly broad niches of twocompeting species (Figure 8-7, top) can become morespecialized (Figure 8-7, bottom) so that the species canshare limited resources. When lions and leopards livein the same area, lions take mostly larger animals asprey, and leopards take smaller ones. Hawks and owlsfeed on similar prey, but hawks hunt during the dayand owls hunt at night.Ecologist Robert H. MacArthur studied the feedinghabits of five species of North American warblers(small insect-eating birds) that hunt for insects andnest in the same type of spruce tree. He found that thebird species minimized the overlap of their niches andreduced competition among the species through resourcepartitioning. They did this by concentratingmuch of their hunting for insects in different parts ofthe spruce trees (Figure 8-8), employing differentNumber of individualsNumber of individualsSpecies 1Regionofniche overlapResource useSpecies 2Species 1 Species 2How Have Some Species Reducedor Avoided Competition? Sharethe Wealth by Becoming MoreSpecializedSome species evolve adaptations that allow them toreduce or avoid competition for resources with otherspecies.Resource useFigure 8-7 Natural capital: resource partitioning and nichespecialization as a result of competition between two species.The top diagram shows the overlapping niches of two competingspecies. The bottom diagram shows that through evolutionthe niches of the two species become separated and more specialized(narrower) so that they avoid competing for the sameresources.150 CHAPTER 8 Community Ecology