Environmental Problems, Their Causes, and Sustainability 1

Animal and vegetable life is too complicated a problem forhuman intelligence to solve, and we can never know howwide a circle of disturbance we produce in the harmonies ofnature when we throw the smallest pebble into the ocean oforganic life.GEORGE PERKINS MARSHThis chapter addresses the following questions:■■■■■10050What determines the number of species in acommunity?What different roles do species play in acommunity?How do species interact with one another in acommunity?How do communities change as environmentalconditions change?Does high biodiversity increase the stability of acommunity?8-1 COMMUNITY STRUCTUREAND SPECIES DIVERSITYWhat Is Community Structure? Appearance,Diversity, and NichesBiological communities differ in their physical appearance,the types and numbers of species they contain,and the ecological roles their species play.3020Ecologists use three characteristics to describe a biologicalcommunity. One is physical appearance: therelative sizes, stratification, and distribution of itspopulations and species, as shown in Figure 8-2 forvarious terrestrial communities. There are also differencesin the physical structures and zones of communitiesin aquatic life zones such as oceans, rocky shoresand sandy beaches, lakes, river systems, and inlandwetlands.The physical structure within a particular type ofcommunity or ecosystem can also vary. Most largeterrestrial communities and ecosystems consist of amosaic of vegetation patches of differing size. Life ispatchy.Community structure also varies around its edgeswhere one type of community makes a transition to adifferent type of community. For example, the edgearea between a forest and an open field may be sunnier,warmer, and drier than the forest interior andhave a different combination of species than the forestand field interiors.However, increased edge area from habitat fragmentationmakes many species more vulnerable tostresses such as predators and fire. It also creates barriersthat can prevent some species from colonizing newareas and finding food and mates.A second characteristic of a community is itsspecies diversity: acombination of its number of differentspecies (species richness) and the abundance of individualswithin each of its species (species evenness). Forexample, suppose we have two communities each witha total of 20 different species and 200 individuals andthus the same species diversity. But these communitiescould differ in their species richness and species evenness.For example, suppose community A has 10 individualsin each of its 20 species. And community B has10 species with 2 individuals each and 10 other species,each with 18 individuals. Which community has thehighest species evenness?A third characteristic is a community’s niche structure:the number of ecological niches, how they resembleor differ from each other, and how species interact10ftmTropicalrain forestConiferousforestDeciduousforestThornforestThornscrubTall-grassprairieShort-grassprairieDesertscrubFigure 8-2 Natural capital: generalized types, relative sizes, and stratification of plant species in various terrestrialcommunities.144 CHAPTER 8 Community Ecology