Environmental Problems, Their Causes, and Sustainability 1

Species Diversity200100Species Diversity1,000100Figure 8-3 Changes inspecies diversity at different latitudes(distances from the equator) in terrestrialcommunities for (a) ants and(b) breeding birds of North andCentral America. As a general rule,species diversity steadily declines aswe go away from the equator towardeither pole. (Modified by permissionfrom Cecie Starr, Biology: Conceptsand Applications, 4th ed., 2000,Brooks/Cole [Wadsworth])0 10(a) Ants90°N 6030 0 30°S 60 80°NLatitude(b) Breeding birds60 40Latitude20 0with one another. Studies indicate that the most speciesrichenvironments are tropical rain forests, coral reefs,the deep sea, and large tropical lakes. Communitiessuch as a tropical rain forest or a coral reef with a largenumber of different species (high species richness) generallyhave only a few members of each species (lowspecies evenness).Several factors affect the species diversity in communities.One is latitude (distance from the equator) interrestrial communities (Figure 8-3). For most plantsand animals, species diversity is highest in the tropicsand declines from the equator to the poles. Another factoris pollution in aquatic systems (Figure 8-4). Other factorsare habitat diversity, NPP, habitat disturbance, andtime.Number of diatom speciesUnpollutedstreamPollutedstreamFigure 8-4 Changes in the species diversity and species abundanceof diatom species in an unpolluted stream and a pollutedstream. Both species diversity and species abundance decreasewith pollution.What Determines the Number of Specieson Islands? Entrances and ExitsThe number of species on an island is determined byhow fast new species arrive and old species becomeextinct, the island’s size, and how far it is from themainland.In the 1960s, Robert MacArthur and Edward O. Wilsonbegan studying communities on islands to discoverwhy large islands tend to have more species of a certaincategory such as insects, birds, or ferns than dosmall islands. To explain these differences in speciesrichness with island size, MacArthur and Wilson proposedwhat is called the species equilibrium model,or the theory of island biogeography. According tothis widely accepted model, a balance between twofactors determines the number of different speciesfound on an island: the rate at which new species immigrateto the island and the rate at which existingspecies become extinct on the island.The model projects that at some point the rates ofimmigration and extinction should reach an equilibriumpoint (Figure 8-5a, p. 146) that determines the island’saverage number of different species. This is afairly complex idea, so study Figure 8-5 carefully. TheCD that comes with this book has a great animation ofthis model. Check it out.According to the model, two features of an islandaffect its immigration and extinction rates and thus itsspecies diversity. One is the island’s size, with a smallisland tending to have fewer different species than alarge one (Figure 8-5b). One reason is that a small islandgenerally has a lower immigration rate because itis a smaller target for potential colonizers. In addition,a small island should have a higher extinction rate becauseit usually has fewer resources and less diversehabitats for its species.http://biology.brookscole.com/miller14145