Environmental Problems, Their Causes, and Sustainability 1

Percentage surviving (log scale)10010100.01Constant lossLate lossEarly lossAgeFigure 9-11 Three general survivorship curves for populationsof different species, obtained by showing the percentages ofthe members of a population surviving at different ages. A lateloss population (such as elephants, rhinoceroses, and humans)typically has high survivorship to a certain age, then high mortality.A constant loss population (such as many songbirds)shows a fairly constant death rate at all ages. For an early losspopulation (such as annual plants and many bony fish species),survivorship is low early in life. These generalized survivorshipcurves only approximate the behavior of species.9-3 EFFECTS OF GENETIC VARIATIONSON POPULATION SIZEWhat Role Does Genetics Play in the Sizeof Populations? The Vulnerability of SmallIsolated PopulationsVariations in genetic diversity can affect the survivalof small, isolated populations.In most large populations genetic diversity is fairly constant.The loss or addition of individuals has little effecton the total gene pool.However, genetic factors can affect the survivaland genetic diversity of small, isolated populations.Several factors can play a role in the loss of genetic diversityand the survival of such populations. One is thefounder effect when a few individuals in a populationcolonize a new habitat that is geographically isolatedfrom other members of the population (Figure 5-7,p. 94). In such cases, limited genetic diversity or variabilitymay threaten the survival of the colonizing population.Another problem is a demograpic bottleneck. Itoccurs when only a few individuals in a populationsurvive a catastrophe such as a fire or hurricane. Lackof genetic diversity may limit the ability of these individualsto rebuild the population. A third factor isgenetic drift. It involves random changes in the genefrequencies in a population that can lead to unequal reproductivesuccess. For example, some individualsmay breed more than others and their genes mayeventually dominate the gene pool of the population.This change in gene frequency could help or hinderthe survival of the population. The founder effect isone cause of genetic drift. A fourth factor is inbreeding.It occurs when individuals in a small population matewith one another. This can increase the frequency ofdefective genes within a population and affect its longtermsurvival.What are Metapopulations? Exchanging GenesNow and ThenVariations in genetic diversity can affect the survivalof small, isolated populations.Some mobile populations that are geographically separatedfrom one another can exchange genes whensome of their membeers get together occasionally andmate. Such collections of interacting local populationsof a species are called metapopulations.Some local populations where birth rates arehigher than death rates produce excess individualsthat can migrate to other local populations. Other localpopulations where death rates are greater than birthrates can accept individuals from other populations.Conservation biologists can map out the locations ofmetapopulations and use this information to providecorridors and migration routes to enhance the overallpopulation size, genetic diversity, and survial of relatedlocal populations.9-4 HUMAN IMPACTS ON NATURALSYSTEMS: LEARNING FROM NATUREHow Have Humans Modified NaturalEcosystems? Our Big FootprintsWe have used technology to alter much of the restof nature in ways that threaten the survival of manyother species and could reduce the quality of life forour own species.In this and the six preceding chapters we have lookedat key concepts of science and ecology. It is time to reviewwhat lessons we can learn from this study of hownature operates and sustains itself. But first let us lookat our environmental impact on the earth.To survive and provide resources for growingnumbers of people, we have modified, cultivated,built on, or degraded a large and increasing area of theearth’s natural systems. Excluding Antarctica, our activitieshave directly affected to some degree about83% of the earth’s land surface (Figure 9-12, p. 172).Figure 9-13 (p. 172) compares some of the characteristicsof natural and human-dominated systems.http://biology.brookscole.com/miller14171