Environmental Problems, Their Causes, and Sustainability 1

Number of reindeer2,0001,5001,0005000CarryingcapacityPopulationovershootscarryingcapacityPopulationcrashes1910 1920 1930 1940 1950YearFigure 9-6 Exponential growth, overshoot, and populationcrash of reindeer introduced to a small island off the southwestcoast of Alaska. When 26 reindeer (24 of them female) were introducedin 1910, lichens, mosses, and other food sourceswere plentiful. By 1935, the herd’s population had soared to2,000, overshooting the island’s carrying capacity. This led to apopulation crash, with the herd plummeting to only 8 reindeerby 1950.About 1 million people died, and 3 million others migratedto other countries.Technological, social, and other cultural changeshave extended the earth’s carrying capacity for humans.We have increased food production and usedlarge amounts of energy and matter resources to makenormally uninhabitable areas habitable. A criticalquestion is how long we will be able to keep doing thison a planet with a finite size and finite resources, andwith a human population whose size and per capitaresource use is growing exponentially.xHOW WOULD YOU VOTE? Can we continue expanding theearth’s carrying capacity for humans? Cast your vote online athttp://biology.brookscole.com/miller14.How Does Population Density AffectPopulation Growth? Some Effects ofClumpingA population’s density may or may not affect howrapidly it can grow.Population density is the number of individuals in apopulation found in a particular space. Density-independentpopulation controls affect a population’s sizeregardless of its density. Such controls include floods,hurricanes, unseasonable weather, fire, habitat destruction(such as clearing a forest of its trees or fillingin a wetland), pesticide spraying, and pollution.For example, a severe freeze in late spring can killmany individuals in a plant population, regardless ofdensity.Some factors that limit population growth have agreater effect as a population’s density increases. Examplesof such density-dependent population controls includecompetition for resources, predation, parasitism,and infectious disease.Infectious disease is a classic type of densitydependentpopulation control. An example is thebubonic plague, which swept through densely populatedEuropean cities during the 14th century. The bacteriumcausing this disease normally lives in rodents. Itwas transferred to humans by fleas that fed on infectedrodents and then bit humans. The disease spread likewildfire through crowded cities, where sanitary conditionswere poor and rats were abundant. At least25 million people in European cities died from thedisease.What Kinds of Population Change CurvesDo We Find in Nature? Variety Is the Spiceof LifePopulation sizes may stay about the same, suddenlyincrease and then decrease, vary in regular cycles, orchange erratically.In nature we find four general types of populationfluctuations: stable, irruptive, cyclic, and irregular (Figure9-7, p. 168). A species whose population size fluctuatesslightly above and below its carrying capacity issaid to have a fairly stable population size (Figures 9-5and 9-7a). Such stability is characteristic of manyspecies found in undisturbed tropical rain forests,where average temperature and rainfall vary littlefrom year to year.Some species, such as the raccoon and feral housemouse, normally have a fairly stable population. However,their population growth may occasionally explode,or irrupt, to a high peak and then crash to amore stable lower level or in some cases to a very lowlevel (Figures 9-6 and 9-7b). Many short-lived, rapidlyreproducing species such as algae and many insectshave irruptive population cycles that are linked to seasonalchanges in weather or nutrient availability. Forexample, in temperate climates, insect populationsgrow rapidly during the spring and summer and thencrash during the hard frosts of winter.The third type consists of cyclic fluctuations of populationsize over a regular time period (Figure 9-7c). Examplesare lemmings, whose populations rise and fallevery 3–4 years, and lynx and snowshoe hare, whosepopulations generally rise and fall in a 10-year cycle.Finally, some populations appear to have irregularbehavior in their changes in population size, withno recurring pattern (Figure 9-7d). Some scientistsattribute this behavior to chaos in such systems.Other scientists contend that it may be orderly behaviorwhose details and interactions are still poorlyunderstood.http://biology.brookscole.com/miller14167