Environmental Problems, Their Causes, and Sustainability 1

In looking at nature ... never forget that every single organicbeing around us may be said to be striving to increase itsnumbers.CHARLES DARWIN, 1859This chapter addresses the following questions:■■■■■How do populations change in size, density,makeup, and distribution in response to environmentalstress?How do species differ in their reproductivepatterns?What role does genetics play in the size and survivalof a population?What are the major impacts of human activitieson populations, communities, and ecosystems?What lessons can we learn from ecology aboutliving more sustainably?9-1 POPULATION DYNAMICSAND CARRYING CAPACITYWhat Are the Major Characteristics of aPopulation? Changing and ClumpingPopulations change in size, density, and age distribution,and most members of populations livetogether in clumps or groups.Population dynamics is a study of how populationschange in size (total number of individuals), density(number of individuals in a certain space), and age distribution(the proportion of individuals of each age in apopulation) in response to changes in environmentalconditions.Populations can also change in how they are distributedin their habitat. Three general patterns of populationdistribution or dispersion in a habitat are clumping,uniform dispersion, and random dispersion (Figure 9-2).The populations of most species live in clumps orgroups (Figure 9-2a). Examples are patches of vegetation,cottonwood trees clustered along streams, wolfpacks, flocks of geese, and schools of fish. Viewedfrom above, most of the world’s landscapes are patchywith clumps of various plant and animal speciesfound here and there. The same thing is found whenwe view the underwater world and the soil beneathour feet.Why clumping? Four reasons. First, the resourcesa species needs vary greatly in availability from placeto place. Second, living in herds, flocks, and schools canprovide better protection from predators. Third, livingin packs gives some predator species such as wolves abetter chance of getting a meal. Fourth, some animalspecies form temporary groups for mating and caringfor their young.Some species maintain a fairly constant distancebetween individuals. By having this patter creosotebushes in a desert (Figure 9-2b) have better access toscarce water resources. Organisms with a random distribution(Figure 9-2c) are fairly rare. The world ismostly clumpy.What Factors Govern Changes in PopulationSize? Entrances and Exits on the Global StagePopulations increase through births and immigrationand decrease through deaths and emigration.Four variables—births, deaths, immigration, and emigration—governchanges in population size. A populationincreases by birth and immigration and decreases bydeath and emigration:Populationchange (Births Immigration) (Deaths Emigration)These variables depend on changes in resource availabilityand other environmental changes (Figure 9-3).A population’s age structure can have a strong effecton how rapidly its size increases or decreases. Agestructures are usually described in terms of organismsthat are not mature enough to reproduce (the prereproductivestage), those that are capable of reproduction(the reproductive stage), and those that are too old to reproduce(the postreproductive stage).Figure 9-2 Generalized dispersionpatterns for individualsin a population throughouttheir habitat. The mostcommon pattern is clumpsof members of a populationthroughout their habitat,mostly because resourcesare usually found inpatches.(a) Clumped (elephants)(b) Uniform (creosote bush) (c)Random (dandelions)164 CHAPTER 9 Population Ecology