Environmental Problems, Their Causes, and Sustainability 1

-Selected SpeciesK-Selected SpeciesCockroachDandelionElephantSaguaroMany small offspringFewer, larger offspringLittle or no parental care and protection of offspringEarly reproductive ageMost offspring die before reaching reproductive ageSmall adultsAdapted to unstable climate and environmental conditionsHigh population growth rate (r)Population size fluctuates wildly above and below carryingcapacity (K)Generalist nicheLow ability to competeEarly successional speciesHigh parental care and protection of offspringLater reproductive ageMost offspring survive to reproductive ageLarger adultsAdapted to stable climate and environmental conditionsLower population growth rate (r)Population size fairly stable and usually close to carryingcapacity (K)Specialist nicheHigh ability to competeLate successional speciesFigure 9-10 Generalized characteristics of r-selected or opportunist species and K-selected or competitorspecies. Many species have characteristics between these two extremes.They are called K-selected species because theytend to do well in competitive conditions when theirpopulation size is near the carrying capacity (K) oftheir environment. Their populations typically followa logistic growth curve.Most large mammals (such as elephants, whales,and humans), birds of prey, and large and long-livedplants (such as the saguaro cactus, oak trees, and mosttropical rain forest trees) are K-selected species. ManyK-selected species—especially those with long generationtimes and low reproductive rates like elephants,rhinoceroses, and sharks—are prone to extinction.Most organisms have reproductive patterns betweenthe extremes of r-selected species and K-selected species, or they change from one extreme tothe other under certain environmental conditions. Inagriculture we raise both r-selected species (crops) andK-selected species (livestock).The reproductive pattern of a species may give it atemporary advantage. But the availability of suitablehabitat for individuals of a population in a particular area iswhat determines its ultimate population size. Regardlessof how fast a species can reproduce, there can be nomore dandelions than there is dandelion habitat andno more zebras than there is zebra habitat in a particulararea.What Are Survivorship Curves?At What Age Is Death Most Likely?The populations of different speciesvary in how long individual memberstypically live.Individuals of species with different reproductivestrategies tend to have different life expectancies. Oneway to represent the age structure of a population iswith a survivorship curve, which shows the percentagesof the members of a population surviving at differentages. There are three generalized types ofsurvivorship curves: late loss, early loss, and constant loss(Figure 9-11). Which type of curve applies to the humanspecies?A life table shows the numbers of individuals ateach age on a survivorship curve. It shows the projectedlife expectancy and probability of death for individualsat each age.Insurance companies use life tables of humanpopulations to determine policy costs for customers.Life tables show that women in the United Statessurvive an average of 6 years longer than men. Thisexplains why a 65-year-old American man normallypays more for life insurance than a 65-year-oldAmerican woman.170 CHAPTER 9 Population Ecology