As we learn more and more about the relationships

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effects of freezing temperatures may, in fact, be density dependent. As we learn more and more about the relationships between species and their environments,we often learn that factors that at first appeared to be independent of density are, in fact, not so. See if you can think of other density-dependent, extrinsic, intrinsic, and density- independent factors that limit populations. K -Selection and r -Selection Are Two Patterns of Population Growth Biologists study population size because they want to predict how it may change. As we have seen,three factors are important:The number of individuals already present,the pop- ulation’s carrying capacity, and its growth rate.The number of individuals changes as the population grows, but the carrying capacity and growth rate are often relatively constant. Some species have adaptations that permit them to live in a state of equilibrium close to their carrying capacities for long periods. Such species are said to be K -selected . Other species have different adaptations that permit them to increase their numbers rapidly when their populations are below their carrying capacities. These species are said to be r -selected . K -selected and r -selected species differ in many ways, as summa- rized in Figure 15-13. Characteristics of K -selected species Characteristics of r -selected species Live in stable environments Live in stressful environments Are ecological specialists Are ecological generalists Have populations stable in size Have populations that fluctuate rapidly in size Do not compete well against other species Compete well against other species Are restricted in where they can live Are widely distributed Are slow to respond to ecological Take rapid advantage of ecological opportunities opportunities Are long lived Are short lived Have few, relatively large young Have many, relatively small young Have long periods of embryonic Have short periods of embryonic development development Reach adulthood slowly Reach adulthood rapidly Invest intensive parental care in young Invest little or no parental care in young Reproduce throughout their lifetime Reproduce once in their lifetime
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504 C HAPTER 15 Population Ecology: How Do Organisms Interact to Form Populations? Human Life Table: Study a life table for a human population. You can probably think of species that fit into each category. You can also proba- bly think of species that do not fit easily into either category.Where, for example, do hu- mans fit? These two categories can be thought of as the extremes of a continuum, with most species fitting in somewhere between the extremes. The concept of K - and r - selection is quite useful in categorizing species and predicting how populations will be- have. For example, most of our endangered species are K -selected. They tend to be sensitive to environmental conditions and intolerant of habitat loss. They tend to have special ecological requirements.Their numbers change slowly. Most of our pests, on the other hand,are r -selected.They seem to be able to live anywhere.Their numbers are hard to control and fluctuate widely.
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