Finally notice that the two graphs do not have the

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Finally, notice that the two graphs do not have the same overall shape. Graph (a) is broad based, indicating a population with a large proportion of young individuals. Graph (b) is less broad based, indicating a much smaller proportion of young population. Pre- vious studies have shown that populations with broad-based, triangular-shaped age- structure population graphs tend to increase in size, while those with bullet-shaped graphs tend to be more stable . In any population, today’s young are tomorrow’s parents. Numerous young today often means even more young in the next generation. (What would be the shape of a population’s age-structure graph that would be expected to decrease in the future?) Using Life Tables Since demographics of human populations are always changing, life tables help to study and predict changes. For the most accurate of predictions, we need to think carefully about the assumptions on which they are based. Our predictions assume (1) no significant immigrations or emigrations and (2) constant birth and death rates. How would large- scale emigrations affect predictions of growth in an otherwise stable population? What factors of population growth would we need to focus on to stabilize a rapidly expanding population? Figure 15-14 Age-structure graphs portray a great deal of information about a population. Each row represents a separate age (or age class). Males are separated from females. Such graphs allow predictions as to future population growth. Populations producing high numbers of young (a) are likely to increase in the near future. Populations producing few young (b) are less likely to do so.
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506 C HAPTER 15 Population Ecology: How Do Organisms Interact to Form Populations? Populations Differ in Their Longevity and Survivorship Patterns There is wide variation in the typical life spans of different species (Figure 15-15). Some insects live only a few days.Annual plants live only a few months. Some mammals live for over a century. Giant tortoises may live for several centuries. Some bristlecone pines have lived for several millennia. How long would an organism live in an environment where nutrients and water are reliably available and where surroundings are comfortable (whatever that may mean for each species), free of predators, diseases, and other stresses? Under these ideal condi- tions, organisms would live until their physiological systems fail. That life span is their physiological longevity , which differs from species to species. At one extreme are the bristlecone pines whose physiological longevity is so long that they may simply live until something kills them. At relatively high altitudes in mountainous deserts in the south- western United States—ideal environments for bristlecone pines—a few individuals have reached ages of over 4200 years. Albeit old and gnarled, they show no signs of dying. Other organisms die even under conditions that are ideal for them. In temperate regions of North America, most praying mantids die with the onset of winter. Even those
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