Brought into the laboratory or classroom in the fall

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brought into the laboratory or classroom in the fall die in midwinter, despite efforts to keep them alive. They, as do most organisms, appear to be programmed to die at some age no matter what. Some animals living in zoos come close to living under ideal conditions and approach their physiological longevity.That animals tend to live longer in zoos than they do in the wild suggests there is another type of longevity at work in natural populations. Ecological longevity is the age to which an individual might be expected to live in a given environment. In nature, predators, diseases, accidents, harsh physical conditions, or a combination of such factors typically end each individual’s life before its physio- logical time runs out. For example, bristlecone pines that live at altitudes lower than Figure 15-15 Some organisms, such as this tiger swallowtail butterfly (a) , live for only a few weeks. Others, such as the bristlecone pine (b) , may live for centuries. (a) (b)
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15-3 How Do Populations Grow? 507 those mentioned previously do not live for millennia partly because population densities are higher,permitting periodic wildfires to sweep through,killing the closely packed trees. Humans have come a long way in controlling their ecological longevity. Most of our predators are gone, our disease organisms are under control (temporarily, at least), and our abilities to produce and distribute food expand faster than our population grows (again, temporarily, at least). Ecological longevity, in most human populations, extends accordingly.What is our physiological longevity? No one knows for sure, but the oldest humans alive today may be approaching it.Can our physiological longevity be extended? If it can be, should it be? These are important questions that society needs to answer soon.What do you think? Species also differ in their patterns of survival. Most plants, invertebrates, fishes, and some reptiles have high mortality in their juvenile stages (seeds and seedlings; eggs and larvae). Relatively few individuals survive these critical early stages.Those that do have a good chance for continued survival. Survivorship of juvenile stages of other rep- tiles and most birds and mammals is enhanced by parental care. Still, mortality rates are relatively high during early life stages and are highest in the first critical months after young disperse from their parents (Figure 15-16). Later, as reproducing adults, these animals have low mortality; that is, most adults survive. Mortality is high again among the very old, perhaps as they approach their potential physiological longevity. Piecing It Together 1. Two opposing forces affect population size: Biotic potential (the population’s ability to increase) and environmental resistance (environmental factors that impede growth).
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