TAILIEUCHUNG - From Individuals to Ecosystems 4th Edition - Chapter 14

Chapter 14 Abundance Introduction Why are some species rare and others common? Why does a species occur at low population densities in some places and at high densities in others? What factors cause fluctuations in a species’ abundance? These are crucial questions. | Chapter 14 Abundance Introduction Why are some species rare and others common Why does a species occur at low population densities in some places and at high densities in others What factors cause fluctuations in a species abundance These are crucial questions. To provide complete answers for even a single species in a single location we might need ideally a knowledge of physicochemical conditions the level of resources available the organism s life cycle and the influence of competitors predators parasites etc. as well as an understanding of how all these things influence abundance through their effects on the rates of birth death and movement. In previous chapters we have examined each of these topics separately. We now bring them together to see how we might discover which factors actually matter in particular examples. The raw material for the study of abundance is usually some estimate of population size. In its crudest form this consists of a simple count. But this can hide vital information. As an example picture three human populations containing identical numbers of individuals. One of these is an old people s residential area the second is a population of young children and the third is a population of mixed age and sex. No amount of attempted correlation with factors outside the population would reveal that the first was doomed to extinction unless maintained by immigration the second would grow fast but only after a delay and the third would continue to grow steadily. More detailed studies therefore involve recognizing individuals of different age sex size and dominance and even distinguishing genetic variants. Ecologists usually have to deal with estimates of abundance that are deficient. First data may be misleading unless sampling is adequate over both space and time and adequacy of either usually requires great counting is not enough estimates are usually deficient studied species may not be typical commitment of time and money. The lifetime of

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