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5.1.5: Chapter Summary

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    Populations are characterized by their population size (total number of individuals) and their population density (number of individuals per unit area). Counting all individuals in a population is the most accurate way to determine its size. However, this approach is not usually feasible, especially for large populations or extensive habitats. Instead, scientists study populations by sampling, which involves counting individuals within a certain area or volume that is part of the population’s habitat. Analysis of sample data enable scientists to infer population size and population density about the entire population. One method used the collect population size data in the field is using quadrat placed at random locations or along a transect in representative habitat. The size of the quadrat depends on the species monitoring. In addition to measuring simple density, further information about a population can be obtained by looking at the distribution of the individuals. Species may have uniform, random, or clumped distribution.

    All species have evolved a pattern of living, called a life history strategy, in which they partition energy for growth, maintenance, and reproduction. These patterns evolve through natural selection; they allow species to adapt to their environment to obtain the resources they need to successfully reproduce. There are different life strategies that species may exhibit. A species may reproduce early in life to ensure surviving to a reproductive age or reproduce later in life to become larger and healthier. A species may reproduce once (semelparity) or many times (iteroparity) in its life.

    Populations with unlimited resources grow exponentially, with an accelerating growth rate. When resources become limiting, populations follow a logistic growth curve. The population of a species will level off at the carrying capacity of its environment. Within populations, phenotypic variation among individuals means that some individuals will be better adapted to their environment than others. The resulting competition between population members of the same species for resources is termed intraspecific competition. Populations are regulated by a variety of density-dependent and density-independent factors. Species are divided into two categories based on a variety of features of their life history patterns: r-selected species, which have large numbers of offspring, and K-selected species, which have few offspring.

    After completing this chapter, you should be able to...
    • Describe how ecologists measure population size and density.
    • Describe three different patterns of population distribution.
    • Describe how life history patterns are influenced by natural selection.
    • Compare and contrast semelparity and iteroparity.
    • Compare and contrast between exponential and logistic growth patterns.
    • Give examples of exponential and logistic growth in natural populations.
    • Describe the role of carrying capacity in population growth.
    • Describe the influences of intraspecific competition in population size.
    • Compare and contrast density-dependent growth regulation and density-independent growth regulation.
    • Compare and contrast K-selected and r-selected species using examples of their life history traits.

    This page titled 5.1.5: Chapter Summary is shared under a not declared license and was authored, remixed, and/or curated by Melissa Ha, Maria Morrow, & Kammy Algiers (ASCCC Open Educational Resources Initiative) .