A population consists of all the individuals of a species that live in a certain area. All organisms belong to a population. Animal populations can range in size from a small, countable population of an endangered species to large populations that can only be estimated (e.g., some insects). The number of plants in a population can be determined by counting individuals in small grids and then making an estimate of the population for the larger area. The total human population, like populations of all other living things, fluctuates every second as new individuals are born and others die.

At A Glance

  • A population is a group of individuals of the same species living in a given area.
  • The density of a population is the number of individuals of a population per unit of area.
  • The dispersion of individuals in a population may be clumped, uniform, or random.
  • Life history traits include significant life events of a species, such as reproductive rate and reproductive lifespan.
  • A life table shows the developmental stages of a single reproductive event of a species. It can be generated using a cohort table, which follows many individuals of a species throughout their lives.
  • Survivorship curves depend on mortality rates and age structure.
  • Populations increase when the ability to reproduce (fecundity) overrides the loss of individuals (mortality).
  • Introduced species, which are species not native to an ecosystem that have been introduced by human activity, actively compete against native species, which are species normally found in an ecosystem. An introduced species can become an invasive species if it reproduces rapidly and spreads widely.
  • Carrying capacity, or the number of individuals that can be supported by available resources, affects populations both positively and negatively.
  • Populations are affected by both abiotic (nonliving) and biotic (living) factors.
  • Exponential and logistic demographic models are used in ecological planning; that is, planning land development in a way to minimize environmental impact.
  • Exponential growth is the result of unlimited expansion in ideal conditions.
  • Logistic growth models adjust for the carrying capacity of a population, which is the number of individuals the environment can support.
  • Human population growth involves birth, death, and age–sex structure.
  • An age-sex structure shows the number of males and females in each age category alive at a given time.