After completing this chapter you should be able to...
- Describe three different population dispersion patterns.
- Differentiate between population size and density.
- Explain how ecologists measure population size.
- Calculate population growth rate (r) and doubling time (t).
- Distinguish between exponential and logistic population growth models, explaining the role of the carrying capacity in logistic growth.
- Provide examples of density-dependent and density-independent factors that regulate populations.
- Compare K-selected and r-selected reproductive strategies.
- Interpret life tables.
- Compare type I, type II, and type III survivorship curves.
Populations are interacting, interbreeding groups of individuals from the same species. Ecologists measure characteristics of populations: dispersion pattern, population size, and population density. Populations with unlimited resources grow exponentially—with an accelerating growth rate. When resources become limiting, populations follow a logistic growth curve in which population size will level off at the carrying capacity. Density-dependent factors limit population growth as they reach their carrying capacity and include biotic factors such as predation, competition, and disease. Density-independent factors, such as storms and fires, are abiotic and decrease population size regardless of density.
Several frameworks explain how life history can influence population dynamics. K-selected species tend to have long life spans and produce few offspring with much parental care whereas r-selected species mature and reproduce rapidly, producing many offspring and offering little parental care. Life tables are useful to calculate life expectancies of individual population members. Survivorship curves show the number of individuals surviving at each age interval plotted versus time.