7.1: Population Growth

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This material in this chapter has been adapted from Donovan and Welden (2002).

Donovan, T. M. and C. Welden. 2002. Spreadsheet exercises in ecology and evolution. Sinauer Associates, Inc. Sunderland, MA, USA.

Chapter Objectives

• Understand the demographic processes that affect population size and growth, including raw birth and death rates, per capita birth and death rates, rates of immigration and emigration, and carrying capacity.

• Explore the derivations of geometric (discrete-time), exponential (continuous-time), and logistic models of populations.

• Investigate the relationships among geometric, exponential, and logistic models.

• Understand the concepts of density dependence and density independence.

The study of population dynamics has been and continues to be an important area of investigation in ecology. A population is a group of individual organisms belonging to the same species living in the same area at the same time. Members of a population are often considered to be actually or potentially interbreeding or exchanging genes.

The term population dynamics means change in population size (number of individuals) or population density (number of individuals per unit area) over time. In general, population dynamics are influenced by four fundamental demographic processes: birth, death, immigration (individuals moving into the population), and emigration (individuals moving out of the population).

We will ignore immigration and emigration so that we may concentrate on births and deaths. For many populations (e.g., the human population of the earth) this is a realistic simplification. Other populations (e.g., the human population of the United States) are more open, however, and immigration and emigration must be considered. Fortunately, the addition of immigration and emigration does not complicate the models very much.

We will focus on two types of models: 1) geometric and exponential models, 2) logistic models. Unlike geometric and exponential models, logistic models take into account a population's carrying capacity (the maximum population size of a biological species that can be sustained by that specific environment).

A population of Imperial cormorants in Chubut, Argentina. Photo taken by Natasha Gownaris.