Describing Populations

Researchers study five important characteristics of a population:

• Geographic range is the area in which a population lives.

• Population density is the number of individuals per unit area.

• Population distribution is how individuals are spaced out in their range.

• Growth rate determines whether a population grows, shrinks, or stays the same size.

• Age structure is the number of males and females of each age in a population.

Population Growth

Populations can grow, shrink, or stay the same size.

• Factors that increase population size include births and immigration, which is the movement of individuals into an area.

• Factors that decrease population size include deaths and emigration, which is the movement of individuals out of an area.

Exponential Growth

When conditions are ideal, the larger a population gets, the faster it grows. When a population’s numbers grow larger with each generation, exponential growth is occurring. Ideal conditions include unlimited resources and absence of predation and disease.

Logistic Growth

Resources become less available as a population grows.

Logistic growth occurs when population growth slows and then stops after a period of exponential growth has occurred.

Population size stabilizes at the carrying capacity, the maximum number of individuals of a given species that an environment can support.

5.2 Limits to Growth

Limiting Factors

A limiting factor is a factor that controls the growth of a population.

Some factors depend on the density of the population. Others do not.

Acting separately or together, limiting factors determine an environment’s carrying capacity.

Limiting factors produce the pressures of natural selection.

Density-dependent Limiting Factors

Density-dependent limiting factors operate strongly when the number of individuals per unit area reaches a certain point.

Examples include:

·  competition

·  predation and herbivory

·  parasitism and disease

·  stress from overcrowding

Density-independent Limiting Factors

Some limiting factors do not necessarily depend on population size.

Density-independent limiting factors depend on population density, or the number of organisms per unit area.

Examples include severe weather, natural disasters, and human activities.

Some of these factors may have more severe effects when population density is high.