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Summary

Population density is the number of individuals of a species per unit of area (or volume). Population size is the total number of individuals in a population.
The region within which a species lives is called the species’ geographic range. Within that range, the species may be restricted to particular suitable habitats; these habitats may occur in patches separated by areas of unsuitable habitat. Review Figure 42.1
Population densities vary over time as well as over space. Review Figure 42.2

According to the BD model, the size of a population at some future time is the current size plus the number of births minus the number of deaths that occur until that time. Equations based on the BD model can be used to calculate a population’s growth rate.
Ecologists can estimate the per capita growth rate (r) by tracking births and deaths in a sample of individuals.
Unless the per capita growth rate is zero, the size of a population must be changing. If r is positive, the population is growing. If r is negative, the population is shrinking.

A species’ life history describes the sequence of key events of growth and development, reproduction, and death that occur during an average individual’s life. Review Figure 42.3
The fraction of individuals that survive to particular life stages or ages is survivorship, and the average number of offspring they produce at those life stages or ages is fecundity. Survivorship and fecundity determine the per capita growth rate, r. Review Table 42.1
Life histories are diverse; they vary both within and among species.
To survive and reproduce, organisms need materials and energy and the time to acquire them—all of which constitute resources. Organisms also need physical conditions they are able to tolerate. Physical conditions differ from resources in that they are experienced, but not used up.
The rate at which an organism can acquire resources increases with the availability of resources in its environment, up to a point. Review Figure 42.4
The principle of allocation states that organisms must divide the resources they obtain among competing life functions. Review Figure 42.5
The environment shapes life histories because it determines the relative costs and benefits of any particular pattern of allocation to different life functions.
Species can persist only in environments in which their per capita growth rate is positive. Review Figures 42.6 and 42.7

Populations grow multiplicatively, increasing or decreasing by a multiple of their current size in each time period. As a result, populations that are increasing in size by a constant multiple have a constant doubling time. Review ACTIVITY 42.1 and ANIMATED TUTORIAL 42.1
As a population becomes denser and the average individual’s share of the resource “pie” shrinks, birth rates decline and death rates increase. Thus the per capita growth rate is said to be density-dependent. Review ANIMATED TUTORIAL 42.2
When r = 0, the population reaches an equilibrium size. That size, called the carrying capacity, or K, is the number of individuals the environment can support indefinitely. Review Figure 42.8
Changes in resource availability or physical conditions cause the carrying capacity to change.
Technology has increased Earth’s carrying capacity for humans, and the human population has responded by growing rapidly. Review Figure 42.9

The regional distribution of a species often takes the form of a metapopulation, a cluster of distinct subpopulations in separate habitat patches. Review ANIMATED TUTORIAL 42.3
The BIDE model adds immigration and emigration to the BD model.
Immigration can repopulate an area where a subpopulation has gone extinct. Immigration also corresponds to gene flow among subpopulations, which combats erosion of genetic diversity through genetic drift.

Knowing the life history of a species makes it possible to identify those life stages that are most important for the species’ population growth rate. That knowledge can be applied to reduce populations of species considered undesirable and to maintain or increase populations of desirable or useful species.
Conservation efforts strive to protect as many large habitat patches as possible, because large patches host large populations with low risk of extinction, and a metapopulation with many subpopulations persists longer than one with few subpopulations.
Providing continuous dispersal corridors of habitat through which individuals can move between patches helps maintain the larger metapopulation and so is important for many conservation efforts. Review Figure 42.11 and ANIMATED TUTORIAL 42.4

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