Concept 43.1 Populations Are Patchy in Space and Dynamic over Time

• Population density is the number of individuals in a population per unit of area (or volume). Population size is the total number of individuals in a population.
   
• The region within which a species occurs is called the species' geographic range. Within that range, the species may be restricted to particular suitable habitats; these may occur in habitat patches separated by areas of unsuitable habitat. Review Figure 43.1
   
• Population densities vary over time as well as over space. Review Figure 43.2
   

Concept 43.2 Births Increase and Deaths Decrease Population Size

• According to the BD model, the population size at some future time is the current size plus the total 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) of a population by tracking births and deaths in a sample of individuals.
   
• If a population's per capita growth rate does not equal zero, then its size must be changing. If r is positive, the population is growing. If r is negative, the population is shrinking.
   

Concept 43.3 Life Histories Determine Population Growth Rates

• A species' life history, which is a quantitative description of its life cycle, can be summarized in a life table. Review Figure 43.3 and Table 43.1
   
• 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 43.1
   
• 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.
   
• The rate at which an organism can acquire resources increases with the availability of resources in its environment. Review Figure 43.4
   
• The principle of allocation states that once an organism has acquired a unit of some resource, it can be used for only one function at a time. Review Figure 43.5
   
• The environment shapes life histories because it determines the relative costs and benefits of any particular pattern of allocation to different functions.
   
• Species can persist only in environments in which their per capita growth rate is positive. Review Figures 43.6 and 43.7
   

Concept 43.4 Populations Grow Multiplicatively, but Not for Long

• Populations grow multiplicatively, increasing or decreasing by a constant multiple of their current size in each interval of time. As a result, populations that are increasing in size have a constant doubling time. Review WEB ACTIVITY 43.1 and ANIMATED TUTORIALS 43.1 and 43.2
   
• As a population becomes denser and resources become scarcer, birth rates decline and death rates increase. Thus the per capita growth rate is said to be density dependent.
   
• 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 43.8
   
• 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 43.9
   

Concept 43.5 Extinction and Recolonization Affect Population Dynamics

• The regional distribution of a species often takes the form of a metapopulation, a cluster of distinct subpopulations in separate habitat patches linked together by the dispersal of individuals among them.
   
• The BIDE model adds immigration and emigration to the BD model. Immigration can rescue an extinct subpopulation.
   

Concept 43.6 Ecology Provides Tools for Managing Populations

• Knowing the life history of a species makes it possible to identify those life stages that are most important for its population growth rate. That knowledge can be applied by managers to reduce populations of species considered undesirable and maintain or increase populations of desirable or useful species.
   
• Conservation efforts give priority to protecting the largest remaining habitat patches because these patches can host the largest populations. Providing continuous corridors of habitat through which individuals can disperse among patches reduces extinction risk. Review Figure 43.11, WORKING WITH DATA 43.1, and ANIMATED TUTORIAL 43.3