Chapter 54

Population Ecology

Studying Populations• A population consists of all the individuals of a species

in a given area.

• Population structure describes the age distribution of individuals, and how those individuals are spread over the environment.

• The number of individuals per unit area or volume is the population density.

• Density has strong influence over how individuals react with one another and with populations of other species.

Studying Populations

• Population structure changes over time due to demographic events: births, deaths, immigration, and emigration.

• These events create population dynamics. Study of these events is called demography.

Studying Populations

• Population ecologists measure number and density of individuals, rates of demographic events, and locations of individuals.

• Individuals are often tagged or marked in some way to facilitate research.

• Tracking devices are also used. They may provide additional physiological and environmental data.

Studying Populations

Figure 54.1 By Their Marks You May Know Them

• A life table can be constructed by tracking a group of individuals born at the same time: a cohort.

• Numbers that are still alive at later dates (survivorship) are determined.

• Some life tables include fecundity: number of offspring produced in a time interval.

Studying Populations

• Life tables can be used to predict future trends.

• The data can be plotted to show survivorship in relation to age.

• Survivorship curves fall into three different general patterns.

Studying Populations

Figure 54.3 Survivorship Curves

• Age distribution reveals information about recent births and deaths.

Example: human population of the U.S.

Studying Populations

Life Histories• An organism’s life history

describes how it allocates time and energy among the various activities throughout its life.

• For example, some animals have a single offspring per reproductive episode, some have many. Some species, such as salmon and agave reproduce only once and then die.

Population Densities• All populations have the potential for explosive

growth.

• Even when per capita growth rate remains constant, as population size increases, number of new individuals added per time unit increases: exponential growth.

• The term ΔN/Δt is the rate of change of the population over time.

• r is the net reproductive rate.

Ndbt

Nr )(

Population Densities

• The highest possible value for r is rmax or the intrinsic rate of increase.

Population Densities

• Real populations cannot maintain exponential growth for long.

• Environmental limits cause birth rates to decrease and death rates to increase.

• The environmental carrying capacity (K) is the number of any particular species that can be supported in an environment.

Population Densities

• Carrying capacity is determined by availability of resources such as food or shelter, plus factors such as diseases and parasites, and social interactions.

• Growth of a population usually slows when it nears carrying capacity.

• A graph of population size over time forms an S-shaped curve, and is known as logistic growth.

Population Densities

Figure 54.8 Logistic Population Growth

• Logistic growth can be modeled by adding a term for carrying capacity to the equation for population growth:

• Growth stops when N = K.

NK

NKr

t

N

Population Densities

Birth rates and death rates are influenced by density-dependent factors:

• As population density increases, food supplies may be depleted, reducing amount of food available to individuals.

• Predators may be attracted to high densities of prey, increasing death rate.

• Diseases can spread more easily.

Population Densities

• Other factors that influence populations are density-independent, such as weather-related phenomena.

Population Densities

• In general, more stable population numbers are seen in species with long-lived individuals and low reproductive rates.

• For example, insect populations tend to fluctuate more than those of birds and mammals.

• Environmental factors can change carrying capacity for species.

Population Densities

Some species tend to be more common thanothers. Four factors have strong influence on thevariation of population density among species:

• Resource abundance• Size of individuals• Length of time a species has lived in an area• Social organization

Population Densities

• Species that use abundant resources generally reach higher population densities than those using scarce resources.

• Animals that eat plants are generally more abundant than animals that eat other animals.

Population Densities

• Species with small body size generally reach higher population densities.

• Small individuals require less energy to survive than large ones.

• This is illustrated by mammal species worldwide.

Population Densities

Figure 54.11 Population Density Decreases as Body Size Increases

• Complex social organization can lead to high population densities.

• Highly social species, such as ants, termites, and humans, can achieve very high densities.

Population Densities

Space & Population Dynamics• Most populations are divided into live in habitat patches.

Managing Populations• Numbers of births and growth of individuals tend

to be highest when population is below carrying capacity.

• If humans wish to maximize the number of individuals harvested from a population, we should try to maintain it below carrying capacity.

• Whaling has also resulted in declining populations.

• Most whale populations have failed to recover.

• Whales are large animals with slow reproductive rates. Many adults are needed to produce a small number of offspring.

Managing Populations

• Humans wish to decrease the size of populations of many pest species.

• Reducing population numbers below carrying capacity stimulates higher birth rates and growth of the population.

• A more effective approach is to remove the resources for the population, (e.g., making garbage unavailable for rats).

Managing Populations

• The size of the human population now contributes to most environmental problems.

• Human social organization and specialization has allowed us to increase the carrying capacity for humans.

Managing Populations

• Earth’s current carrying capacity for humans is set in part by the biosphere’s ability to absorb our by-products, especially CO2 from fossil fuels; also by water availability and our willingness to cause extinction of other species to accommodate our increasing use of Earth’s resources.

Managing Populations