Population Ecology Chapter 3

Population Ecology Definitions

• Population ecology is the study of how members of a population (same species living in a given area) interact with their environment.
• It focuses on factors that influence a population's: distribution, dispersion, size, density, abundance, dynamics, and structure.

Population Distribution

• Ecologists use observations and experiments to test explanations for the distribution and abundance of species affected by environmental components.
• Biogeography is the study of the geographic distribution of organisms throughout the landscape.
• It provides a good starting point for understanding what limits the geographic distribution of species.

Factors that Influence Species Distribution

• Dispersal tendency: the movement of individuals away from centers of high population density or from their area of origin that contributes to the global distribution of organisms.
• Habitat tolerance: the ability of a species to adapt to different environmental conditions.
• Local population size: the number of individuals in a specific area.
• Rarity can be divided into three types: Rarity I, II, and III, based on factors such as geographic range, habitat tolerance, and local population size.

Measuring Population Density

• Measuring density of populations is a difficult task.
• Ecologists use sampling techniques to estimate population density, such as quadrat sampling and life tables.

Sampling Techniques

• Quadrat sampling: involves randomly selecting square areas of a specific size within a study site and collecting data of interest within these quadrats.
• Static (or time-specific) life table: counts all individuals alive at a given time and records the age of each, and records age at death of individuals.
• Age distribution: calculates the difference in proportion of individuals in each age class, assuming differences from mortality.

Age Structure and Population Growth

• Age structure diagrams can reveal a population's growth trends and point to future social conditions.
• The age structure of a population can help us predict the future growth of populations.
• Survivorship curves: a graphic way of representing the data, which plots the number of individuals in a cohort still alive at each age.

Reproductive Rates

• Demographers focus on females because only females give birth to offspring.
• A reproductive table is an age-specific summary of the reproductive rates in a population.
• Birth rate: the number of young born per female.

Population Growth

• Population growth (N) = Births (B) minus Deaths (D).
• Population growth rate: the change in a population size per time interval.
• Per capita birth rate (b): the number of offspring produced per unit time by an average member of the population.
• Per capita death rate (m): the number of deaths per unit time by an average member of the population.
• Exponential population growth: the size of a population that is growing exponentially increases at a constant rate, resulting in a J-shaped growth curve.

Carrying Capacity (K)

• It is the number of individuals in a population that the environment can just maintain (“carry”) with no net increase or decrease.
• Growth limiting factors shape the carrying capacity.
• Long periods of exponential increases are not common in the real world, as the carrying capacity would eventually be reached.