Population Ecology

Populations are described by density, spatial distribution, and growth rate.

Population Characteristics

Populations are groups of the same species in the same area.

Characteristics of population include: Density Spatial Distribution Growth rate

Population density The number of organisms per unit area

is population density.

Spatial distribution Dispersion is the pattern of spacing of

individuals of a population within an area.

Three kinds: clumped, uniform, and random

Population-Limiting Factors

Limiting factors are biotic or abiotic forces that keep population from increasing indefinitely.

Limiting factors are either density-independent or density-dependent.

Density-independent factors Any factor in the environment that does

not depend on the number of members in a population per unit area is a density-independent factor.

Usually abiotic, and include:▪ Weather events▪ Fire▪ Human alteration of the landscape▪ Air, land, and water pollution

Density-dependent factors Any factor in the environment that

depends on the number of members in a population per unit area is a density-dependent factor.

Often biotic, and include:▪ Predation▪ Competition▪ Disease▪ Parasites

Population growth rate The population growth rate (PGR)

describes how fast a given population grows.▪ Natality: birthrate of a population in a given

year.▪ Mortality: death rate of a population in a

given year▪ Emigration: number of individuals moving

away from the population▪ Immigration: number of individuals moving

into a population

Births Deaths

Immigration Emigration

Births and immigrationadd individuals toa population.

Deaths and emigrationremove individualsfrom a population.

Exponential growth model▪ Occurs when growth rate is proportional to

population size.▪ All populations grow exponentially until they

encounter a limiting factor.

Logistic growth model:▪ Occurs when a population’s growth slows or

stops following exponential growth.▪ A population stops increasing when the

number of births < number of deaths, or when emigration > immigration.

Carrying capacity▪ Carrying capacity is the maximum number of

individuals of a species (pop size) that an environment can support.

Reproductive patterns Species vary in the number of births per

reproduction cycle, age that reproduction begins, and in the life span.

Plants and animals are placed into groups based on these patterns.