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

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

Deaths and emigration remove individuals from a population. Births Deaths Deaths and emigration remove individuals from a population. Births and immigration add individuals to a population. Figure 53.3 Population dynamics. Immigration Emigration 13

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.