APES Unit 3

Population: Individuals of a particular species that inhabit an area

Population size: Number of individuals present at a given time

Population density: The number of individuals in a population per unit area

Easier to find mates

Increased competition and vulnerability to predation

Increased transmission of diseases

Sometimes causes organisms to leave an area if too dense

Population distribution (dispersion): Spatial arrangement of organisms

Random: Haphazardly located individuals, with no pattern

Uniform: Individuals are evenly spaced

Clumped: Organisms found close to other members of population

Sex ratio: proportion of males to females

In monogamous species, a 1:1 sex ratio maximizes population growth

Most species are not monogamous, so ratios vary

Age distribution (structure): The relative numbers of organisms of each age in a population

In species that continue growing as they age, older individuals reproduce more (trees)

Three age categories

Pre-Reproductive age

Reproducing (can contribute to population growth)

Post-Reproductive age

Natality: births within the population

Mortality: deaths within the population

Immigration: arrival of individuals from outside the population

Emigration: departure of individuals from the population

Births and immigration add individuals; deaths and emigration remove individuals

Crude birth (death) rates: number of births (or deaths) per 1000 individuals per year

Population change = (Births + Immigration) - (Deaths + Emigration)

Natural rate of population increase = (Crude birth rate) − (crude death rate)

Population growth rate = (Crude birth rate + immigration rate) − (Crude death rate + emigration rate)

Growth rate as a percent: Population growth rate × 100

Exponential growth: When a population increases by a fixed percent

Limiting factors: Physical, chemical, and biological attributes of the environment that restrain population growth

Carrying capacity: The maximum population size of a species that its environment can sustain

Density-dependent factors: limiting factors whose influence is affected by population density

Increased density increases the risk of predation, disease, and competition

Larger populations have stronger effects of limiting factors

Density-independent factors: limiting factors whose influence is not affected by population density

Events such as floods, fires, and landslides

Biotic potential: an organism’s capacity to produce offspring

K-selected species: species with long gestation periods, few offspring, and strong parental care

Have a low biotic potential

Stabilize at or near carrying capacity; good competitors

r-selected species: species that reproduce quickly and offer little or no care for offspring

Have a high biotic potential

Populations fluctuate greatly

Survivorship curves: graphs that show that the likelihood of death varies with age

Type I: higher death rate at older ages

Larger animals (e.g., humans)

Type II: same death rate at all ages

Medium-sized animals (e.g., birds)

Type III: higher death rate at young ages

Small animals, plants

Rule of 70 (doubling time)

Doubling time of population can be predicted by dividing 70 by % population growth rate

DT (years) = 70/GR (%)

Thomas Malthus’s An Essay on the Principles of Population (1798)

Humans will outstrip food supplies

War, disease, starvation reduce populations

Biocapacity: the amount of biologically productive land and sea available to us

Ecological deficit: ecological footprint > biocapacity

Ecological reserve: ecological footprint < biocapacity

Total fertility rate (TFR): The average number of children born to each female

Replacement fertility: The TFR that keeps the size of a population stable (about 2.1)

Natural rate of population change: Change due to birth and death rates alone (no migration)

Life expectancy: average number of years that an individual is likely to continue to live

Has increased with reduced rates of infant mortality

Demographic transition: a model of economic and cultural change

Explains the declining death and birth rates in industrializing nations

Populations undergo four stages

Pre-industrial stage: in pre-industrial societies, both birth and death rates are high

High birth rate to compensate for high infant mortality

Population growth is slow

Transitional stage: declining death rates due to increased food production and medical care

Birth rates remain high since people are not used to the low infant mortality rates

Population grows quickly

Industrial stage: birth rates fall as jobs provide opportunities for women outside the home and children are not needed in the workforce

Difference between birth and death rates shrinks

Population growth slows

Post-industrial stage: birth and death rates are low and stable

Population stabilizes or even shrinks