Population Ecology Cheat Sheet

The core ideas of Population Ecology distilled into a single, scannable reference — perfect for review or quick lookup.

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Quick Reference

Exponential Growth

A pattern of population growth in which the population size increases at a constant per-capita rate, producing a J-shaped curve. It occurs when resources are unlimited and there are no environmental constraints on reproduction or survival.

Logistic Growth

A model of population growth that incorporates carrying capacity, producing an S-shaped (sigmoidal) curve. Growth rate slows as the population approaches the maximum number of individuals the environment can sustain.

Carrying Capacity (K)

The maximum population size that an environment can sustain indefinitely given the available resources such as food, water, habitat, and space. It is a dynamic value that can shift with environmental changes.

Density-Dependent Factors

Regulatory forces whose effects on a population intensify as population density increases. These factors include competition for resources, predation, disease transmission, and parasitism, and they tend to stabilize populations near carrying capacity.

Density-Independent Factors

Environmental factors that affect population size regardless of population density. These include natural disasters, extreme weather events, and seasonal climate changes that can cause sudden population declines.

r-Selected and K-Selected Species

A classification framework for life history strategies. r-selected species prioritize high reproductive rates, early maturity, and many small offspring, while K-selected species invest in fewer offspring with greater parental care and longer lifespans.

Survivorship Curves

Graphical representations of the pattern of survival in a population from birth to maximum age. Type I curves show high survival until old age, Type II show constant mortality, and Type III show high early mortality with few surviving to adulthood.

Metapopulation

A group of spatially separated populations of the same species that interact through immigration and emigration. Local populations may go extinct and be recolonized, and the persistence of the species depends on the balance of these dynamics across patches.

Life Table

A demographic tool that records age-specific survival rates, mortality rates, and reproductive output (fecundity) for a cohort of individuals in a population. Life tables are used to calculate key parameters such as net reproductive rate and generation time.

Allee Effect

A phenomenon in which individual fitness or population growth rate decreases at low population densities. This positive relationship between density and fitness can create a critical threshold below which populations decline toward extinction.

Key Terms at a Glance

Population:A group of individuals of the same species living in a defined area at a given time.

Population Density:The number of individuals per unit area or volume of habitat.

Birth Rate (Natality):The number of new individuals produced per unit time, often expressed per capita.

Death Rate (Mortality):The number of individuals dying per unit time, often expressed per capita or age-specific.

Immigration:The movement of individuals into a population from another area.

Emigration:The movement of individuals out of a population to another area.

Carrying Capacity:The maximum population size that a given environment can sustain indefinitely with available resources.

Exponential Growth:Unrestricted population growth at a constant per-capita rate, described by the equation dN/dt = rN.

Logistic Growth:Population growth that decelerates as population size approaches carrying capacity, described by dN/dt = rN((K-N)/K).

Intrinsic Rate of Increase (r):The maximum per-capita growth rate of a population under ideal, unlimited resource conditions.

Life Table:A table of age-specific survival, mortality, and fecundity data for a population cohort.

Survivorship Curve:A graph showing the proportion of individuals surviving to each age in a population, classified as Type I, II, or III.

Fecundity:The potential reproductive capacity of an individual or population, measured as the number of offspring produced.

Net Reproductive Rate (R0):The average number of female offspring produced per female over her lifetime.

Density-Dependent Regulation:Population regulation by factors whose effects scale with population density, such as competition, disease, and predation.

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