AP Biology Unit 8 Ecology Notes: Understanding Populations and What Regulates Them

Population

A group of individuals of the same species living in the same area at the same time.

Population ecology

The branch of ecology that studies how many individuals are in a population, where they are, how their numbers change over time, and why.

Population size

The total number of individuals in a population.

Population density

The number of individuals per unit area (or volume), often used to infer encounter rates for competition, mating, and disease transmission.

Population dispersion (distribution)

How individuals are spaced within their habitat (commonly clumped, uniform, or random).

Clumped dispersion

Individuals occur in patches; often caused by uneven resource distribution or social behavior (the most common pattern in nature).

Uniform dispersion

Individuals are evenly spaced; often caused by territoriality or intense competition for resources.

Random dispersion

Unpredictable spacing; tends to occur when resources are relatively uniform and individuals have little interaction.

Demographic processes

The four processes that change population size: births and immigration increase size; deaths and emigration decrease size.

Immigration

Movement of individuals into a population, increasing population size.

Emigration

Movement of individuals out of a population, decreasing population size.

Population change equation (ΔN)

An accounting model for population size change over a time interval: ΔN = (B + I) − (D + E), where B=births, I=immigration, D=deaths, E=emigration.

Intrinsic rate of increase (r)

The per capita growth rate under ideal conditions; r > 0 indicates growth, r < 0 indicates decline.

Exponential growth

Growth at a constant proportion per unit time under ideal conditions (abundant resources); modeled by dN/dt = rN, producing J-shaped growth.

Carrying capacity (K)

The maximum population size an environment can sustain over time given resources, space, and other constraints; K can change with seasons, disturbances, and human impacts.

Logistic growth

Growth that slows as a population nears carrying capacity due to density-dependent limits; modeled by dN/dt = rN(1 − N/K), often producing an S-shaped curve.

Overshoot and dieback

When a population exceeds carrying capacity (often due to time lags), depletes resources, and then declines sharply (sometimes below K).

Mark-recapture

A population sampling method using two capture events: individuals are captured and marked, then later a second sample is captured to estimate total population size.

Mark-recapture estimator (Lincoln–Petersen)

Population estimate: N ≈ (M × C) / R, where M=number marked first, C=total caught second, R=marked individuals recaptured second.

Mark-recapture assumptions

Key requirements to avoid bias: marks don’t affect survival/catchability, marked individuals mix back into the population, and the population is closed between samples (no major births, deaths, immigration, emigration).

Life history

Traits that shape demography (e.g., age at first reproduction, number of offspring, parental investment, lifespan) by affecting birth and death rates.

Survivorship curves (Type I, II, III)

Graphs of survival over age: Type I—high survival until old age (many mammals); Type II—constant mortality rate (some birds/reptiles); Type III—high juvenile mortality with high survival of survivors (many fish/plants).

Density-dependent limiting factor

A factor whose impact increases as population density increases, creating negative feedback that slows growth (e.g., competition, disease/parasitism, many predation effects, stress/waste).

Density-independent limiting factor

A factor whose impact does not depend on population density, often abiotic disturbances (e.g., drought, floods, fires, extreme temperatures, hurricanes, some pollution events).

Allee effect

A pattern where very low population density reduces per capita growth (e.g., difficulty finding mates, reduced group defense, reduced cooperative hunting), potentially preventing recovery of small populations.