Summary

Levels of organisation: Ecology is studied at the levels of organism, population, community, and biome. A population is a group of individuals of the same species living in a defined geographical area, capable of interbreeding.

Population attributes: Unlike an individual organism, a population has properties like birth rate, death rate, sex ratio, and age distribution/age pyramid. Population density — the size of a population relative to a given space — is a key attribute, measured in different ways depending on the species (total count, per-unit-area count, or an indirect index like number of nests or pug marks).

Population growth: Density changes through natality, immigration, mortality, and emigration: Nt+1 = Nt + [(B + I) - (D + E)]. Exponential growth (dN/dt = rN, unlimited resources) produces a J-shaped curve. Logistic growth (dN/dt = rN × (K-N)/K, limited resources, bounded by carrying capacity K) produces an S-shaped curve and is the more realistic model in nature.

Life history variation: Species differ in when they reproduce, how many offspring they produce, and how much parental care they invest, as natural selection shapes different reproductive strategies for different environments — e.g., oysters and pelagic fishes produce many small offspring with little care, while birds and mammals produce few large offspring with substantial care. …