The population structure of a species
A population is a set of individuals of one species with a common origin that live for a long time in a part of the range, freely interbreed and are relatively isolated from other populations; it has a shared gene pool, changes under genetic drift, mutation and selection, and is the elementary unit of evolution. A population tends to keep its size fairly constant (homeostasis). Main indicators: number (count of individuals), density (number or biomass per unit area or volume), sex structure (ratio of females to males; the share of breeding females matters), age structure (immature – pre-reproductive, breeding – reproductive, old – post-reproductive groups; shown as an age pyramid), birth rate and death rate. If births exceed deaths the population grows, if equal it is stable, if lower it declines; a declining population needs protection. The carrying capacity of the environment (shortage of resources, disease) limits numbers, so growth is often S-shaped (logistic), while a population below its minimum size may go extinct. Numbers fluctuate periodically (seasonal, multi-year) and non-periodically (drought, cold winter) – «population waves».
Draw age pyramids: for three populations (growing, stable, declining) draw a three-tier diagram with percentages of your choice for the pre-reproductive, reproductive and post-reproductive groups, and write the likely future of each. Capturing and marking animals in nature is done only by specialists with permission.