What is a population?

Definition: A group of organism of the same species which live in the same habitat at the same time where they can freely interbreed.

Describing a population

• Size: Numbers
• Density: Numbers per unit area (or volume in aquatic ecosystems)
• Distribution: Random, clumped, uniform

• Growth: Changes with time: (immigration – emigration) + (natality – mortality)

Phase 1: Log or exponential phase. Unlimited population growth. Abundant food, no disease, no predators etc

Phase 2: Decline or transitional phase. Limiting factors slowing population growth.

Phase 3: Plateau or stationary phase. No growth. The limiting factors balance the population’s capacity to increase. The population reaches the Carrying Capacity (K) of the environment. Added limiting factors will lower K. Removing a limiting factor will raise K.

Measuring populations

• Total count (census)
• Sampling
• Random sampling
• Systematic sampling (grid, transect)
• Sample size, quadrats
• Numbers of samples

Estimating population density
Measuring a population’s area
Calculating population size (density x area)

Influences on a population

• Abiotic factors (e.g. climate, nesting sites) often independent of density.

• Biotic factors (e.g. disease, food supply, predation) often density dependent.