Organisms and Populations

About this unit

Ecology studies how organisms interact with each other and with their physical environment. This unit focuses on the population level: a group of individuals of the same species living in a defined area and sharing resources. It covers population attributes such as birth rate, death rate, sex ratio and age structure, the ways populations grow (exponential and logistic), how life-history traits vary, and the different interactions between populations of different species, from predation and competition to mutualism.

Key ideas

• Populations have attributes that individuals do not, such as birth rate, death rate and age distribution.
• Population density changes through natality, mortality, immigration and emigration.
• Exponential growth gives a J-shaped curve; logistic growth gives an S-shaped (sigmoid) curve that levels off at the carrying capacity K.
• r is the intrinsic rate of natural increase.
• Interactions are classified by their effect (+, −, 0) on each species.
• Gause's competitive exclusion principle and resource partitioning explain coexistence of competitors.

How to prepare

Learn the growth equations dN/dt = rN and dN/dt = rN(K − N)/K and practise simple calculations. Memorise the sign pattern for each interaction with one Indian or textbook example each. Be able to sketch the three age-pyramid shapes and say what each means.