An ecosystem is a major ecological unit that has structure and function. Structurally, ecosystem is the entire biotic community in a given area with its abiotic (nonliving) components such as light, temperature, soil etc. It functions by converting the solar energy into chemical energy. The study of ecosystem is called ecosystem ecology.

As already pointed, individuals of a species do not live in isolation. They always have their own groups in a specific habitat.

The group of individuals of the same species occupying a particular habitat at the given time is called population. The study of population is known as population ecology.

There arc hence three ecological levels of organisations. Population (autoecology), community (synecology) and ecosystem with their complexities these organisations are influenced by a number of ecological factors. These factors that influence the populations, communities or ecosystems are described below.