Ecology is the study of the inter-relationships between organisms and their environment, including both non-living (abiotic) and living (biotic) factors.

A community of living organisms interacting with each other and their physical environment in a defined area.

Ecosystems can range in size from very small, like a rockpool, to very large, like a woodland.

Biotic factors are the living components of an ecosystem, like competition and predation.

Abiotic factors are the non-living, physical components of an ecosystem, like temperature and rainfall.

Higher light intensity increases the rate of photosynthesis, which allows plants to grow faster and to produce more spores or seeds, increasing population size.

1. In plants and cold-blooded animals - at low temperatures, enzymes work more slowly, reducing metabolic rate and so population size. At temperatures above the optimum, enzymes are less efficient because they undergo denaturation, reducing population size.
2. In warm-blooded animals - as the temperature of the external environment moves further from the optimum temperature, the organisms use more energy trying to maintain their body temperature, reducing their reproductive rate and population size.

When water is scarce, populations are small.

Low humidity increases the transpiration rate in plants and the evaporation of water from the bodies of animals, leading to lower population sizes due to water loss.

Each enzyme has an optimum pH at which they operate most effectively, and population size is larger when the appropriate pH exists for an organisms' enzymes.

Population size is smaller when the pH differs from the optimum.

A population is a group of individuals of one species occupying the same habitat at the same time.

A community is all the populations of different species living and interacting within an ecosystem.

A habitat is the physical location where an organism lives.

A microhabitat is a smaller habitat with its own microclimate within a larger habitat.

A niche describes the role of an organism in its ecosystem, including all the biotic and abiotic conditions to which an organism is adapted to survive, reproduce, and maintain a viable population.

The competitive exclusion principle states that no two species can occupy the same niche in the same environment simultaneously.