Ecology: Definitions, Branches and Hierarchy

What Ecology Studies

Ecology is the branch of biology that studies how living organisms relate to one another and to their surroundings. Unlike physiology, which looks inward at how an organism's own body works, ecology looks outward — at the web of relationships an organism is embedded in. The subject is conventionally split into two broad branches:

• Autecology — the study of an individual species and its relationship with its environment. An autecological study of, say, a single fish species would look at its temperature tolerance, feeding habits, breeding season and the specific factors that limit its distribution.
• Synecology — the study of groups of organisms (populations, communities) in relation to their environment and to each other. Synecology is concerned with interactions between species and with the organisation of whole communities, rather than the biology of one species in isolation.

The Ecological Hierarchy

Ecological organisation is built up in nested levels, each level being made of units of the level below it:

1. Individual/Organism — a single living being.
2. Population — all individuals of one species living in a defined area at a given time, capable of interbreeding.
3. Community — all the populations of different species living and interacting together in a given area.
4. Ecosystem — a community together with its physical (abiotic) environment, functioning as a unit through energy flow and nutrient cycling.
5. Biome — a very large regional ecosystem characterised by a particular climate and a characteristic type of vegetation and animal life (for example, tropical rainforest, desert, grassland, tundra).
6. Biosphere/Ecosphere — the sum of all biomes on Earth; the global zone of life encompassing all ecosystems, from the depths of the oceans to the upper atmosphere.

Each level shows properties that do not exist at the level below it — a population has a birth rate and death rate that an individual does not, and a community has species diversity that a population does not. This is why ecologists insist on studying whole systems rather than isolated organisms.

Habitat versus Niche

Two terms that are frequently confused but mean very different things:

• Habitat is the physical address of an organism — the actual place where it lives, for example a pond, a forest floor, or the intertidal zone of a rocky shore. It answers the question "where does this organism live?"
• Niche is the functional role an organism plays within its habitat — its feeding pattern, its position in the food chain, the resources it uses, the time of day it is active, and how it interacts with other species. It answers the question "what does this organism do, and how does it live?"

Two species can share the same habitat yet occupy different niches by feeding on different food, being active at different times, or exploiting different microhabitats within the same area — this is precisely what allows many species to coexist without direct competition driving one to extinction. A niche is therefore best thought of as an organism's multidimensional "way of life" rather than a physical location.