5.11—Population, community, habitat and ecosystem

A habitat is where an organism lives; a population is all members of one species there; a community is interacting populations; an ecosystem is the community interacting with the non-living parts of its habitat.

These levels are nested, so the correct term depends on what is included. Ecosystems have energy flow and nutrient cycling, but they are not sealed boxes: organisms and materials can move between them.

A pond’s frogs form a population; frogs, fish and pond plants form a community; adding water chemistry, light and sediment makes the pond an ecosystem.

Do not call every group of organisms a population, and do not treat an ecosystem as self-contained. Scale and the stated interactions decide the term.