Biology 9700/13 — May/June 2012

Background Concept

Ecology has a precise hierarchy of terms that describe living things at different scales. The key terms are:

• Population — all the individuals of a single species living in the same area at the same time, and (potentially) interbreeding.
• Community — all the organisms (i.e. all the populations of all different species) living together in the same area at the same time.
• Ecosystem — a community of organisms together with the abiotic (non-living) environment they interact with, e.g. soil, water, atmosphere.
• Habitat — the place where a particular organism (or population) lives.

The crucial distinction between community and ecosystem is the inclusion of the non-living environment in an ecosystem but not in a community.

Understanding the Question

This is a one-mark multiple-choice question asking for the correct ecological definition of the term community. The command word "is" simply requires a definition to be matched to the right word; no explanation or application is needed beyond recognising the definition.

Approach

Match each option against the standard definitions above and select the one that exactly describes a community.

Step-by-Step Reasoning

• Option A ("all the food webs in an ecosystem") — A food web is a description of the feeding relationships within a community; it is not the community itself. Reject.
• Option B ("all the individuals of one species in an area") — This is the definition of a population, restricted to a single species. Reject.
• Option C ("all the organisms in an area") — This correctly states that a community includes organisms of all species present in an area, with no restriction to a single species and no inclusion of non-living components. Accept.
• Option D ("the living organisms and their non-living environment") — This is the definition of an ecosystem, which adds the abiotic environment to a community. Reject.

Key Takeaways

• Community = all the living organisms (every species) in an area.
• Population, community and ecosystem form a hierarchy: individuals → populations → community → ecosystem (with abiotic factors added).
• Watch for the trap of confusing community with ecosystem, especially when "environment" is mentioned.

Common Mistakes

• Confusing community with population (option B) — many students drop the phrase "of one species" and think any group of organisms is a community.
• Confusing community with ecosystem (option D) — adding the non-living environment turns a community into an ecosystem.

Things to Be Careful About

The word "all" in option C is doing important work: it means all species, not just all individuals of one species. Read each option carefully for these qualifier words.