A community reaches a climax when

Step 1: Analyze the question The question asks what defines a climax community in ecological succession.

Step 2: The concept Ecological succession is the gradual replacement of one community by another over time:

• Pioneer community: the first organisms to colonize a bare area (e.g., lichens, mosses on bare rock).
• Intermediate (seral) stages: grasses, shrubs, and small trees replace the earlier ones.
• Climax community: the final, stable stage. The community is in equilibrium with its environment. Species composition and population size stay fairly constant over a long period, unless a disturbance occurs (e.g., bush burning or flooding). In Nigeria, the tropical rainforest in the south is a typical climax community.

Step 3: Evaluate each option

• A. Only pioneer organisms are present: Wrong. This is the beginning of succession, not the end.
• B. New habitats are constantly being formed: Wrong. This describes an unstable, changing environment, which is characteristic of early succession.
• C. Introduction of new plant and animal species: Wrong. Constant introduction of new species means the community is still changing, so it has not reached climax.
• D. The composition and size of a community remains constant over a long period: Correct. This is stability, or equilibrium, the defining feature of a climax community.

Step 4: Answer The correct option is D.

Exam tip: Link the word "climax" with "stable". Pioneer means first and changing, while climax means final and steady. If an option mentions stability, constancy, or equilibrium, it is likely the answer for climax questions.