Which of these is a starting point for primary succession?

• A. a surface exposed by a retreating glacier
• B. a surface with mature soil
• C. a lake that gradually fills with sediment
• D. a habitat with established vegetation

Answer: (A)

Primary succession starts on surfaces that have no soil and little to no life. A surface exposed by a retreating glacier fits perfectly: bare rock with no soil, providing a clean slate for pioneer organisms. Lichens and mosses are well suited to such harsh, nutrient-poor conditions and begin the process by slowly breaking down the rock and adding organic material to form a thin initial soil. As that soil thickens, more plants can take root, and the community becomes progressively more complex over time.

In contrast, a surface that already has mature soil already hosts established communities, so it isn’t the initial stage of primary succession. A lake filling with sediment involves a transition from an aquatic environment and doesn’t represent the first colonization of bare land. An area with established vegetation is a later stage after pioneers have created soil, not the starting point.