Life Science Practice Exam

Ecological succession describes how a community changes in composition and structure over time as species colonize, interact, and gradually modify the environment. It starts with pioneers that can survive harsh conditions, followed by intermediate communities as more species arrive and conditions become more favorable, and may lead to a mature or climax community. There are two main forms: primary succession, which begins on bare or barren substrates with no soil, and secondary succession, which follows disturbance that leaves soil intact.

This concept fits the question because it focuses on the long-term change in which species composition and interactions shift the ecosystem's makeup, rather than pinpointing a single species or a single event. Keystone species describe organisms with outsized impacts on the environment, not the overall temporal change. Extinction is the loss of a species from a region, not the process by which communities evolve. Invasive species are about non-native organisms that disrupt communities, potentially altering trajectories, but they represent a factor influencing change rather than the process itself.