Practice question

Grasses colonizing a sand dune can initiate autogenic succession because the organisms themselves modify the habitat and influence later community development. Grass roots bind mobile sand, while stems reduce wind speed and trap additional particles. Dead leaves and roots add organic matter, microbes release nutrients, and improved water retention makes the substrate less hostile. These changes permit herbs, shrubs, and eventually trees to establish where regional climate allows. The succession is autogenic to the extent that biological engineering drives the transition, although wind deposition and storms remain allogenic influences. A forest recovering after fire is classified mainly as secondary succession initiated by an external disturbance. Colonization of a new volcanic island and bare rock are examples of primary succession, but those descriptions alone do not specifically demonstrate organism-caused environmental modification. Sand-dune grasses provide the clearest mechanism among the choices: colonists stabilize and enrich their own substrate. Their positive feedback between vegetation and soil development illustrates how ecosystem engineers can redirect physical conditions and produce a sequence of community replacement.