Practice question

Secondary succession retains preexisting soil, often together with seeds, roots, spores, organic matter, microbes, and surviving organisms. These biological and edaphic legacies distinguish it from primary succession and permit vegetation and nutrient cycling to recover without first constructing soil from bare substrate. Organisms can drive succession autogenically by changing their own habitat, while floods, erosion, sedimentation, or climate can drive allogenic change. Biological agents such as grazers and pathogens may also redirect succession by altering competitive relationships. Distinguishing the driver from the observed sequence clarifies mechanism: community turnover is the response, whereas substrate development, resource preemption, facilitation, or external forcing explains why replacement occurs. This distinction prevents every temporal fluctuation from being mislabeled as ecological succession. In this context, the keyed term, Presence of preexisting soil, identifies the relevant mechanism or quantitative relationship and links the observed pattern to its underlying ecological cause. The distinction is testable by measuring changes in organisms, resources, or process rates through time rather than relying on the label alone.