What differentiates secondary from primary succession?
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. Disting
Ref: Ecology: Concepts and Applications, Molles, 9th Ed., Ch. 20
