In allogenic succession, the change is due to:
Allogenic succession is driven mainly by forces external to the resident community, including erosion, sediment deposition, flooding, climatic shifts, or changes in hydrology. Such abiotic processes alter habitat conditions independently of biological modification and can redirect community composition. A disturbance changes the availability of space, resources, and biological legacies, creating opportunities for colonization. Whether succession is primary or secondary depends chiefly on the starting substrate and survival of soil, not simply on the identity of the disturbance. Colonists must
Ref: Ecology: Concepts and Applications, Molles, 9th Ed., Ch. 20