Q.(a) Tabulate the differences between primary and secondary succession. OR

Plant succession is the gradual, orderly replacement of one plant community by another on the same site, seen whenever a barren or disturbed area is re-colonised over time. The first colonisers are pioneers; the intermediate communities that replace one another in sequence are seral communities; and the final, stable community is the climax community (the end-state itself called the climax). Succession is driven by three classes of cause acting in sequence — initiating causes (abiotic/biotic factors that create bare or disturbed ground, determining whether primary or secondary succession follows), continuing causes (migration, aggregation, competition, reaction — the ongoing processes reshaping the community), and stabilizing causes (chiefly climatic, and what eventually locks the community in place). It shares recognisable characteristics wherever it occurs: it is systematic, driven by changing abiotic/biotic factors, converts an unstable community into a stable one, and steadily increases species diversity, biomass, niche specialisation and soil humus while food relationships grow from a simple chain into a complex web. It is classified along several axes: primary (starting on genuinely barren ground, no prior community, slow) versus secondary (starting on a disturbed site with soil already present, faster); autogenic (driven by the vegetation's own biotic effects on itself) versus allogenic (driven by external abiotic factors); and autotrophic (green plants dominate early on, energy content rises) versus heterotrophic (bacteria/fungi/animals dominate early on, energy content falls). The process itself runs through seven fixed stages — nudation (bare ground appears), invasion/migration (propagules arrive), ecesis (the species establishes and can reproduce), aggregation (population grows), competition (for water, nutrients, light, space), reaction (the community modifies its own habitat and gets di …