Which of the following is the correct sequence of phases of Biotic succession ?

Question

UnlockIAS · Accepted Answer

Correct — D, Migration – Ecesis – Reaction – Stabilisation. This is the textbook order of ecological (biotic) succession, and each step physically depends on the one before it. Migration is the arrival of propagules — seeds, spores, fruits, rhizome fragments — carried into the bare area by wind, water, animals or gravity. Ecesis is the establishment of those arrivals: germination, growth to maturity and successful reproduction at the new site. Only a small fraction of what migrates manages to ecise, and the survivors form the pioneer community. As their numbers rise, aggregation and then competition and coaction set in, and the community begins to Reaction — the phase in which the organisms themselves modify the habitat by adding humus, holding moisture, changing soil chemistry and altering light and temperature. Reaction is the engine of succession, because the modified habitat usually suits the next community better than it suits the one that made it, so one seral community replaces another. When the modification no longer produces a change of community, the sequence reaches Stabilisation — the climax, a self-perpetuating community in equilibrium with the regional climate. Nothing can arrive after it has established, and nothing can stabilise before the habitat has been reworked, which is exactly what the three wrong options violate.

Why the other options are wrong — (a) It puts establishment before arrival. Ecesis is the germination and successful growth of a propagule that has already reached the site, so it cannot precede migration. The last two terms are in the right order, which makes this the most tempting wrong option.  (b) Wrong twice. It inverts migration and ecesis, and it also places stabilisation before reaction — the climax cannot arrive before the habitat modification that drives the community changes leading to it.  (c) It opens correctly with migration but then leaves ecesis until after the climax. Arrivals that never establish cannot modify a habitat or build a stable community, so establishment must come second, not last.

Concept: Ecological or biotic succession is the orderly, directional replacement of one biotic community by another in the same area until a relatively stable climax community is reached. The full sequence taught in Indian syllabi runs: nudation (creation of a bare area) → invasion, which itself has three steps of migration, ecesis and aggregation → competition and coaction → reaction → stabilisation or climax. The whole series of communities that occupy the site one after another is called a sere, and each intermediate community is a seral stage.

State PCS 2024 Q26 — Environment & Ecology PYQ with Answer

State PCS 2024 Q26 — Environment & Ecology PYQ with Answer