Question
Describe ecological succession in a pond (hydrarch succession). Trace the stages from pioneer community to climax community.
(NEET, CBSE Class 12 — Organisms and Populations / Ecosystem)
Solution — Step by Step
Succession in water (hydrarch) begins with phytoplankton (algae, diatoms) colonising a bare water body. They are the pioneer species — first to arrive. Their dead remains accumulate at the bottom, gradually making the water shallower.
As sediment accumulates, rooted submerged plants (Hydrilla, Vallisneria) establish. Their roots trap more sediment, further reducing water depth. Organic matter builds up on the bottom.
Water becomes shallow enough for floating plants (Nelumbo, Nymphaea) and later marsh plants (Typha, Sagittaria) that are rooted but emerge above water. The pond edges begin to look like a marsh.
As the area dries further, grasses and sedges colonise (meadow stage), followed by shrubs. The once-aquatic habitat is now terrestrial in character.
Finally, trees establish and a stable forest community develops. This is the climax community — self-sustaining and in equilibrium with the climate. For most of India’s climate, the climax community is a forest.
graph LR
A["Bare pond<br/>Pioneer: Phytoplankton"] --> B["Submerged plants<br/>Hydrilla"]
B --> C["Floating plants<br/>Nelumbo, Nymphaea"]
C --> D["Marsh plants<br/>Typha"]
D --> E["Meadow<br/>Grasses, Sedges"]
E --> F["Shrubs"]
F --> G["Climax: Forest"]Why This Works
Succession is a directional, predictable change in community composition over time. In hydrarch succession, the environment changes from aquatic to terrestrial — each stage modifies the habitat, making it suitable for the next stage.
The key principle: species modify the environment to the point where they are replaced by species better adapted to the new conditions. Phytoplankton make the water shallower (bad for themselves, good for submerged plants). Submerged plants trap more sediment (good for marsh plants), and so on.
The opposite process — xerarch succession — starts on bare rock and progresses through lichens, mosses, grasses, shrubs, to forest. Both hydrarch and xerarch converge on the same climax: a mesic (moderately moist) community.
Alternative Method — Compare Hydrarch and Xerarch
Hydrarch: starts wet, gets drier. Xerarch: starts dry, gets moister. Both converge at a mesic climax. This convergence is a key NEET concept.
For NEET, remember: hydrarch = water to land (pioneer: phytoplankton), xerarch = rock to land (pioneer: lichens/crustose). The climax community depends on the climate, not the starting point. In the same region, both types of succession lead to the same climax forest.
Common Mistake
Students confuse primary and secondary succession. Primary succession occurs on bare, lifeless surfaces (new rock, new pond). Secondary succession occurs where a community was destroyed but soil/seed bank remains (after fire, flood). Secondary succession is faster because soil and seeds are already present. Hydrarch succession is an example of primary succession.