Community

A community is the set of populations that live together in one area, and each species holds its own ecological niche. In a forest, plants share light by height, forming layers from canopy to ground. Dominant, keystone, and indicator species play different roles, and primary versus secondary succession differ in start and speed. Explore the canvas of forest layers and the classification tables for species roles and succession.

Many populations together form a community

🌳 What is a community?

①Community = the set of all populations living together in one area

②The populations are linked through food chains and food webs

③Each species holds a unique ecological niche (role) within the community

Forest stratification — sharing the light

🔆 Why do layers form?

①Higher up the light is strong, so tall plants (trees) take it

②Lower down the light weakens, so shade plants grow

③Sharing the resource of light by height creates the layered structure

The species that make a community

Member species

Classified by their role

Ecological niche

Niche and competition

Succession — change over time

Succession

ChartPrimary vs secondary succession

Type	Starting environment	Pioneer	Speed
Primary (dry)	soilless lava plateau / bare rock	lichens	slow
Secondary	after fire or logging, soil remains	herbs (grasses)	fast

Example 1 — order of succession

In succession that begins on a soilless lava plateau, which pioneer first arrives and helps build soil, and what is the final stable stage of succession?

Succession beginning where there is no soil is dry primary succession.

The pioneer is lichens, which live even in harsh sites; it stabilizes at the shade-tree forest that tolerates low light.

▸ pioneer = lichens; final stable stage = shade-tree forest (climax)

Sun-tree forest turns into shade-tree forest because the grown sun-trees' shade stops young sun-trees, while young shade-trees grow better.

Summary

Order of dry primary succession

bare lava → lichens → grassland → shrubs → heliophyte forest → mixed forest → sciophyte forest (climax)

the pioneer is lichens; the final stable stage is the climax (shade-tree forest)

Competitive exclusion principle

two species with fully overlapping niches → competition → one is eliminated

partitioning the niche lets the two species coexist

CSAT-style (adapted, Biology I)

Succession occurred in two regions A and B. A started in a burned forest, B on a new volcanic island with no soil. Choose the correct statement.

①In A, lichens are the first pioneer to arrive.

②B is secondary succession, so it proceeds faster than A.

③A still has soil, so it starts from herbs and proceeds faster than B.

④In both regions the climax is a sun-tree forest.

⑤As succession proceeds, species diversity always decreases.

▸ ③ A still has soil, so it starts from herbs and proceeds faster than B.

A (after fire) keeps soil → secondary succession, starts from herbs, fast. B (volcanic island) has no soil → primary succession, starts from lichens, slow.

①, ② swap the succession types of A and B (wrong); the climax in ④ is normally a shade-tree forest (wrong).

Diversity generally rises in the early–middle stages, so ⑤ is wrong. Hence ③ is correct.

🎯 Exam Points

①Community = set of populations, linked by a food web

②Dominant (represents), keystone (large impact), indicator (signals environment)

③Niche = food niche + spatial niche; full overlap → competitive exclusion, partitioning → coexistence

④Primary (lichen pioneer, slow) vs secondary (herb pioneer, soil present, fast)

⑤Climax = shade-tree forest (stable); sun→shade forest shift is due to light competition

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Matter Cycling & Ecosystem Stability

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