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Grade 10-11 (age 15-17)

high school Community

A community is all the populations of different species living together in one area, and each species differs in what it eats and where it lives. In a forest, plants split the light by height, so layers form from the canopy at the top down to the forest floor. Species play different parts: dominant species characterize the community, keystone species shape it far beyond their numbers, and indicator species reveal the state of their surroundings. Over time the community changes through succession until it settles at a stable climax. The diagram shows the five forest layers, what typically lives in each one, and how the light fades from top to bottom.

Populations Together Make a Community

🌳 What Is a Community?
①A community is every population of every species living together in one area
②Its populations are tied together by who eats whom, forming a food web
③Each species has its own ecological niche, set by what it eats and where it lives

Layers of a Forest

🔆 Why Layers Form
①The top of the forest gets full sunlight, so the tallest trees take it
②Lower down, the leaves above block more and more light, and only plants that grow in dim light remain
③Sharing out a limited resource, light, by height produces the layering: canopy, subcanopy, shrub layer, herb layer and ground layer
④The ground layer holds mosses, fungi and fallen leaves, and decomposers break the litter down and return its nutrients to the soil

The Roles of Species

Members of a community

Species by role

Ecological niche

Niche and Competition

Succession: How a Community Changes over Time

Succession

Primary vs Secondary Succession
TypeStarting environmentPioneerSpeed
Primary (dry)soilless lava plain or bare rocklichensslow
Secondaryafter fire or logging; the soil remainsherbs (grasses)fast
Example 1: The Order of Succession
In succession that begins on a lava plain with no soil at all, which pioneer arrives first and helps build soil, and what is the final stable stage?
1
Succession that starts where there is no soil is dry primary succession.
2
Lichens, which can live on bare rock, arrive first, wear the rock down and leave organic matter that becomes soil. The community then passes through grassland, shrubland, sun-tolerant forest and mixed forest, and settles as a shade-tolerant forest, where young trees can grow in dim light.
▸ pioneer = lichens; final stable stage = shade-tolerant forest (climax)
A sun-tolerant forest turns into a shade-tolerant one because the grown sun-tolerant trees shade the forest floor. Their own seedlings cannot grow in that shade, while seedlings of shade-tolerant trees can, and they take over. It is the same fading light you see from top to bottom in the layer diagram.

Summary

Order of dry primary succession
bare lava → lichens → grassland → shrubland → sun-tolerant forest → mixed forest → shade-tolerant forest (climax)
lichens are the pioneers, and the climax, the stable stage that no longer changes much, is a shade-tolerant forest
Competitive exclusion principle
two species with fully overlapping niches → competition → one is eliminated
by dividing the niche (resource partitioning), the two species can coexist
CSAT-style problem (Life Science I), adapted
Succession took place in two regions, A and B. A began in a forest after a fire, and B on a new volcanic island with no soil. Choose the correct statement.
①In A, lichens arrive first as pioneers.
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-tolerant forest.
⑤As succession proceeds, species diversity always decreases.
▸ ③ A still has soil, so it starts from herbs and proceeds faster than B.
1
A (after a fire) keeps its soil, so it is secondary succession that starts from herbs and moves fast; B (a volcanic island) has no soil, so it is primary succession that starts from lichens and moves slowly.
2
① and ② swap the succession types of A and B, and ④ calls the climax a sun-tolerant forest, when it is usually a shade-tolerant forest.
3
Species diversity generally rises from the early to the middle stages, so ⑤ is also wrong. The answer is ③.
🎯 Exam Points
①A community is the set of populations in one area, linked by a food web
②The dominant species has the highest importance value; a keystone species shapes the community despite small numbers; an indicator species reveals the state of its environment
③A niche combines a food niche and a spatial niche; full overlap leads to competitive exclusion, while partitioning allows coexistence
④Primary succession starts on soilless ground with lichens and is slow; secondary succession starts where soil remains, with herbs, and is fast
⑤A sun-tolerant forest gives way to a shade-tolerant one because of low light on the forest floor, and the shade-tolerant climax stays stable for a long time
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