A volcano erupts and buries a hillside in bare lava. A century later it's a forest. How? Nature rebuilds in a predictable sequence, and the first colonists aren't oak trees — they're lichens breaking rock into soil, one crumb at a time. That sequence, ecological succession, is one of the most testable ideas in Unit 2 because it's visual and logical. The flip side is what happens on isolated patches — literal islands, or "habitat islands" like a park surrounded by city. Island biogeography predicts how many species a patch can hold based on two variables: how big it is and how far from a source of colonists. Two clean models, lots of exam points.
Ecological succession is the gradual, somewhat predictable change in community composition following a disturbance or on newly exposed ground.
Primary succession starts on ground with no soil — bare rock from a retreating glacier, a new volcanic island, a lava flow. The first colonizers are pioneer species, typically lichens and mosses that can grow on bare rock. They weather the rock and, when they die, add organic matter, gradually building soil. Once thin soil exists, grasses, then shrubs, then trees follow. Because soil must form first, primary succession is very slow (centuries to millennia).
Secondary succession starts where a disturbance (fire, farming abandonment, flood, hurricane, logging) removed the community but left the soil intact. Because soil, seeds, and roots survive, recovery is much faster (decades). An abandoned farm field going from weeds → grasses → shrubs → pine → hardwood forest is the classic example.
[DIAGRAM: Two panels. Top: PRIMARY succession — bare rock → lichens/mosses (pioneers) → small soil + grasses → shrubs → conifers → deciduous forest, with a long time arrow (100s–1000s yrs). Bottom: SECONDARY succession — cleared field (soil present) → weeds/grasses → shrubs → fast-growing trees → mature forest, with a shorter time arrow (decades). Y-axis note: species richness and biomass rise over time toward the climax community.]
The relatively stable end stage is the climax community, historically thought to be a single stable endpoint. Modern ecology treats it as dynamic — subject to ongoing disturbance, so communities are often a shifting mosaic rather than a fixed endpoint.
Early- vs. late-succession species: Pioneers are typically r-selected (fast-growing, many offspring, tolerant of harsh conditions, poor competitors). Late-succession species are typically K-selected (slow-growing, good competitors, shade-tolerant). Succession generally increases species richness, biomass, and soil development over time — until a disturbance resets it.
Disturbance and keystone effects: Periodic disturbances (fire, grazing) can maintain diversity by preventing any one competitor from dominating. Some ecosystems (chaparral, some grasslands, longleaf pine) are fire-adapted and depend on periodic fire.
Island biogeography theory predicts the number of species an island (or isolated habitat patch) supports as a balance between two rates: - Immigration of new species (arriving from a mainland source), and - Extinction of species already present.
Two geographic variables drive the outcome:
[DIAGRAM: Immigration rate curve (declines as # species present rises) and extinction rate curve (rises as # species present rises) crossing at an equilibrium species number S. Show that near islands shift the immigration curve up (higher S) and small islands shift the extinction curve up (lower S*). Separate small species–area plot: richness increasing with island area (curve rising and leveling).]
"Large-and-near" islands hold the most species; "small-and-far" islands the fewest.
Why it matters for conservation: Habitat fragmentation turns continuous habitat into "islands." The theory predicts that large, connected reserves near other habitat conserve more species than small, isolated ones — the basis for wildlife corridors that connect fragments and boost effective immigration.
Succession appears as sequence questions ("what comes first after a glacier retreats?") and as primary-vs-secondary distinctions. Island biogeography appears as size/distance predictions and as the theoretical justification for reserve design and corridors (Unit 5).
A forest fire burns a woodland to the ground but leaves the soil. Which type of succession follows, and roughly how fast?
Solution: Soil remains → secondary succession, which is relatively fast (decades) because seeds, roots, and nutrients survive.
Interpretation: The presence or absence of soil is the deciding question every time.
A glacier retreats, exposing bare rock. What are the likely pioneer species, and what critical function do they perform?
Solution: Lichens and mosses are the pioneers. They break down rock (weathering) and add organic matter as they die, building the first soil that later plants require.
Interpretation: No soil → primary succession → lichens/mosses first.
Two islands lie off a coast: Island A is 500 km² and 20 km from the mainland; Island B is 50 km² and 300 km away. Predict which supports more species and explain using both variables.
Solution: Island A supports more species. It is larger (more habitat, larger populations, lower extinction) and closer to the mainland (higher immigration). Island B is both small (higher extinction) and far (lower immigration), so it holds the fewest species.
Interpretation: Large + near = maximum richness; small + far = minimum.
A highway splits a forest reserve into two halves. Explain, using island biogeography, why building a wildlife corridor between them would help conserve species.
Solution: The highway fragments habitat into two smaller "islands," each with higher extinction rates (smaller populations) and reduced immigration between them. A corridor reconnects the fragments, effectively increasing area and immigration, lowering extinction risk, and maintaining gene flow — raising the number of species each fragment can sustain.
Interpretation: Corridors convert "small-and-isolated" back toward "large-and-connected."
(C) Fire maintains diversity and triggers seed release in fire-adapted systems.
Bare lava/rock (no soil) → pioneer species (lichens, mosses) weather rock and build soil → grasses/herbs once thin soil forms → shrubs → fast-growing (r-selected) trees → mature climax forest. Rate is limited by soil formation, which is slow, so primary succession takes centuries to millennia.
The 20-ha patch loses species faster: it is smaller (smaller populations → higher extinction rate) and, being isolated, receives less immigration to replace losses. Both effects lower the equilibrium species number.
FRQ rubric (10 pts): - (a) 1 pt IV = patch size/area; 1 pt DV = bird species richness. (2) - (b) 1 pt sample multiple patches spanning a range of sizes; 1 pt use a standardized survey (point counts / transects) of equal effort in each; 1 pt count distinct species over the same time/season. (3) - (c) 1 pt names a valid control/covariate (distance to source habitat, habitat type/quality, survey effort, season); 1 pt explains it could otherwise confound the area–richness relationship. (2) - (d) 1 pt richness rising with area matches the species–area relationship (larger area → more species); 1 pt leveling off reflects the balance of immigration and extinction reaching equilibrium. (2) - (e) 1 pt recommends corridors connecting patches / enlarging patches / adding stepping-stone habitat, with a justification (raises immigration / lowers extinction). (1)
Researchers want to test whether habitat patch size affects bird species richness in a fragmented landscape.
(a) State the independent and dependent variables. (2 pts) (b) Describe a method for collecting data across patches of different sizes. (3 pts) (c) Identify one variable that should be controlled or accounted for, and explain why. (2 pts) (d) The data show richness increases with patch area, then levels off. Explain how this supports island biogeography theory. (2 pts) (e) Recommend one landscape-design change to increase richness in the smallest patches, and justify it. (1 pt)
MC: 1. (B) Primary — no soil. 2. (B) Lichens and mosses. 3. (B) Soil/seeds/roots survive, so recovery is faster. 4. (D) Large and near. 5. (C) Richness and biomass increase toward the climax. 6. (B) Pioneers are r-selected, fast-growing. 7. (C) Corridors reconnect fragments. 8. (C) Abandoned farm field retains soil → secondary. Others start on bare, soil-less ground. 9. (B) More species with more area. 10. (C) Fire maintains diversity and triggers seed release in fire-adapted systems.
Bare lava/rock (no soil) → pioneer species (lichens, mosses) weather rock and build soil → grasses/herbs once thin soil forms → shrubs → fast-growing (r-selected) trees → mature climax forest. Rate is limited by soil formation, which is slow, so primary succession takes centuries to millennia.
The 20-ha patch loses species faster: it is smaller (smaller populations → higher extinction rate) and, being isolated, receives less immigration to replace losses. Both effects lower the equilibrium species number.
FRQ rubric (10 pts): - (a) 1 pt IV = patch size/area; 1 pt DV = bird species richness. (2) - (b) 1 pt sample multiple patches spanning a range of sizes; 1 pt use a standardized survey (point counts / transects) of equal effort in each; 1 pt count distinct species over the same time/season. (3) - (c) 1 pt names a valid control/covariate (distance to source habitat, habitat type/quality, survey effort, season); 1 pt explains it could otherwise confound the area–richness relationship. (2) - (d) 1 pt richness rising with area matches the species–area relationship (larger area → more species); 1 pt leveling off reflects the balance of immigration and extinction reaching equilibrium. (2) - (e) 1 pt recommends corridors connecting patches / enlarging patches / adding stepping-stone habitat, with a justification (raises immigration / lowers extinction). (1)
⭐ Exam strategy: For succession, your first question is always "is there soil?" — that alone determines primary vs. secondary. For island biogeography, always cite BOTH size (extinction) and distance (immigration) — full-credit answers use both levers.
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