Community Dynamics, Disturbance, and Succession

1. Disturbance and the Disturbance Regime

A disturbance is a relatively discrete event — such as fire, flood, windthrow, drought, or human clearing — that removes organisms or changes resource availability. Its regime is described by frequency (how often it occurs), intensity (how severe each event is), and extent (how large an area it affects). Communities are shaped as much by the pattern of disturbance as by steady conditions.

2. Succession and Its Mechanisms

Succession is the directional change in community composition over time. Primary succession begins on newly exposed surfaces with no soil (lava, glacial till, bare rock); pioneer species — hardy early colonizers such as lichens, mosses, and some grasses — arrive first. Secondary succession occurs after a disturbance that leaves soil and surviving seeds or roots (abandoned farmland, post-fire forest). Three mechanisms drive the sequence: facilitation, in which early species modify conditions to make them suitable for later species; tolerance, in which later species simply outlast or outgrow earlier ones because they tolerate resource-poor conditions; and inhibition, in which early species actively slow or block later arrivals.

3. Stability, Equilibrium, and Alternate States

The climax community was the classical idea of a single, predictable endpoint toward which succession proceeds. Modern ecology treats this as a useful simplification at best: soil development and species arrival are partly chance-driven, and recurring disturbance keeps many communities in a dynamic equilibrium — a state that persists but shifts around a central condition. Some systems can exist in alternative stable states, different configurations that are each self-sustaining under the same conditions and can flip between one another after a large disturbance. Stability has two distinct components: resistance, the ability to withstand a disturbance without changing, and resilience, the speed of recovery after change.

How it works

1. A disturbance (natural or human-caused) removes organisms or alters resources.
2. Colonization begins, either from a bare surface (primary) or surviving soil and seeds (secondary).
3. Pioneer species establish first and modify conditions.
4. Later species arrive through facilitation, tolerance, or despite inhibition.
5. Species composition shifts directionally until it reaches a dynamic equilibrium or is disturbed again.
6. The community's stability is judged by its resistance and resilience to future disturbance.

Common confusions

Memory aids

Remember "FIE" for a disturbance regime: Frequency, Intensity, Extent. For succession mechanisms, think "FIT": Facilitation, Inhibition, Tolerance. And "R & R": Resistance = Refuses to change, Resilience = Recovers fast.

Quick review

Topic Recap

• Community dynamics are driven by disturbance and succession.
• A disturbance regime is defined by frequency, intensity, and extent.
• Primary succession begins on bare, soil-free surfaces; secondary succession begins where soil and propagules remain.
• Facilitation, tolerance, and inhibition are the three mechanisms that move succession forward (or hold it back).
• The intermediate disturbance hypothesis predicts maximum diversity at intermediate disturbance.
• The climax community is an oversimplification; dynamic equilibrium and alternative stable states better describe real systems.
• Resistance and resilience are distinct, complementary measures of stability.

Knowledge Check

1. A new volcanic island emerges with bare lava. What kind of succession will occur, and why?
2. List the three components of a disturbance regime and define each.
3. Name the three mechanisms of succession and state how facilitation differs from inhibition.
4. A grassland is scorched by fire but its soil and seed bank survive; another area is stripped to bedrock. Which shows primary and which shows secondary succession?
5. Under the intermediate disturbance hypothesis, why does diversity drop at very low and very high disturbance levels?

Answers and Rationales

1. Primary succession, because the bare lava has no soil and no surviving seeds or roots; the community must build from scratch.
2. Frequency (how often disturbance occurs), intensity (how severe each event is), and extent (the area affected). Together they define the disturbance regime.
3. Facilitation, tolerance, and inhibition. Facilitation means early species make conditions better for later species; inhibition means early species slow or block later arrivals. They push succession in opposite directions.
4. The fire-scorched grassland with surviving soil and seed bank shows secondary succession; the area stripped to bedrock shows primary succession (no soil remains).
5. At very low disturbance, the best competitors exclude other species (low diversity); at very high disturbance, only a few stress-tolerant species survive (also low diversity). Diversity peaks at intermediate levels where disturbance prevents exclusion but is not so severe that most species are wiped out.