After a volcanic eruption covers land with bare lava, lichens and mosses begin to colonize the rock. Which type of succession is this, and why?
ASecondary succession, because the eruption was a disturbance to a previously occupied landscape
BPrimary succession, because colonization begins on bare substrate with no soil or seed bank
CInhibition succession, because the eruption creates conditions hostile to most species
DClimax succession, because lichens are highly specialized for this environment
The defining criterion for primary vs. secondary succession is not whether a disturbance occurred but whether soil and biological legacies (seed bank, organic matter) remain afterward. Fresh lava has neither — organisms are starting from scratch on bare rock. A common error is classifying any post-disturbance sequence as secondary succession; secondary requires that soil and seeds survived the disturbance (as after a fire or field abandonment).
Question 2 True / False
Most ecosystem, if left undisturbed long enough, will converge on a single predictable climax community determined largely by regional climate.
TTrue
FFalse
Answer: False
The classical Clementsian climax community concept has been largely revised. Modern ecology recognizes that alternative stable states can persist indefinitely — different communities can occupy identical climates depending on historical contingency (which species arrived first), legacy effects, and ongoing disturbance regimes. Two identical forest patches can follow different successional trajectories and settle into genuinely different community types. Climate sets broad constraints but does not fully determine a unique endpoint.
Question 3 Short Answer
The intermediate disturbance hypothesis predicts that species diversity peaks at moderate disturbance frequency. Explain why diversity declines at both very low and very high disturbance levels.
Think about your answer, then reveal below.
Model answer: At very low disturbance, competitive exclusion proceeds unchecked — dominant species outcompete and eliminate subordinate ones over time. At very high disturbance, most species cannot establish or reproduce between disturbance events, leaving only disturbance-tolerant specialists. Moderate disturbance interrupts competitive exclusion often enough to allow coexistence of many species, while still permitting enough recovery time for non-specialists to persist.
This tests mechanistic understanding rather than pattern recall. The key insight is that disturbance acts differently at each extreme: too little favors competitive dominants; too much favors only stress-tolerant specialists. The middle range creates a diversity-promoting balance between these opposing pressures.