Questions: Community Stability: Resistance and Resilience

Question 1

An old-growth temperate forest has remained essentially unchanged in species composition for centuries. After clear-cutting, the site has not recovered its original composition after 200 years. What does this pattern reveal about the forest's stability properties?

• A. The forest has high resistance and high resilience — centuries of stability followed by eventual recovery
• B. The forest has low resistance and low resilience — it failed to resist disturbance and cannot recover
• C. The forest has high resistance but low resilience — it withstood minor disturbances for centuries but recovers very slowly after major clearing
• D. The forest has low resistance but high resilience — its slow recovery proves it resists disturbance more than it recovers

Answer: (C)

Resistance is the ability to remain unchanged *during* disturbance; resilience is the ability to *recover after* disturbance. The centuries of compositional stability under natural stressors demonstrate high resistance. But the 200+ year recovery timeline after clear-cutting reveals extremely low resilience. This combination — high resistance, low resilience — is a key insight: the two properties are distinct and can trade off. It also has conservation implications: once a highly resistant ecosystem is overcome, restoration may take generations.

Question 2

A diverse grassland maintains relatively stable biomass production across several drought years, while an adjacent species-poor grassland crashes. According to the insurance hypothesis, what mechanism explains the diverse grassland's stability?

• A. More species means more total photosynthetic capacity, which offsets drought losses through sheer abundance
• B. Diverse communities have more predators, which prevent herbivore outbreaks that would amplify drought stress
• C. Functional redundancy allows drought-tolerant species to compensate when drought-sensitive species decline, maintaining overall community function
• D. Species-rich communities have deeper root networks on average, giving them better access to groundwater during drought

Answer: (C)

The insurance hypothesis holds that diversity buffers against environmental variability because different species respond differently to the same stressor. When drought causes drought-sensitive species to decline, drought-tolerant species with similar ecological roles expand and maintain overall ecosystem function. This functional redundancy — multiple species performing similar roles — is the mechanism, not raw abundance (A), predator effects (B), or average root depth (D). The analogy is insurance: you don't benefit most of the time, but when something goes wrong, backup capacity prevents collapse.

Question 3

A community can have high resistance to disturbance but low resilience after disturbance — these two properties are independent enough to trade off within the same ecosystem.

• T. True
• F. False

Answer: True

Resistance and resilience are distinct properties that measure different aspects of stability. Old-growth forests exemplify high resistance (centuries of compositional stability under chronic stress) combined with low resilience (extremely slow recovery if major disturbance eliminates the community). Grasslands often show the opposite: easily disturbed by fire but recovering within a few years. This independence is ecologically important because you cannot infer resilience from resistance or vice versa — conservation strategies must assess each property separately.

Question 4

Highly productive ecosystems are generally more stable than less productive ones because greater biomass provides a larger buffer against disturbance losses.

• T. True
• F. False

Answer: False

High productivity does not confer stability and can actually reduce it. A monoculture cornfield is among the most productive agricultural ecosystems but is catastrophically vulnerable to a single pathogen or weather event because it lacks the functional redundancy and diversity that buffer diverse communities. Highly productive systems are often species-poor and optimized for performance in one condition, not robustness across varied conditions. The diversity-stability relationship — not the productivity-stability relationship — is the reliable empirical pattern.

Question 5

Explain the difference between resistance and resilience, and describe why understanding both properties matters for conservation decisions.

Model answer: Resistance is a community's ability to remain unchanged during a disturbance — to absorb stress without shifting in composition or function. Resilience is its ability to return to its original state after disturbance has altered it. They matter independently for conservation because they suggest different management strategies: a highly resistant ecosystem rewards protection (prevent disturbance from reaching the threshold that overcomes resistance), while a highly resilient ecosystem is more forgiving and rewards restoration investment. An ecosystem with low resistance and low resilience needs both urgent protection of intact areas and pessimistic forecasts for restoration.

The two properties can trade off (old-growth forests: high resistance, low resilience; grasslands: low resistance, high resilience), so 'is this ecosystem stable?' is an incomplete question. Conservation ecologists must ask: stable in which sense, and against what disturbances? This framing also helps predict responses to climate change — a resistant ecosystem may persist for decades under gradually intensifying stress, then collapse suddenly with little capacity to recover, demanding intervention before the threshold is reached.