Questions: Ecosystem Stability, Resilience, and Tipping Points

Question 1

A shallow lake remained clear for decades despite gradually increasing agricultural nutrient runoff. Then, abruptly, it shifted to a turbid state dominated by algae. When nutrient inputs were reduced back to the original level, the lake stayed turbid. What concept best explains why the reduction failed to restore the clear state?

• A. Functional redundancy — algae and rooted plants perform equivalent roles, so the system has no reason to prefer one state
• B. Hysteresis — the turbid state is self-reinforcing through feedbacks (no plant nutrient uptake, sediment resuspension, restructured fish community) that require nutrient reduction far below the original threshold to reverse
• C. Low resilience — the lake is returning to its natural turbid equilibrium state after the disturbance
• D. High resistance — the turbid state is resisting the change back to the clear-water regime

Answer: (B)

Hysteresis is the defining feature of tipping points: the path back to the original state is different from the path into the alternative state. The turbid lake has entered a self-reinforcing feedback loop — no rooted plants to absorb excess nutrients, sediment disturbance by bottom-feeding fish, and high phytoplankton biomass shading out any recovering plants. Simply removing the original stressor (reducing nutrients to pre-threshold levels) is insufficient to reverse these feedbacks. Restoration requires driving nutrients far below the original tipping point, or active intervention like fish removal. This asymmetry is what makes tipping points so dangerous in climate and conservation contexts.

Question 2

An ecologist compares ecosystem recovery after severe drought between a species-rich and a species-poor grassland. Both eventually recover similar biomass, but the species-rich grassland recovers faster. Which mechanism most directly explains this?

• A. The species-rich grassland has more total individuals, giving it greater biomass in reserve to draw upon during recovery
• B. Functional redundancy: multiple species performing similar roles ensure that drought-sensitive species can be replaced by drought-tolerant ones with equivalent function
• C. The species-poor grassland has fewer competitors, so individual plants recover more slowly due to reduced competitive pressure
• D. Species-rich grasslands have deeper average root systems that access more water during recovery

Answer: (B)

Functional redundancy is the ecological insurance mechanism: multiple species performing similar ecological roles (nutrient cycling, primary production, decomposition) mean that losing one to drought does not eliminate that function — other species with different environmental tolerances compensate. A species-poor grassland has no such backup — losing the dominant species can severely impair the function that species performed. Note that functional redundancy depends on which species are present and what roles they play, not just species count. Two equally species-rich grasslands could differ substantially in redundancy if one is dominated by many similar species and the other by functionally diverse ones.

Question 3

Resistance and resilience are positively correlated properties — an ecosystem that strongly resists disturbance will also bounce back quickly once pushed past its limits.

• T. True
• F. False

Answer: False

Resistance and resilience are somewhat independent properties that can be decoupled. A highly resistant ecosystem — one that maintains function during a disturbance — may be catastrophically fragile once its threshold is exceeded, recovering slowly or collapsing into an alternative state (analogous to a rigid structure that doesn't bend but shatters). Conversely, an ecosystem can be easily disturbed but bounce back rapidly (like a flexible structure that deforms and springs back). Managing for one property does not guarantee the other. This distinction matters for conservation: a highly resistant ecosystem in a changing climate may offer false security if its tipping point threshold is crossed.

Question 4

Tipping points in ecosystems are particularly concerning because, once crossed, restoring the original state typically requires reducing the stressor to levels far below the original tipping point — not simply removing the stressor.

• T. True
• F. False

Answer: True

This is the key management implication of hysteresis. Because the alternative state is self-reinforcing through positive feedbacks, simply removing the stressor that caused the shift is insufficient. Restoration requires driving conditions well below the original threshold to overcome the self-reinforcing feedbacks maintaining the alternative state, or actively dismantling those feedbacks through intervention (e.g., sediment treatment, food web manipulation in turbid lakes; active replanting in deforested areas). This asymmetry explains why early detection and prevention are so much more effective than post-collapse restoration.

Question 5

Explain why two ecosystems with identical species richness might differ substantially in their stability, and what this reveals about the limits of using species count alone as a stability predictor.

Model answer: Stability depends on functional composition, not just species number. Two ecosystems with equal richness can differ in functional redundancy (how many species perform each ecological role) and functional complementarity (how completely species partition the available niche space). If one high-richness ecosystem has many species performing the same function, it has high redundancy and will resist losing that function to perturbation. If another has equal richness but each species performs a unique function, losing any species eliminates that function despite high richness. Additionally, resistance and resilience are distinct and independently influenced by community composition — the same species may enhance resistance but contribute little to resilience or vice versa. Species count is a rough proxy; what matters is which species are present and what functional roles they fill.