Ecosystem resilience

1. What Is Resilience?

  • The lecturer frames resilience as “incredibly fascinating… ecosystem resilience” and notes that the concept now appears in economics, psychology, and politics but originated in ecology.

  • Resilience relates to how ecosystems maintain functioning, resist change, recover, or adapt when disturbed.

Why resilience matters

  • We have an intuitive sense of when an ecosystem is “not doing what it should be doing.”

    • Quote: “We have expectancy of what an ecosystem should be doing… if it starts no longer having that… something’s going wrong.”

  • Ecosystems have intrinsic properties (temperature, salinity, rainfall, biodiversity, competition) that define a functional regime.

2. Regime Shifts

A regime shift is a dramatic, persistent change in ecosystem structure and function.

Examples

  • Mongolian grasslands shifting to desert due to overgrazing.

  • Pacific fish populations:

    • Pacific salmon and Japanese sardine show alternating boom–bust cycles.

    • Quote: “Real good time… then all of a sudden it appears… wasn’t there before.”

  • Kelp forest collapse in NW USA:

    • Orcas → eat otters → otters no longer control urchins → urchins overgraze kelp → kelp beds collapse.

    • Quote: “The kelp beds completely collapsed.”

  • Black Sea collapse:

  • Overfishing + eutrophication → loss of predatory fish → boom in planktivorous fish → zooplankton decline → jellyfish increase → oxygen decline.

Coral reef example

  • Six possible states: healthy coral → stressed → algal-dominated → turf → urchin barren → slime.

  • Multiple drivers can push reefs between states.

3. Why Regime Shifts Matter

  • They cause major ecological and socioeconomic impacts:

  • Loss of fisheries, tourism, coastal protection, biodiversity.

  • Quote: “You can guarantee politicians are going to look up… and try to get back to where we were before.”

4. What Triggers Regime Shifts?

Students identified several triggers; lecturer confirms:

Abiotic triggers

  • Temperature increases (e.g., coral bleaching).

  • Nutrient input / eutrophication.

  • Sediment load, salinity changes.

  • Storms, heatwaves.

Biotic triggers

  • Keystone species removal (e.g., otters).

  • Predation changes, grazing pressure.

  • Disease outbreaks.

Extrinsic vs intrinsic

  • Extrinsic = outside the system (runoff, climate oscillations).

  • Intrinsic = internal dynamics (competition, species interactions).

5. Thresholds and Tipping Points

Optimal range → stress → collapse

  • Ecosystems have a tolerance spectrum.

  • Quote: “If we start straying outside… we will start seeing symptoms… eventually a collapse.”

Two types of regime shift dynamics

  1. Gradual (linear) shift

    • Slow environmental change → slow ecosystem change.

    • System can move back if conditions reverse.

  2. Abrupt (non-linear) shift / tipping point

  • System appears stable until a threshold is crossed → sudden collapse.

  • Quote: “Gradual change… and then all of a sudden boom it collapses.”

6. Bottom-Up and Top-Down Regime Shifts

Bottom-up

  • Triggered by changes at the base of the food web (phytoplankton).

  • Example: Pacific Decadal Oscillation altering temperature/upwelling → sardine/salmon cycles.

  • Example: Nutrient limitation → phytoplankton decline → zooplankton decline → fish decline.

Top-down

  • Triggered by changes at the top of the food web.

  • Example: Orcas → otters → urchins → kelp collapse.

Combined bottom-up + top-down

  • Coral reefs:

  • Nutrients (bottom-up) + predator removal (top-down) → diagonal shift toward algal/urchin/slime states.

7. The “Cup and Ball” Model of Resilience

A central conceptual model in resilience theory.

Key ideas

  • The ball = current ecosystem state.

  • The valley = stability domain (how strongly the system returns to its state).

  • The landscape = shaped by biology, physics, and feedbacks.

Interpretation

  • Deep valley = high resilience (system resists change).

  • Shallow valley = low resilience (system easily pushed into alternative state).

  • Quote: “This whole landscape is generated by the biology… everything that forces the ball to want to go back.”

Two ways regime shifts occur

  1. Gradual erosion of the valley → system becomes precarious → small disturbance flips it.

  2. Single violent event → system pushed directly into a new valley.

8. Practical Examples of the Cup-and-Ball Model

Coral reef

  • Overfishing + eutrophication erode resilience → bleaching or storms push reef into algal state.

Clear-water lake

  • Phosphorus loading → resilience erodes → algal bloom → turbid lake state.

Seagrass bed

  • Grazers removed + salinity change → heatwave triggers collapse → phytoplankton-dominated state.

9. Hysteresis

A crucial concept in resilience theory.

Definition

  • The path to recovery is not the same as the path to collapse.

  • Quote: “We have to bring the conditions even much, much, much better… before the system will switch back.”

Implication

  • Restoration is harder than preventing collapse.

  • Systems may remain stuck in degraded states even after conditions improve.

10. Engineering vs Ecological Resilience

Engineering resilience

  • Focuses on speed of return to a single equilibrium.

  • Does not consider alternative stable states.

Ecological resilience

  • Accepts multiple stable states.

  • Focuses on how much disturbance a system can absorb before shifting.

11. Biodiversity and Resilience

Redundancy

  • Multiple species performing similar roles → system can compensate if one is lost.

  • Quote: “If we lose one grazer, the others can compensate… the whole food web will not collapse.”

Functional diversity

  • Different species traits → greater adaptability, resistance, and recovery capacity.

Biodiversity underpins the three resilience traits

  • Resistance (withstand change)

  • Recovery (bounce back)

  • Adaptability (shift functionally to cope)