Ecosystem Stability:Components and Models

Ecosystems are Complex Adaptive Systems

*bottom-up self-organization leads to stability

•A system•Many parts•Interactions•Bottom-up•Self-organization•Emergence•Change•Feedbacks•Adaptive•Memory•Open to outside•Fuzzy boundaries•Non-equilibrium•Non-linear•Thresholds•Tipping points•Surprises•Stability

Ecosystem Stability

• The vast majority of natural ecosystems experience regular environmental change, or disturbances.

• Most ecologists describe ecosystem stability as the ability of an ecosystem to maintain its structure and function over long periods of time and despite disturbances.

Temporal, spatial and structural features of complex system

Amand et al. (2010)

Tweets (social interactions) in Japan in response to the 2011 Tsunami have a

scale-free pattern

Resistance and Resilience• There are two main components to ecosystem stability: resistance

and resilience.

• An ecosystem displays resistance if keeps its structure and continues normal functions even when environmental conditions change.

• An ecosystem displays resilience if, following a disturbance, it eventually regains its normal structure and function.

Resilience, the amount of energy that the system can absorb without leaving the cup for an alternative stability domain.

Stability, the speed at which the ball returns to homeostasis; correlated with productivity

Ball-and-cup model of system stabilityBall=Current state of system

stability

alternative domain

resilience

currentstability domain

stability

alternative domain

resilience

currentstability domain

Cup = Current stability domain

Tundra: low stability, low resilience

ICH: high stability, high resilience

CWH: high stability, low resilience

Garry Oak: low stability, high resilience

Managing ecosystems within the range of natural variability (RONV)

• RONV= resilience=range of possible locations of the ball within the cup

• Resilience: “the capacity of a system to absorb disturbance and reorganise while undergoing change so as to still retain essentially the same function, structure, identity, and feedbacks”.

• Management goal: make sure you stay in the cup and that it remains as wide and deep as possible

Maintaining stability

• Species diversity is often the key to both ecosystem resistance and resilience.

• An ecosystem rich in biodiversity will likely be more stable than one whose biodiversity is low.

How does environmental change affect ecosystem stability?

• Populations respond in ways that reflect the success or failure of members of the population to survive and reproduce.

• Species respond to environmental change in ways that enable them to maintain homeostasis.

• Communities respond to environmental change in ways that reflect the responses of the species and populations in the community.

FeedbacksNegative: stabilizes ecsosytems

Positive: destabilizes ecosystems

Ehrenfeld et al. (2005)

Loss of biodiversity can reduce stability

•Changing environmental conditions can cause the decline of local biodiversity. If this happens, an ecosystem’s resistance and/or resilience may decline. The end result is that the ecosystem loses stability.

•Ecosystems that are less stable may not be able to respond to a normal environmental disturbance, which may damage ecosystem structure, ecosystem function, or both.

Three types of changeT

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Tre

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%

Tre

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Precipitationdry wet

Precipitationdry wet

Precipitationdry wetDry Wet Dry Wet Dry Wet

% F

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over

Non-reversible

•Resistance: Ability of system to absorb small disturbances and prevent amplification

•Resilience: Ability of system to return to its original state

•Robustness: amount of disturbance system can absorb without flipping to alternative state

•Response: Magnitude of change•Recovery: Extent of return to original state

Ecosystem stability or response to disturbance depends on:

Alive then dead: shifting stability domains

Perry’s cup vs peak models of system stability1. Destabilization of ball

depends on force (cup) versus type or foreignness (peak) of disturbance.

2. Ecosystem has plenty of warning (cup) for threshold disturbances versus surprises (falls off peak) (tipping points)

3. Ball movement in cup reversible once disturbance removed, but not once knocked off peak (domino effects common)

4. Cup model suggests equilibrium, but ecosystems are always in disequilibrium

Gunderson & Holling 2002

Adaptive cycle of recovery (succession) after disturbance

r=growth (pioneer; stand initiation)K=carrying capacity (competition, niche specialization) (seral; stem exclusion)Ω=release, new opportunities (young climax; stand re-initiation)α=re-organization and recovery (late climax; old-growth)

Complex system cycle

Complex system cycle and threshold changesr K

Ωα

Panarchy: all-encompassing nested system

Panarchy in natural ecosystemsT

empo

ral s

cale

Spatial scale

Tim

e

Summary• Complex adaptive systems are inherently stable• Stable systems change but are homeostatic, like a dancer• Stables systems have resistance, where small

disturbances are contained, and resilience, where the system returns to the same stability domain

• Complex system cycles and panarchy are stabilizing characteristics

• Positive feedbacks and crossing tipping points can lead to loss of stability