Ecosystem stability and change

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Lesson notes

Ecosystem Stability

**Ecosystem stability** (or equilibrium) is the ability of an ecosystem to return to its original state after a disturbance.
A stable ecosystem can resist change or recover quickly from disruptions like fires, floods, or human activities.
Stability includes **resilience** (speed of recovery) and **resistance** (ability to avoid change).
Not all ecosystems are stable; some undergo constant change (e.g., predator-prey cycles on Isle Royale).

Components of Stability

**Constancy**: little change in ecosystem characteristics over time (e.g., species abundance remains steady).
**Persistence**: the ecosystem continues to exist over long periods despite disturbances.
**Resilience**: how quickly an ecosystem returns to equilibrium after a perturbation.
An ecosystem can be stable in some properties (e.g., biomass) but unstable in others (e.g., biodiversity).

Factors Affecting Stability

**Biodiversity** often increases stability: more species provide functional redundancy and buffer against change.
**Food web complexity** can enhance resilience by providing alternative pathways for energy flow.
**Physical environment** (climate, soil, water) influences how ecosystems respond to disturbances.
Human activities like deforestation, pollution, and climate change can reduce stability.

Disturbances and Change

**Perturbations** are events that disrupt ecosystem structure or function (e.g., drought, wildfire, invasive species).
Ecosystems may shift to a **new equilibrium** if disturbances are severe or persistent.
**Ecological succession** is the gradual process of change in species composition after a disturbance.
Some changes are cyclic (e.g., seasonal variations) and do not threaten long-term stability.

Measuring Stability

Scientists use mathematical models (e.g., differential equations) to analyze stability at equilibrium points.
The **Jacobian matrix** (or community matrix) helps determine if a system returns to equilibrium after small changes.
Robert May's work in the 1970s used random matrix theory to explore diversity-stability relationships.
Field studies monitor species abundance, biomass, and nutrient cycling over time to assess stability.

Examples of Stable and Unstable Ecosystems

**Stable**: Grassland plant communities often show high resilience to grazing and fire.
**Stable**: Microbial communities in soil can maintain function despite temperature fluctuations.
**Unstable**: The wolf-moose system on Isle Royale exhibits large population oscillations.
**Unstable**: Coral reefs can undergo phase shifts to algae-dominated states after bleaching events.

Energy pyramid showing trophic levels. Energy decreases at each level, affecting ecosystem stability.

A simplified food web illustrating energy flow and trophic relationships that contribute to ecosystem stability.

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Practice questions

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1.What term describes the ability of an ecosystem to return to its equilibrium state after a disturbance?
Easy
- AResilience
- BResistance
- CPersistence
- DConstancy
2.Which of the following best defines ecological stability?
Easy
- AAn ecosystem that never changes
- BAn ecosystem that returns to equilibrium after a disturbance or does not experience unexpected large changes
- CAn ecosystem with high biodiversity
- DAn ecosystem that is constant in all properties
3.According to the source, which of the following is an example of an ecosystem that is not stable?
Medium
- AA grassland plant community
- BA microbial community
- CThe wolf and moose population on Isle Royale
- DA predator-prey system that oscillates cyclically
4.Which of the following is NOT one of the more specific terms proposed by Grimm and Wissel to replace 'ecological stability'?
Medium
- AConstancy
- BResilience
- CPersistence
- DDiversity
5.In the context of ecological stability, what does 'persistence' mean?
Hard
- AThe ability to return to equilibrium after a disturbance
- BThe tendency to remain unchanged over time
- CThe continued existence of a system over a long period
- DThe resistance to change in species composition
6.Which of the following describes a stable ecosystem?
Easy
- AAn ecosystem that experiences large changes frequently
- BAn ecosystem that can recover from disturbances and maintain its characteristics
- CAn ecosystem with no predators
- DAn ecosystem that never loses species
7.A vegetation community in response to a drought might conserve biomass but lose biodiversity. This shows that:
Medium
- AThe community is stable in all properties
- BThe community is unstable in all properties
- CAn ecosystem can be stable in some properties and unstable in others
- DDrought always causes loss of biomass
8.According to the source, what did Robert May investigate using the Jacobian matrix?
Hard
- AThe relationship between diversity and stability of an ecosystem
- BThe effect of noise on biological systems
- CThe definition of ecological stability
- DThe predator-prey cycle

Revision notes

## Ecosystem Stability and Change ### Key Concepts - **Ecological stability (equilibrium)**: An ecosystem's ability to return to its original state after a disturbance (resilience) or to avoid unexpected large changes over time. - **Resilience**: Capacity to recover after a perturbation. - **Persistence**: Ability to remain over time despite disturbances. - **Constancy**: Lack of change in a property (e.g., biomass, biodiversity). ### Stability vs. Instability - Not all ecosystems are stable. Example: wolves and moose on Isle Royale show fluctuating populations. - An ecosystem can be stable in some properties but unstable in others. Example: a vegetation community may conserve biomass but lose biodiversity during drought. ### Types of Stability (Grimm & Wissel, 1997) - Over 70 definitions exist. To avoid confusion, use specific terms like **constancy**, **resilience**, and **persistence**. - Example: A predator-prey cycle is persistent and resilient, but not constant. ### Stability Analysis (Robert May, 1970s) - Uses a **Jacobian matrix** (community matrix) to test stability near an equilibrium point. - Linearizes the system of differential equations: \[ \frac{dN_i}{dt} = g_i(N_1, \ldots, N_S), \quad i=1,\ldots,S \] At a fixed point \(N_1^\star, \ldots, N_S^\star\): \[ \frac{dN_i}{dt} = \sum_{j=1}^S J_{ij}(N_j - N_j^\star) \] The fixed point is **linearly stable** if all eigenvalues of the Jacobian have negative real parts. ### May's Random Matrix Theory - May applied random matrix ideas to large ecosystems with \(S\) species. - Stability depends on the strength and pattern of interactions. - **Key insight**: Highly diverse or strongly interacting systems may be less stable. ### Takeaway for Exam - Understand the difference between resilience, persistence, and constancy. - Know that stability is not a single property; an ecosystem can be stable in one aspect and unstable in another. - Remember that stability analysis uses eigenvalues of the Jacobian matrix. - Be able to give examples: stable grasslands vs. unstable predator-prey dynamics.

Ecological stability: The ability of an ecosystem to return to its equilibrium state after a perturbation (resilience) or to avoid unexpected large changes over time.
Resilience: The capacity of an ecosystem to recover after a disturbance.
Persistence: The ability of an ecosystem or community to continue existing over time despite disturbances.
Constancy: Lack of change in a particular property of an ecosystem (e.g., biomass, species richness) over time.
Jacobian matrix: A matrix of partial derivatives used in stability analysis to determine if an equilibrium point is stable by checking eigenvalues.
Linear stability: A fixed point is linearly stable if all eigenvalues of the Jacobian matrix have negative real parts, meaning small perturbations decay over time.

Flashcards

What is ecological stability?

Ecological stability is the ability of an ecosystem to return to its equilibrium state after a disturbance (resilience) or to avoid unexpected large changes over time.

What is resilience in an ecosystem?

Resilience is the capacity of an ecosystem to return to its equilibrium state after a perturbation or disturbance.

Can an ecosystem be stable in some properties but unstable in others?

Yes, for example, a vegetation community during a drought might conserve biomass (stable) but lose biodiversity (unstable).

Give an example of a naturally unstable ecosystem.

The wolf and moose populations on Isle Royale are an example of an unstable ecosystem, as they do not remain constant over time.

What role does noise play in biological systems?

Noise (random fluctuations) can significantly influence biological systems and, in some cases, fully determine their temporal dynamics.

What are three specific terms that can replace 'ecological stability' for clarity?

Constancy, resilience, and persistence are more specific terms that can describe different aspects of stability.

Define constancy in ecology.

Constancy refers to an ecosystem's ability to remain unchanged or vary within narrow limits over time.

Define persistence in ecology.

Persistence is the ability of an ecosystem to continue existing over time, even if it undergoes fluctuations.

How would you describe an ecosystem that oscillates cyclically around a fixed point (e.g., predator-prey cycles)?

Such an ecosystem would be described as persistent and resilient, but not constant.

What is the Jacobian matrix used for in ecology?

The Jacobian matrix (or community matrix) is used in stability analysis to test whether an ecosystem's equilibrium point is stable by examining its eigenvalues.

What did Robert May investigate using stability analysis in the 1970s?

Robert May used stability analysis to investigate the relationship between the diversity and stability of an ecosystem.

How did May analyze large ecosystems?

May used random matrix theory, assuming the ecosystem's interactions could be approximated by a random matrix, to study stability properties.

What is a fixed point in an ecosystem model?

A fixed point is an equilibrium state where species abundances remain constant over time, such that dN_i/dt = 0 for all species.

What condition must be met for a fixed point to be linearly stable?

All eigenvalues of the Jacobian matrix must have negative real parts for the fixed point to be linearly stable.

What does it mean if an ecosystem is 'linearly stable'?

Linear stability means that after a small perturbation, the system will return to its equilibrium state.

What is a perturbation in ecology?

A perturbation is a disturbance or change that affects an ecosystem, such as a drought, fire, or introduction of a new species.

How can changes to physical components of an ecosystem affect populations?

Changes like temperature shifts or water availability can alter habitats, food sources, and survival rates, leading to population changes.

How can changes to biological components of an ecosystem affect populations?

Changes like introduction of a predator or disease can reduce prey populations, while removal of a species can cause cascading effects.

What is dynamic equilibrium in an ecosystem?

Dynamic equilibrium is a state where ecosystem properties fluctuate around a stable average, maintaining overall balance despite changes.

What is ecosystem resilience?

Ecosystem resilience is the speed and ability of an ecosystem to recover from a disturbance and return to its original state.

What is the difference between resilience and constancy?

Resilience is the ability to recover after disturbance, while constancy is the ability to remain unchanged.

Why is biodiversity important for ecosystem stability?

Higher biodiversity can provide functional redundancy, making ecosystems more resilient to disturbances.

What happens when an ecosystem loses its resilience?

It may shift to a different state (e.g., grassland to desert) and not return to the original equilibrium.

Give an example of a human activity that can disrupt ecosystem stability.

Deforestation can remove habitat, alter water cycles, and reduce biodiversity, destabilizing the ecosystem.

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