Ecosystem stability and change

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课程笔记

Ecosystem Balance

Ecosystem balance refers to the number of animals an area can support for long periods.
If the balance is upset, the whole system could fail.
All organisms rely on resources the area can supply; any area can only support a limited number of animals.
If all grass and trees die, herbivores like zebra and elephants would die, and then carnivores like cheetah and hyena would also die.
Population growth of species over time is a factor used to see if an ecosystem is balanced.

Limiting Factors

Limiting factors are resources that limit the growth of a population, such as food, water, space, light, nutrients, oxygen, disease, temperature, and predation.
Low food supply or lack of space are examples of limiting factors that can cause population decline.
When resources are limited, populations may decline due to less offspring, higher death rates, or emigration.
Human activities like pesticides, herbicides, and habitat destruction can also limit population growth.
Weather can be a limiting factor; for example, rainfall limits reproduction of the Agave Americana plant.

Limiting factors threatening the Riverine Rabbit population

Carrying Capacity

Carrying capacity is the population size that an ecosystem can support indefinitely.
When population is small, resources are plentiful, birth rates are high.
As population increases, resources decrease, leading to higher death rates and lower birth rates.
Population levels off at carrying capacity when birth and death rates become stable.

Population size levelling off at carrying capacity

Natural Factors Disrupting Ecosystems

Natural disasters like floods, hurricanes, and droughts can severely disrupt ecosystems.
Ecosystems may recover from sudden natural disasters, but long-term changes like climate change may cause irreversible damage.
The extinction of dinosaurs is theorized to be due to a sudden climate change caused by a meteor strike.
Droughts and famines can cause death of many organisms, especially species dependent on water for reproduction like amphibians.

The dry landscape in Ethiopia and people collecting water during a drought and famine

Human Factors Disrupting Ecosystems

Modern humans have a huge impact on nature through land clearing, pollution, waste, and litter.
Poaching and over-harvesting of animals cause lasting damage to ecosystems.
Over-hunting of small animals can force carnivores to prey on domestic animals, leading to further killing.
Poaching of species like abalone (perlemoen) and Mopani worms can lead to local extinction and disrupt food chains.
The Riverine Rabbit is critically endangered due to habitat destruction and human invasion.

Abalone (perlemoen) in its natural environment and served as a delicacy

Pollution

Pollution is a major human-caused disruption to ecosystems.
Types of pollution include water pollution, air pollution, and land pollution.
Pollution can harm organisms directly and degrade habitats.
Individuals can reduce pollution by changing habits and reducing waste.

Posters encouraging reduction of pollution

Case Study: Yellowstone Wolves

In the early 1900s, wolves were killed off in Yellowstone, leading to elk overpopulation and decline of aspen trees.
Reintroduction of wolves in 1995 allowed aspen to regrow, increasing biodiversity.
Wolves controlled elk populations, which allowed vegetation to recover, benefiting beavers and other species.
The return of wolves also affected coyote populations, increasing rabbits and mice, which supported hawks, foxes, and badgers.
This case shows how changes in one population can affect many others in the ecosystem.

Food Chains and Food Webs

A food chain shows the flow of energy from one organism to the next.
A food web is a network of interconnected food chains.
Most organisms eat multiple things, so food webs are more realistic.
All organisms depend on two global food webs: aquatic (based on phytoplankton) and land (based on plants).
Humans are top predators in both food webs.

A caterpillar feeding on a plant

Energy Flow

Energy flows through an ecosystem: from the sun, to producers, to consumers — unlike matter, it is not recycled.
Some energy is lost as heat at every step of a food chain, so fewer organisms can be supported at each higher level.
If a disruption removes producers or breaks a link in a food chain, the flow of energy to the rest of the ecosystem is disrupted too.

Sunlight over a grassy field, illustrating the sun as the primary energy source for ecosystems

Stability and Change

Stability of populations is affected by changes to living and nonliving components of an ecosystem.
Small changes can cause large effects elsewhere in the ecosystem.
Natural and human disruptions can cause imbalances, but ecosystems may find a new balance.
Every part of an ecosystem has a role; removing one part can have cascading effects.
Nature tries to adjust to disruptions, but sometimes new balances cannot be found.

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练习题

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1.What is the main source of energy for most ecosystems?
Easy
- AThe sun
- BThe moon
- CWind
- DSoil nutrients
2.Which of the following is a natural factor that can disrupt a balanced ecosystem?
Easy
- AFlooding
- BDeforestation
- CPollution
- DPoaching
3.What is the term for the maximum population size that an ecosystem can support over a long period?
Easy
- ACarrying capacity
- BLimiting factor
- CPopulation growth
- DBiodiversity
4.Which of the following are limiting factors in an ecosystem? (Select all that apply)
Easy
- AFood
- BWater
- CPredation
- DCarrying capacity
- EBiodiversity
5.Match the term to its definition.
Easy
- Producer
- Consumer
- Decomposer
- Makes its own food
- Eats other organisms
- Breaks down dead organisms
6.Arrange the following events in the order they occurred in Yellowstone National Park after wolves were removed.
Easy
- Elk populations increased
- Aspen tree populations declined
- Wolves were killed
- Beavers left the park
7.What happened to the elk population when wolves were removed?
Medium
- AIt increased
- BIt decreased
- CIt stayed the same
- DIt became extinct
8.The reintroduction of wolves increased the biodiversity of Yellowstone National Park.
Medium
True or false?

复习笔记

## 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.

闪卡

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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