Chapter D4.2: Stability and Change in Ecosystems
Welcome to D4.2 Stability and Change! In this chapter, we explore how ecosystems balance continuity with change. Why do some ecosystems remain stable for hundreds of years, while others suddenly collapse? How do human activities tip this balance, and what can we do to restore it? Whether you are studying at Standard Level (SL) or Higher Level (HL), these notes break down every concept step-by-step.
Note: Content marked with [HL Only] is required exclusively for Higher Level students. All other sections apply to both SL and HL.
---1. Ecosystem Stability and Its Requirements
An ecosystem is considered stable when its structure, species composition, and overall biological functions remain relatively constant over long periods, or return to equilibrium after a disturbance.
Key Requirements for Ecosystem Stability
For an ecosystem to maintain its long-term stability, three fundamental requirements must be met:
1. Continuous energy supply: Ecosystems require an uninterrupted input of energy (primarily light energy from the sun) because energy flows through trophic levels and is progressively lost as heat (as seen in C4.2). Without continuous energy, biological work cannot occur.
2. Nutrient cycling: Unlike energy, matter cannot be continuously imported from space. Chemical elements (such as carbon and nitrogen) must be recycled continuously between the living (biotic) organisms and the non-living (abiotic) environment by producers, consumers, and decomposers.
3. High genetic and species diversity: A community with high biodiversity contains complex food webs and functional redundancy. If one species declines, others can fill its functional role, preventing the collapse of the entire system.
Analogy: Think of an ecosystem like a suspension bridge. The cables are the energy input, the recycled steel joints are the nutrient cycles, and the multiple supporting pillars represent biodiversity. If you have only one pillar (low diversity), losing it collapses the whole bridge!
Key Takeaway: Stability depends on continuous energy input, unbroken nutrient cycles, and sufficient biodiversity to buffer disturbances.
---2. Tipping Points and Ecosystem Modelling: Amazon Deforestation
Ecosystems can absorb a certain amount of stress before they change drastically. However, every ecosystem has limits.
What is a Tipping Point?
A tipping point is a critical ecological threshold. When an ecosystem is pushed beyond this threshold by environmental change or human disturbance, even a tiny additional change triggers a rapid, irreversible shift into a completely different ecological state.
Amazon Deforestation as a Tipping Point
The Amazon rainforest is a classic real-world example named in the IB guide:
- The stable cycle: The vast canopy of the Amazon generates a significant portion of its own rainfall through transpiration (water evaporating from leaves). This water vapour condenses to form rain clouds, maintaining the humid, wet conditions that rainforest trees require.
- The disruption: Widespread deforestation removes trees, decreasing local and regional transpiration rates. Less moisture enters the atmosphere, leading to severe droughts and longer dry seasons.
- The shift: Once deforestation crosses a critical tipping point, the remaining forest can no longer generate enough rainfall to sustain itself. The humid tropical rainforest begins an irreversible transition into a degraded, dry savanna ecosystem.
Ecosystem Modelling
Scientists use mathematical and computer-based modelling to simulate these complex ecological dynamics. Models help ecologists:
- Predict how ecosystems respond to varying rates of deforestation, habitat fragmentation, and climate pressures.
- Identify where potential tipping points lie before catastrophic collapse occurs.
- Test different conservation strategies virtually to see which interventions are most effective.
Quick Review: Crossing an ecological tipping point means a system shifts to an alternate state from which it cannot easily recover naturally.
---3. Keystone Species
Not all species in an ecosystem have the same ecological impact. Some play a uniquely dominant role relative to their abundance.
Definition and Significance
A keystone species is a species that has a disproportionately large effect on its environment relative to its biomass or population size.
- Keystone species prevent one or two dominant species from monopolizing resources like food or space.
- If a keystone species is removed, the balance of the ecosystem is shattered, often resulting in a dramatic loss of biodiversity and radical community restructuring.
Common Mistake to Avoid: Do not confuse a dominant species with a keystone species. A dominant species has a big effect simply because there is a huge mass of it (like forest trees). A keystone species may have a low population size or low biomass, but its presence is vital to keeping the entire community structure intact.
Key Takeaway: Keystone species exert strong top-down or structural control, keeping community diversity high despite having relatively low abundance.
---4. Human Impacts on Ecosystem Stability
Human activities frequently destabilize ecosystems by disrupting nutrient cycles, polluting food chains, or overexploiting natural populations.
A. Sustainable Harvesting and Sustainable Agriculture
- Sustainable harvesting: Taking biological resources (such as fish or timber) at a rate that does not exceed the natural replenishment rate of the population. This maintains population sizes at or near maximum sustainable yields without causing population crashes.
- Sustainable agriculture: Farming methods that conserve soil structure, prevent excessive nutrient depletion, minimize chemical run-off, and preserve surrounding biodiversity, allowing food production to continue indefinitely without degrading the land.
B. Eutrophication
Eutrophication is the nutrient enrichment of aquatic ecosystems, frequently caused by agricultural fertilizer run-off or untreated sewage entering waterways.
The process occurs in distinct, sequential steps:
1. Nutrient enrichment: Excess nitrates and phosphates leach from agricultural soils into rivers and lakes.
2. Algal bloom: High nutrient availability causes rapid population growth of microscopic algae on the water surface.
3. Light blockage: Dense layers of algae prevent sunlight from penetrating deeper water layers, causing submerged aquatic plants to die because they cannot photosynthesize.
4. Decomposition: The algae and plants eventually die. Saprotrophic bacteria multiply rapidly as they decompose this massive quantity of dead organic matter.
5. Oxygen depletion: The aerobic cellular respiration of these decomposers consumes dissolved oxygen in the water.
6. Anoxia and death: The water becomes anoxic (devoid of oxygen), leading to widespread suffocation and death of fish and other aquatic organisms.
C. Biomagnification
Biomagnification is the increasing concentration of a persistent, non-biodegradable substance (such as fat-soluble pollutants or heavy metals) in the tissues of organisms at progressively higher trophic levels.
- Contrast with Bioaccumulation: Bioaccumulation occurs within a single organism over its lifespan as it takes in a toxin faster than it can excrete it. Biomagnification occurs across the entire food chain.
- Why it happens: Trophic transfer efficiency is low; large amounts of biomass at lower trophic levels are consumed to sustain a smaller biomass of predators at higher trophic levels (as covered in C4.2). Because the toxin is retained in fatty tissue and not broken down or excreted, apex predators accumulate the highest, most toxic concentrations.
D. Plastic Pollution
Plastics are synthetic polymers that persist in the environment for decades or centuries. They impact marine and terrestrial ecosystems in several critical ways:
- Macroplastics: Large plastic debris causes entanglement of wildlife, physical injury, and fatal gut blockages when mistaken for food.
- Microplastics: Physical weathering and ultraviolet radiation break plastics down into microscopic fragments. Microplastics are ingested by filter feeders and plankton, carrying toxic chemical additives directly into aquatic food webs and accumulating along trophic chains.
Key Takeaway: Eutrophication deprives waterways of oxygen, biomagnification concentrates persistent toxins in apex predators, and plastic pollution causes mechanical harm and chemical contamination.
---5. Rewilding
Rather than merely managing degraded landscapes, modern conservation increasingly turns to rewilding.
Principles of Rewilding
- Rewilding is a large-scale conservation approach aimed at restoring natural ecosystem processes and wilderness areas.
- It often involves reintroducing missing keystone species or apex predators to re-establish natural ecological cascades.
- It reduces active human management, allowing natural ecological dynamics—such as natural regeneration, predation, and natural disturbance cycles—to take over and restore dynamic equilibrium.
Key Takeaway: Rewilding steps back from active control and allows natural biological processes and reintroduced keystone species to restore degraded habitats.
---6. Ecological Succession [HL Only]
Ecosystems are not static; communities change over time through a structured process called ecological succession.
Causes of Succession
Succession is driven by the interaction between organisms and their abiotic environment:
- When species colonize an area, their metabolic activities alter abiotic variables (for example, accumulating organic soil, increasing moisture retention, or casting shade).
- These changes often make the environment less suitable for the existing species and more suitable for new, competitor species, which gradually replace them.
Primary Succession vs. Secondary Succession
- Primary succession: Begins in a completely lifeless area with no pre-existing soil (such as bare rock exposed by a retreating glacier, or newly cooled volcanic lava).
- Pioneer species: The first organisms to colonize bare rock (typically lichens and mosses). They secrete acids that weather rock into mineral particles and die to contribute the first layer of organic humus, slowly building the soil.
- Secondary succession (for comparison) occurs when an established community is cleared by a disturbance (such as fire or farming) but the soil layer remains intact, leading to much faster recovery.
Cyclical Succession
Not all succession is linear. In cyclical succession, a community experiences recurring changes driven by cyclic natural disturbances or life histories of key species:
- For example, a dominant plant species matures, becomes vulnerable to a specific pathogen, windthrow, or fire, dies, and temporarily opens space for earlier successional species to recolonize before the cycle repeats.
Climax Communities and Arrested Succession
- Climax community: The final, stable stage of ecological succession. The community is in dynamic equilibrium with the prevailing regional climate and abiotic factors. Biomass, soil depth, and biodiversity reach their highest steady-state values.
- Arrested succession (Plagioclimax): Occurs when succession is prevented from reaching its natural climatic climax by a continuous, ongoing disturbance. Human activities (such as regular livestock grazing, controlled burning, or agricultural mowing) keep the vegetation permanently at an intermediate successional stage (such as permanent heathland or pasture).
HL Summary Box:
- Primary succession: Starts on bare rock with no soil; pioneer lichens/mosses create soil.
- Cyclical succession: Periodic disturbances create a repeating loop of successional states.
- Climax community: Stable, mature ecological equilibrium determined by climate.
- Arrested succession: Human or natural intervention halts progression before the climatic climax is reached.
Chapter Review & Practice Guidance
Paper 1 & Paper 2 Quick Exam Tips
- Eutrophication sequences: Paper 2 questions frequently ask you to outline or explain eutrophication. Always structure your answer logically: nutrient run-off \(\rightarrow\) algal bloom \(\rightarrow\) shading \(\rightarrow\) plant death \(\rightarrow\) bacterial decomposition \(\rightarrow\) oxygen consumption \(\rightarrow\) death of aerobic organisms.
- Deforestation and tipping points: Connect Amazon deforestation directly to disrupted transpiration loops and rainfall loss when discussing tipping points.
- Distinguish concepts carefully: Ensure you clearly distinguish bioaccumulation (within an individual) from biomagnification (across trophic levels).