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D4.2.2—Requirements for stability

Ecosystem stability depends on continuing energy input, nutrient recycling, biological diversity, and abiotic conditions that do not exceed species tolerances.

Syllabus
First assessment 2025
Objective
D4.2.2
Level
SL

Exam analysis

Chance of appearing1%of analysed past papers
Latest appearanceMay 2023
Most common paperPaper3
Typical marks2

Common command terms

  • Outline
  • Describe

Scoring notes

Common mistake
Treating sustainability as a list of organisms only, without explaining energy input or nutrient recycling.

Recent exam appearances

May 2023Paper3 ["SL"] · TZ214(b)[ 2 ]D4.2.2—Requirements for stability

Coverage 2023–2023 · Updated 16 Jul 2026

Energy Flows through Ecosystems while Nutrients Must Be Recycled

Requirement Why stability depends on it
continual energy input replaces energy dissipated as heat during metabolism
producer biomass captures light or chemical energy into organic molecules
decomposers and nutrient cycling return mineral nutrients from waste and dead biomass for reuse
matter inputs and retention supply elements needed to build biological molecules

Energy is not recycled: it enters, is transferred and is lost as heat. Chemical elements can cycle repeatedly between organisms and the abiotic environment.

Diversity and Tolerance Ranges Spread Ecological Risk

Stability support Mechanism
species diversity multiple species can occupy different niches or partly replace lost functions
genetic diversity varied alleles increase the chance that some individuals tolerate disease or environmental change
abiotic conditions within tolerance key species can survive, reproduce and maintain their ecological roles

Higher diversity often supports resilience, but it is not a guarantee. Stability still depends on interaction strengths, climate, energy supply and the type of disturbance.

Two beetle communities contain the same three species, but the more even community has higher diversity because no single species dominates.

Stability Emerges from Flows, Diversity and Feedback

continued energy input + nutrient recycling + diversity + tolerable abiotic conditions → ecological functions persist or recover; disturbance that weakens these supports can trigger reinforcing change and a new state

Question Evidence to seek
Is the ecosystem stable? persistence or recovery of named structures and processes over a stated timescale
Is a tipping point plausible? threshold-like response plus reinforcing feedback
Does one species have a keystone role? disproportionate community change after its removal
Does a mesocosm support the mechanism? replicated treatment–control difference with variables held constant

Do not equate stable with unchanged, old with invulnerable, or correlated change with a demonstrated feedback mechanism.

Requirements for stability

[ 4 ]

Outline the features of ecosystems that make them sustainable.

Concept essentials

  • Energy must continually enter ecosystems because it is transferred and lost rather than recycled.
  • Decomposers support stability by returning inorganic nutrients from organic matter and wastes.
  • Limiting factors such as light, temperature, water, and nutrients restrict productivity when outside tolerance ranges.