Ecosystems, Populations and Nutrient Cycles
Productivity, trophic transfer, population interactions, succession, decomposition, carbon and nitrogen cycling and sustainable management.
How to study A-level Biology
Move between scales deliberately: identify the biological process, trace matter or information, use quantitative evidence, then evaluate the method and conclusion.
Core concepts
Concept 1
Energy and biomass decline through trophic levels because not all material is consumed, digested or converted into new biomass.
Exam cue: State whether a quantity is energy, biomass, number or rate and include area and time units where required.
Concept 2
Population size reflects density-dependent and density-independent factors, interactions and changing carrying capacity.
Exam cue: Use gross and net productivity definitions before calculating transfer efficiency.
Concept 3
Decomposers and microorganisms recycle elements; succession and human intervention alter community structure over time.
Exam cue: Evaluate management by combining biological outcome, timescale, uncertainty and stakeholder costs.
Risk pitfalls and guardrails
Treating energy as recycled through an ecosystem.
Guardrail: Do not replace a mechanism with a vague purpose statement, or infer causation from an association without experimental support.
Assuming population growth remains exponential indefinitely.
Guardrail: Do not replace a mechanism with a vague purpose statement, or infer causation from an association without experimental support.
Describing succession as a fixed march toward one inevitable endpoint in every habitat.
Guardrail: Do not replace a mechanism with a vague purpose statement, or infer causation from an association without experimental support.
Memory anchors
Gross Primary Productivity
GPP is the rate at which producers fix chemical energy before respiratory losses.
Net Primary Productivity
NPP equals GPP minus producer respiration.
Transfer Efficiency
Transfer efficiency is energy or biomass passed on divided by that available at the previous level.
Carrying Capacity
Carrying capacity is the population size an environment can sustain under current conditions.
Decomposition
Decomposers digest dead organic material and release mineral nutrients through respiration and mineralisation.
Checkpoint rule
Do the check-up only after you can summarize each concept in one sentence and identify one dangerous pitfall from memory.
Knowledge Check (after reading)
Short check-up to confirm understanding of this module.
Check-up Questions
Why does available energy decrease between trophic levels?
Gross primary productivity is 24 MJ m⁻² year⁻¹ and producer respiration is 9 MJ m⁻² year⁻¹. What is NPP?
Answer all questions to submit.
Next step personalized recommendations
Continue learning
Move forward only after this module is stable.
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