GCSE BIOLOGY • SPECIFICATION 1.7

Ecology

Aligned to AQA GCSE Biology (8461/8464). Wording consistent with AQA mark schemes.

Mastering interdependence, nutrient cycles, and sampling precision.

1. Communities & Adaptations

Exam Definition (Ecosystem):

"The interaction of a community of living organisms (biotic) with the non-living (abiotic) parts of their environment."

Factor TypeExamples (Must Learn)
Abiotic (Non-living)Light intensity, Temperature, Moisture levels, Soil pH, Wind intensity, $CO_2$ levels.
Biotic (Living)Availability of food, New predators, New pathogens, One species outcompeting another.

Interdependence

Within a community, each species depends on others for food, shelter, pollination, and seed dispersal. If one species is removed, it can affect the whole community.

2. Required Practical: Sampling Techniques

To measure the abundance and distribution of organisms, we use specific sampling methods. Wording here is strictly marked.

Random Sampling (Quadrats):
  • Use a random number generator to get coordinates to avoid bias.
  • Count the number of organisms (recorded as number per $m^2$).
  • Calculate a mean and multiply by the total area to estimate population size.
Systematic Sampling (Transects):
  • Used to see how distribution changes along a gradient.
  • Stretch a transect line (tape measure) across the area.
  • Place quadrats at regular intervals along the line.

Q: Why must coordinates be generated randomly? (1 Mark)

To prevent bias in the results, ensuring a representative sample.

3. Trophic Levels & Biomass (Triple Only)

Exam Definition (Biomass):

"Biomass is the dry mass of living material in organisms at a trophic level."

In a pyramid of biomass, each level is usually smaller than the one below it. Only about 10% of biomass is transferred to the next level.

Reasons for Biomass Loss

  • Not all parts of the organism are eaten (e.g. bones).
  • Some material is egested as faeces.
  • Some is lost as waste products of metabolism (e.g. $CO_2$ and water in respiration, Urea in urine).
4. The Carbon and Water Cycles

The Carbon Cycle

  • Photosynthesis: The main biological process that removes $CO_2$ from the atmosphere.
  • Respiration: Animals, plants, and decomposers release $CO_2$.
  • Combustion: Burning fossil fuels or wood releases $CO_2$.
  • Decay: Microorganisms (decomposers) break down dead matter, releasing $CO_2$ via aerobic respiration.
[Image of the carbon cycle]
Decomposition: Fast decay requires Oxygen, Warmth, and Moisture. Anaerobic decay produces methane, a potent greenhouse gas.
[Image of the water cycle]
5. Biodiversity & Human Impact

Biodiversity is the variety of all the different species of organisms on earth or within an ecosystem.

Human ImpactEnvironmental Effect
DeforestationLoss of habitat; increases $CO_2$ (less photosynthesis + burning trees).
Peat BogsReleases $CO_2$ when peat decays or is burned for compost/fuel.
Global WarmingCaused by increased levels of greenhouse gases such as carbon dioxide and methane.

Explain how high biodiversity helps maintain the stability of an ecosystem (2 Marks).

High biodiversity provides alternative food sources and habitats, so if one species declines, others can survive, making the ecosystem more stable.

Final Exam Guardrail

• Biomass: Always define it as dry mass to avoid losing marks on precision.
• Sampling: Use quadrats for abundance and transects for distribution changes.
• Decomposition: Decomposers (bacteria/fungi) release $CO_2$ through respiration.
• Greenhouse Gases: Methane ($CH_4$) and Carbon Dioxide ($CO_2$) are the two you must name.
• Stability: Linked to reduced dependence on single food sources.

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