6.3.1 Ecosystems

Define population and habitat. (2 marks)

- Population: A group of organisms of the same species living in the same habitat.

- Habitat: The part of an ecosystem where a particular species lives.

Define community, ecosystem and niche. (3 marks)

- Community: All populations of different species living in the same area at the same time.

- Ecosystem: A community plus the non-living (abiotic) components of their environment.

- Niche: The role of an organism within an ecosystem, shaped by adaptations to both biotic and abiotic conditions.

Define factors affecting ecosystems. (3 marks)

- Carrying capacity: The maximum population size an ecosystem can support.

- Abiotic factors: Non-living environmental conditions, e.g. temperature, light.

- Biotic factors: Effects of interactions between organisms.

Describe how biomass is transferred through ecosystems. (3 marks)

- Plants (producers) make carbohydrates from CO₂ and water using photosynthesis.

- Energy is lost between trophic levels mainly through respiration and excretion.

- The remaining energy is stored as biomass and can be measured as carbon or dry mass per area.

State abiotic factors that increase ecosystem productivity. (3 marks)

- Plenty of water availability.

- Sufficient light and warmth.

- Healthy green plants with high photosynthesis rates.

State the formula used to calculate the efficiency of biomass transfer between trophic levels. (1 mark)

Efficiency = (biomass transferred ÷ biomass intake) × 100

Describe human methods to improve biomass transfer efficiency. (4 marks)

- Restrict animal movement to reduce respiration energy loss.

- Provide animals with high-energy food to increase energy input.

- Keep animals indoors to reduce heat loss.

- Remove competition and predators by controlled growing environments.

Draw a labelled diagram summary of the nitrogen cycle. (6 marks)

Describe nitrogen fixation. (3 marks)

- Nitrogen-fixing bacteria, e.g. Rhizobium and Azotobacter, convert atmospheric nitrogen to ammonium ions.

- Azotobacter are free-living; Rhizobium live in root nodules in a mutualistic relationship with legumes.

- Provides plants with nitrogen compounds for growth.

Describe nitrification. (2 marks)

- Nitrifying bacteria (Nitrosomonas and Nitrobacter) convert ammonium → nitrite → nitrate.

- This is a two-stage oxidation process in soil.

Describe denitrification. (2 marks)

- Anaerobic denitrifying bacteria convert nitrogen compounds back to nitrogen gas.

- Reduces nitrogen available to plants.

Describe ammonification. (2 marks)

- Decomposition of organic nitrogen compounds by saprobionts.

- Produces ammonium ions returned to the soil.

Describe the carbon cycle. (4 marks)

- Photosynthesis fixes atmospheric CO₂ into carbohydrates, proteins, and lipids.

- Respiration releases CO₂ back to the atmosphere.

- Decomposers break down dead material, releasing CO₂ via respiration.

- Imbalances caused by deforestation and fossil fuel combustion increase CO₂ levels, causing global warming and ocean acidification.

Define succession. (1 mark)

The change in an ecological community over time.

Describe primary succession. (6 marks)

1. Pioneer species (e.g. lichen) colonise bare rock or sand.

2. They survive harsh conditions and, when they die, decompose to form humus.

3. Mosses and small plants grow, increasing soil depth and nutrients.

4. Larger plants establish, further changing conditions.

5. Existing species are outcompeted as conditions favour new species.

6. Final stage is the climax community, dominated by trees.

State the changes during succession. (3 marks)

- Species richness and number of organisms increase.

- Larger plants and animals colonise.

- Food webs become more complex.

Define deflected succession. (2 marks)

- When human activities prevent the natural progression of succession so a climax community is not reached.

- Can occur due to grazing animals, trampling, controlled burning, or vegetation removal.

Explain why deflected succession is used in conservation. (3 marks)

- Maintains earlier stages of succession, increasing habitat variety and species diversity.

- Supports conservation goals by sustaining a wider range of species.

- Can help balance human resource use (e.g. coppicing for timber) with habitat preservation.