Topic 3 of 18Pearson EdExcel

Ecology & the Environment

Food chains, ecosystems, and environmental impact

Key ecological terms:

Ecosystem — all living organisms in an area + their non-living environment

Habitat — where an organism lives

Population — all organisms of one species in an area

Community — all populations of different species in an area

Niche — an organism's role in the ecosystem

Food chains and webs:

Show the direction of energy transfer (arrows point from food to feeder)

Producers — plants; make food via photosynthesis

Primary consumers — herbivores eating plants

Secondary consumers — carnivores eating herbivores

Decomposers — bacteria and fungi; break down dead organisms

Energy transfer: Only ~10% of energy passes from one trophic level to the next. Rest lost as heat/respiration.

Nutrient cycles: The **carbon cycle** — photosynthesis removes CO₂, respiration/combustion release it. **Nitrogen cycle** — nitrogen-fixing bacteria (in root nodules) → nitrates for plants → decomposers return nitrogen.

Human impacts: Deforestation reduces photosynthesis, increases CO₂. **Eutrophication** — excess fertilisers → algal bloom → oxygen depletion → fish die.

Key Points to Remember

1Ecosystem: living + non-living environment
2Energy transfer: ~10% per trophic level
3Carbon cycle: photosynthesis removes, respiration releases CO2
4Eutrophication: fertilisers → algal bloom → dead zone

Pakistan Example

Mangroves of Sindh — Pakistan's Ecological Treasure Under Threat

The Indus Delta mangrove forests near Thatta are Pakistan's largest. They support fisheries, protect coastlines, and store carbon. Deforestation for firewood is disrupting food webs — shrimp populations collapse without mangroves as nurseries. Eutrophication from agricultural run-off in the delta is also causing algal blooms, depleting oxygen and killing fish. EdExcel ecology topics are playing out in real time in Pakistan.

Quick Revision Infographic

Biology — Quick Revision