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Unit 2 · Ecology & the Environment

Unit 2 — Ecology and the Environment — Edexcel IGCSE Biology Modular - UNIT 2 Revision Notes & Topical Questions

5 min read 9 parts Lesson 1 of 5 Unit quiz

📚 Edexcel IGCSE Biology Modular - UNIT 2 Hub / Topical Revision
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Lesson summary

Ecology and the Environment: Overview — EDEXCEL MODULAR - OL notes: Summarise ecosystem terms, food webs, energy flow and pyramids, nutrient cycles, human impacts and conservation.TermMeaningPopulationAll the organisms of one species in an areaCommunityAll the populations of diff…

  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Worked example 4
  • Trophic level
  • Eutrophication

Section 1 of 9Ecosystem terms and sampling

TermMeaning
PopulationAll the organisms of one species in an area
CommunityAll the populations of different species in an area
HabitatThe place where an organism lives
EcosystemA community and its non-living environment together

To compare population sizes in two different areas, use quadrats: place quadrats randomly (random coordinates), count the organisms, take at least 10 samples per area, and calculate the mean per square metre. Move leaves aside with a pencil or stick to check for organisms underneath. Estimated population = mean per quadrat x total area / area of one quadrat.

Section 2 of 9Food chains, webs and pyramids

A food chain shows the transfer of energy: producer, primary consumer, secondary consumer, tertiary consumer.

1
A food web shows the many feeding relationships.
2
If the mouse population in a web were suddenly wiped out, the fox and the hawk populations might decrease but would not be wiped out,
  • as mice are not their only food source.
3
A pyramid of biomass shows the mass of living matter at each trophic level and is always pyramid shaped,
  • because energy is lost at each level.
    • (respiration, movement, heat, uneaten parts, waste)
4
Typically only about 10% of the energy passes to the next level.

Section 3 of 9Nutrient cycles and pollution

Nitrogen is cycled through ecosystems by nitrogen fixation, nitrification, uptake by plants, feeding, death and decay by decomposers, and denitrification.

1
The carbon cycle involves photosynthesis, respiration, feeding, decay and combustion.
2
Air pollution has several biological consequences from a range of pollutant gases, such as acid rain from sulfur dioxide.
3
The greenhouse effect: gases such as carbon dioxide and methane absorb heat radiated from the Earth; some is reflected back to the Earth,
  • which warms the atmosphere and leads to global warming.
4
Minerals from agricultural fertilisers can be leached into water bodies such as lakes and streams, resulting in eutrophication.
5
Lichens are indicators: they grow only where the air is clean, so their presence indicates low pollution.

Section 4 of 9Biotic factors, competition and disease

Scientists investigate the effect of introducing grey squirrels into a woodland occupied by red squirrels.

Biotic factors, competition and disease
Grey squirrels compete for food and nesting sites and carry a virus that red squirrels cannot resist, so red squirrel numbers fall.
If a new pathogen enters an ecosystem, the populations living there may have no immunity; ash dieback, caused by the fungus Hymenoscyphus fraxineus, has infected millions of ash trees in Europe.

Section 5 of 9Practical skills in ecology

Investigating the effect of an abiotic factor: measure light intensity with a light meter, temperature with a thermometer and soil moisture or pH with probes at each quadrat position, so that species counts can be linked to conditions.

Practical skills in ecology
Evaluate the investigation: the sample size is small, quadrats may be unrepresentative, and a few results cannot show cause and effect.
Mark-release-recapture estimates the size of a mobile animal population: population = (number in first sample x number in second sample) / number of marked animals recaptured.
Example: 40 snails marked, second sample 50 snails with 10 marked: population = 40 x 50 / 10 = 200.

Section 6 of 9Food webs: worked reasoning

In a web, if a predator is removed, its prey may increase and then overgraze producers; if a producer is removed, all dependent consumers decrease. Always say what happens, then why, naming the trophic links.

Section 7 of 9Worked examples

  1. Explain why a pyramid of biomass is always pyramid shaped. [2]2 marks

  2. Describe how to find the population of a plant in a field using quadrats. [4]4 marks

  3. Explain what eutrophication is. [3]3 marks

  4. Why are lichens used as pollution indicators? [1]1 mark

Section 8 of 9Key terms

TermMeaning
PopulationAll organisms of one species in an area.
CommunityAll populations in an area.
EcosystemA community and its environment.
Trophic levelA feeding level in a food chain.
EutrophicationExcess nutrients causing algal growth and oxygen depletion.
Indicator speciesA species showing the quality of an environment.

Section 9 of 9Exam technique

1
Saying energy is recycled in ecosystems - it flows through and is lost as heat.
2
Saying a population includes different species.
3
Counting quadrats chosen by hand.
4
Saying fertiliser pollution kills fish directly.

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