This topic covers how human activity changes ecosystems and how biologists judge the evidence: habitat loss, pollution such as eutrophication, conservation choices and the limits of sampling. Questions rarely ask for a definition alone. They give you a short scenario or data table and ask you to explain a cause, compare options or decide what the data support.
It sits inside Cambridge IGCSE Biology and builds on ecology and energy flow and nutrient cycles and populations. Check the current Cambridge syllabus for your exam year to confirm the exact content.
What should you know first?
You should be comfortable with four things before starting.
- Food chains and webs, and what happens to other organisms when one population changes.
- Photosynthesis and respiration, because oxygen and light matter in polluted water.
- Decomposition and nutrient recycling.
- Mean, percentage change and a quadrat estimate.
If any of these feels shaky, revisit the earlier modules first. The explanations here assume them.
One orienting example
An invented survey counts mangrove saplings in ten 1 m² quadrats at the same site in two years. The mean is 14 per quadrat in the first year and 9 in the second.
The change is 9 − 14 = −5 saplings per quadrat. As a percentage of the starting value, −5 ÷ 14 × 100 = −35.7%, or about 36% fewer.
That is the evidence. Why it fell (clearing, salt change, a dry year) is a separate step that needs more information. Every lesson in this module practises keeping those two steps apart.
In what order should you study the lessons?
- Assess evidence for a habitat change: start here to learn to read change, calculate it and state what it shows.
- Explain eutrophication through a causal chain: the most tested pollution example, built link by link.
- Compare conservation options using specified ecological criteria: practise judging options against stated criteria rather than preference.
- Interpret biodiversity sampling limitations: learn what a quadrat result can and cannot tell you.
- Separate environmental science evidence from political recommendations: finish by telling data, interpretation and opinion apart.
What are the common traps?
- Writing “it is bad for the environment” instead of naming the organism, the process and the result.
- Skipping a link in a chain, for example going from fertiliser straight to dead fish.
- Stating a cause as certain when the data only show a change.
- Forgetting that a small or badly placed sample may not represent the whole area.
- Answering a question about evidence with a personal opinion.
How should you use the practice set?
Work through the human influence on ecosystems practice set after the lessons, on paper first and with the answers closed. Mark each one honestly, and use the routing at the end to return to the matching lesson. The mistake log and retest queue helps you keep track of repeated slips.
If you would like someone to look at your own answers, our teachers offer online one-to-one Biology tuition.