Human Influences on Ecosystems

This is one of the most straightforward topics on the syllabus to understand and one of the easiest to lose marks on. The biology is not difficult — the marks go to explaining chains of consequences rather than naming problems.

A question asking you to explain how deforestation leads to extinction wants the steps in between, not the two endpoints.

1. Food supply

A growing human population needs more food. The main ways of increasing food production are:

Modern agricultural machinery — allows larger areas to be farmed
Chemical fertilisers — improve crop yields
Insecticides and herbicides — reduce losses to pests and weeds
Selective breeding — improves yields of crops and livestock
Monoculture — growing a single crop over a large area

Monoculture

Monoculture is growing one type of crop over a large area.

Advantages	Disadvantages
Efficient — planting, treating and harvesting can all be mechanised	Reduces biodiversity — one crop replaces a varied habitat
High yields of a single product	Pests that attack that crop can spread rapidly through the whole area
Simpler to manage	Often needs more fertiliser and pesticide

Intensive livestock production

Keeping animals in controlled conditions increases food production: animals are kept warm and their movement is restricted, so less energy is lost and more is available for growth. The concerns are animal welfare, the increased risk of disease spreading in crowded conditions, and the use of antibiotics contributing to resistance.

2. Habitat destruction

Habitats are destroyed for several reasons, and questions often ask you to list them:

Increased area for housing and farming as the population grows
Extraction of natural resources — mining, quarrying, logging
Marine pollution — including oil spills and waste dumping
Freshwater pollution

Why the population is growing

Worth being able to explain: birth rate exceeds death rate, largely because improvements in medicine, sanitation and food supply mean fewer people die young and life expectancy has increased.

The consequence: reduced biodiversity

Biodiversity is the number of different species living in an area.

When habitats are destroyed, species lose their homes and food sources. Populations fall, some species become extinct, and food chains and food webs are disrupted — because removing one organism affects everything that feeds on it, and everything it feeds on.

3. Deforestation

Deforestation is the removal of forests, usually to obtain timber or to clear land for farming or building.

Its undesirable effects form a set of chains, and the marks are in the chains.

Extinction

Forests are the habitat of very large numbers of species. Removing the forest destroys their habitat and removes their food source, so populations fall and species may become extinct. Loss of one species then affects others in the same food web.

Loss of soil / soil erosion

Tree roots normally bind the soil together
With the trees gone, there are no roots to hold it
The soil is directly exposed to rain and wind
The fertile topsoil is washed or blown away

The topsoil matters because it contains humus — decomposed remains of plants and animals — which makes it the most fertile layer. Once it is gone, soil fertility falls and little will grow.

Flooding

With no trees to intercept rainfall and no roots to absorb water, more water runs straight off the land into rivers. Eroded soil is deposited in rivers, raising the riverbed, so rivers overflow more easily and flooding becomes more likely.

Increased carbon dioxide in the atmosphere

Two effects at once:

Fewer trees means less photosynthesis, so less carbon dioxide is removed from the atmosphere
Burning the felled trees releases carbon dioxide

This links deforestation directly to the enhanced greenhouse effect.

Disruption of the water cycle

Trees take up water and lose it by transpiration. Fewer trees means less transpiration, so less water vapour enters the atmosphere, fewer clouds form and rainfall decreases — leaving the area drier.

4. Pollution

Eutrophication

This is the single most examined process in the topic, and it must be given as a sequence. Learn it in order:

Excess fertiliser (containing nitrates and phosphates) is applied to fields, or untreated sewage enters the water
It is leached into rivers and lakes by rainwater
The extra nutrients cause rapid growth of algae at the surface — an algal bloom
The algal layer blocks light from reaching plants below
Aquatic plants cannot photosynthesise, so they die
Decomposers (bacteria) feed on the dead plants and algae, and their populations increase rapidly
These decomposers respire aerobically, using up the dissolved oxygen in the water
Oxygen concentration falls, so fish and other aquatic organisms die

Step 6–7 is where marks are won and lost. The oxygen is used up by decomposing bacteria respiring, not by the algae themselves. Answers that skip the decomposers rarely get full marks.

It is fertilisers that cause eutrophication, not pesticides. The two get mixed up constantly. Fertilisers add nitrates and phosphates — that is what drives the algal bloom.

Non-biodegradable plastics

Non-biodegradable means the material cannot be broken down by decomposers, so it persists in the environment.

Effects:

In water: aquatic animals may become entangled in plastic waste, or eat it, which can block the digestive system and cause starvation
On land: plastic accumulates, taking up space in landfill and remaining for centuries

Untreated sewage

As well as causing eutrophication, sewage contains pathogens that can cause disease in humans, such as cholera.

Greenhouse gases and climate change

The greenhouse effect is natural and necessary. Greenhouse gases in the atmosphere absorb heat that would otherwise escape into space, keeping the Earth warm enough to support life.

The problem is the enhanced greenhouse effect — human activities raising the concentration of greenhouse gases, so more heat is trapped and average global temperature rises.

The two greenhouse gases named in the syllabus:

Gas	Main sources
Carbon dioxide	Burning fossil fuels; deforestation
Methane	Cattle (and other livestock); rice paddy fields; decaying waste in landfill

Consequences of climate change:

Melting of ice caps and glaciers, causing sea levels to rise and low-lying land to flood
Changes in rainfall patterns, affecting where crops can be grown
Extreme weather events becoming more frequent
Species unable to adapt fast enough to changing conditions, reducing biodiversity

Acid rain (caused by sulfur dioxide and oxides of nitrogen from burning fossil fuels) appears in older past papers and in some other specifications. Check whether it is on your version of the syllabus before spending time on it — and note that carbon dioxide is not the answer to an acid rain question.

5. Conservation and sustainable resources

A sustainable resource is one which is produced as rapidly as it is removed, so it does not run out.

Sustainable development is providing for the needs of an increasing human population without harming the environment, so that resources remain available for future generations.

Sustainable forestry

Replanting — planting a new tree for every one felled
Education about the importance of forests
Quotas limiting how much timber can be taken
Selective felling rather than clearing whole areas

Sustainable fishing

Quotas on the mass of fish that may be caught
Larger net mesh size — so young fish escape and can breed before being caught
Closed seasons — no fishing during breeding periods
Protected areas where fishing is banned

A frequent multiple-choice trap: you want to increase the mesh size, not decrease it. Larger holes let small, immature fish escape.

Why species need conserving

To maintain biodiversity and the stability of ecosystems
To protect species that may be a source of medicinal drugs
To preserve genetic variation for future crop and livestock breeding
To maintain resources for future generations

Methods of conservation

Protected habitats — national parks and nature reserves
Captive breeding programmes in zoos, with release back into the wild
Seed banks, storing seeds of many varieties
Monitoring and protecting endangered species
Education about the importance of conservation

Recycling

Recycling paper, plastic, glass and metal reduces the amount of waste going to landfill, reduces the extraction of raw materials, and uses less energy than producing the material from scratch. Recycling paper in particular means fewer trees felled.

6. Mistakes that cost marks

Giving eutrophication without the decomposers. The oxygen is used by bacteria respiring as they decompose dead material.

Saying pesticides cause eutrophication. It is fertilisers — nitrates and phosphates.

Naming a problem instead of explaining it. “Deforestation causes extinction” scores little; the chain of habitat loss → loss of food and shelter → population decline → extinction scores.

Saying the greenhouse effect is bad. The natural greenhouse effect keeps the Earth habitable. The problem is the enhanced greenhouse effect.

Saying “pollution” as a single answer. Name the pollutant and its effect.

Decreasing net mesh size to conserve fish. Increase it.

Confusing biodiversity with population size. Biodiversity is the number of different species, not how many individuals there are.

Frequently asked questions

What is eutrophication in simple terms? Fertiliser or sewage enters water, algae grow rapidly and block the light, plants die, bacteria decomposing them use up the oxygen, and fish and other aquatic organisms die from lack of oxygen.

Why does deforestation increase carbon dioxide? Two reasons: fewer trees photosynthesise, so less carbon dioxide is removed; and burning the felled trees releases more.

What is the difference between the greenhouse effect and global warming? The greenhouse effect is the natural trapping of heat that makes the Earth habitable. Global warming is the rise in average temperature caused by increasing the concentration of greenhouse gases.

Which gases should I name for climate change? Carbon dioxide and methane.

Why is monoculture bad for biodiversity? A single crop replaces a varied habitat, so the range of species that can live there falls sharply.

What does non-biodegradable mean? It cannot be broken down by decomposers, so it remains in the environment for a very long time.

Quick revision checklist

These notes cover topic 20 of the Cambridge IGCSE Biology (0610) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across many one-to-one IGCSE Biology lessons, with particular attention to the errors students make most often and the wording examiners reward.

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Human influences on ecosystems