Understanding Human influences on ecosystems: IGCSE Biology Explained

Imagine your neighbourhood as an ecosystem

Picture the street you live on. There are trees along the pavement, birds nesting in the gutters, insects buzzing around the bins, and maybe a fox that raids the rubbish on Tuesdays. Now imagine someone bulldozes half the street to build a shopping centre. The trees go. The birds leave. The fox has nowhere to hide. That, scaled up to forests, oceans and grasslands, is what this topic is really about: what happens to living systems when humans change the rules.

For your IGCSE Biology exam, you need to understand four big areas: how we produce food, how we destroy habitats, how we pollute the environment, and what we can do to fix the damage. Every one of these comes up regularly, and the examiners love questions that ask you to weigh advantages against disadvantages. So you'll want to think in trade-offs, not absolutes.

Increasing food production

Feeding eight billion people takes more than a few allotments. Over the past century, humans have developed five main strategies to grow more food from the same (or less) land. Each one boosts yield, but each one also carries a cost to ecosystems.

Monocultures and intensive farming

A monoculture is a field planted with a single crop species, stretched across as much land as possible. It is efficient for harvesting and chemical application, but it creates a fragile system.

Advantages of monocultures: easier mechanised harvesting, uniform product quality, economies of scale for fertiliser and pesticide use.

Disadvantages: rapid spread of pests and diseases, soil nutrient depletion (same minerals removed every season), loss of biodiversity, heavy dependence on chemical inputs.

Intensive livestock production follows a similar logic. Animals are kept in confined spaces and fed controlled diets to maximise growth rate. This lowers costs and increases output but raises concerns about animal welfare, antibiotic resistance (from routine use of antibiotics), and pollution from concentrated animal waste.

Habitat destruction

Biodiversity is the number of different species living in an area. When humans clear land for housing, farming, or resource extraction, we reduce that number. Three main drivers come up in the exam.

• Increased land for housing, crops and livestock: expanding cities and farmland replaces forests, wetlands and grasslands. Species that depend on those habitats lose their home.
• Extraction of natural resources: mining, logging and drilling physically remove parts of the ecosystem. Quarrying a hillside doesn't just remove rock; it destroys every organism on and in that rock.
• Freshwater and marine pollution: chemicals, sewage and plastics degrade aquatic habitats even when the land itself is untouched. Coral reefs, for instance, bleach under thermal and chemical stress.

The exam sometimes gives you a scenario (a forest cleared for palm oil, a wetland drained for a housing estate) and asks you to explain the consequences for named organisms. The key is to trace the effect through the food web: if the producer is removed, every consumer that depends on it is affected.

Pollution

Pollution is the addition of substances to the environment at a rate faster than they can be broken down or dispersed. The IGCSE Biology syllabus focuses on two categories: water pollution and air pollution.

Eutrophication step by step

This process comes up so often that it's worth learning the chain by heart:

1. Fertiliser (containing nitrates and phosphates) is washed off fields into rivers or lakes.
2. The extra minerals cause rapid growth of algae on the water surface (an algal bloom).
3. The algal bloom blocks light to plants below the surface, so those plants die.
4. Bacteria decompose the dead plants and algae, using up dissolved oxygen through aerobic respiration.
5. Oxygen levels fall so low that fish and other aquatic organisms suffocate and die.

The exam will sometimes ask you to put these steps in order or fill in missing stages. Get comfortable with the logic: nutrients in, algae up, light down, plants die, bacteria multiply, oxygen drops, animals die.

Conservation

Conservation is the management of ecosystems to maintain biodiversity and protect endangered species. Your IGCSE exam expects you to understand why conservation matters and which strategies are used.

Why conserve?

• Maintaining biodiversity keeps ecosystems stable. A diverse food web is more resilient to change than a simple one.
• Protecting resources for future generations. Sustainable development means using resources at a rate that allows them to be replenished.
• Preserving species that may have undiscovered medical or economic value. Many medicines originate from tropical plants that haven't been fully studied yet.

Conservation strategies

• Protected areas and nature reserves: legally restricting human activity in sensitive habitats.
• Captive breeding programmes: breeding endangered species in zoos or research centres and reintroducing them to the wild.
• Seed banks: storing seeds from rare or threatened plant species to preserve genetic diversity.
• Legislation: laws banning hunting, controlling emissions, or limiting deforestation.
• Education and community involvement: helping local populations understand the economic and ecological value of conservation.

Sustainable development

Sustainable development means meeting the needs of the current generation without compromising the ability of future generations to meet their own needs. In practice, this includes replanting forests after logging, rotating crops to maintain soil fertility, setting fishing quotas to prevent overfishing, and using renewable energy to reduce emissions.

Examiners often present a scenario where economic pressure conflicts with environmental protection and ask you to evaluate both sides. The trick is to acknowledge the genuine benefits of development (jobs, food, infrastructure) before explaining the ecological costs. A one-sided answer rarely picks up full marks.

Common exam mistakes

1. Confusing eutrophication steps: students often say fertiliser kills fish directly. It doesn't. The chain runs through algal bloom, light loss, plant death, bacterial decomposition, and oxygen depletion first.
2. Writing "chemicals kill everything": too vague. Name the specific chemical (insecticide, herbicide, fertiliser) and explain how it disrupts the ecosystem.
3. Forgetting bioaccumulation: insecticides like DDT accumulate along food chains. Organisms at the top (birds of prey) receive the highest concentrations, not the lowest.
4. Mixing up sulfur dioxide and carbon dioxide: SO₂ causes acid rain. CO₂ contributes to the greenhouse effect. Different pollutant, different problem.
5. One-sided evaluation answers: if the question says "discuss" or "evaluate," you must present both advantages and disadvantages. Leaving out either side costs marks.
6. Saying conservation means "leaving nature alone": conservation is active management, not abandonment. Captive breeding, seed banks and legislation all require human effort.

Self-check questions

1. Name three methods humans use to increase food production and give one disadvantage of each.
2. Explain the process of eutrophication in the correct sequence, starting from fertiliser application.
3. Why does bioaccumulation mean that top predators are most affected by insecticides?
4. Describe two differences between the effects of sulfur dioxide and carbon monoxide on the environment and human health.
5. Give two examples of conservation strategies and explain how each helps to maintain biodiversity.