Topic 20: Human influences on ecosystems

As the human population grows, the demand for food, land and energy puts enormous pressure on the natural world. Some of our actions damage ecosystems through pollution and habitat loss, while others — conservation and sustainable resource use — aim to protect them. This topic examines both the harm we cause and the steps we can take to reduce it.

Increasing food supply and its problems

To feed a growing population, farmers use modern methods to increase food production:

• Agricultural machinery to farm larger areas with less labour.
• Chemical fertilisers to add nitrates and improve plant growth.
• Insecticides and herbicides to kill pests and weeds.
• Selective breeding to produce higher-yielding varieties.

These methods raise yields but bring problems. Monocultures (growing a single crop over a large area) reduce biodiversity and let pests spread easily. Intensive livestock production can raise yields but raises concerns about animal welfare and the spread of disease. Overuse of chemicals can pollute soil and water.

Deforestation and habitat loss

Deforestation is the large-scale cutting down of forests for timber, farmland or building. Its consequences are serious:

• Loss of biodiversity — species lose their habitats and may become extinct.
• Soil erosion — roots no longer hold the soil, which is washed away by rain.
• Flooding and leaching — without trees to absorb water, land floods and nutrients drain from the soil.
• More carbon dioxide — fewer trees means less photosynthesis, so less carbon dioxide is removed from the air, adding to the greenhouse effect.

Pollution and eutrophication

Human activity releases many pollutants. Air pollution includes sulfur dioxide (causing acid rain) and excess carbon dioxide and methane, which enhance the greenhouse effect and contribute to climate change. Water pollution comes from sewage, chemical waste and fertilisers, and from non-biodegradable plastics that harm wildlife.

Eutrophication is an important sequence to learn:

• Excess nitrate from fertilisers or sewage drains into rivers and lakes.
• This causes rapid growth of algae — an algal bloom — that covers the surface.
• Light is blocked, so water plants below die.
• Decomposing bacteria multiply and feed on the dead plants, using up oxygen in aerobic respiration.
• The reduced oxygen kills fish and other aquatic organisms.

Conservation and sustainability

A sustainable resource is one that is produced as quickly as it is used, so it does not run out. Sustainable development meets the needs of people today without harming the ability of future generations to meet their own needs.

Ways to conserve resources and protect species include:

• Recycling materials such as paper, plastic and metals to reduce waste and demand on raw materials.
• Sustainable forestry and fishing — replanting trees, using quotas, controlling net mesh size and protecting breeding seasons.
• Conservation programmes using national parks, captive breeding, seed banks and zoos to protect endangered species and maintain biodiversity.
• Reducing extinction risk preserves the genetic variety needed for future medicines, food crops and healthy ecosystems.