Human influences on ecosystems

Cambridge IGCSE Biology 0610 · Core and Extended

Syllabus points

• Describe how humans have increased food production and the negative impacts of doing so.
• Describe the reasons for habitat destruction and its undesirable effects.
• Describe the effects of pollution, including eutrophication and the enhanced greenhouse effect.
• Describe the need for conservation and the methods used.
• Explain the risks of unsustainable resource use and how resources can be managed sustainably.

Increasing food production

Modern agriculture uses monoculture, growing a single crop over a large area, along with fertilisers, pesticides and herbicides, machinery, and intensive livestock farming.

Each raises yield and each carries a cost:

• Monoculture reduces biodiversity and removes habitats, and because every plant is identical the whole crop is vulnerable to one disease or pest.
• Fertilisers raise yield but run off into water and cause eutrophication.
• Pesticides kill pests but also harm other species, can accumulate along the food chain, and select for resistant pests.
• Herbicides kill weeds and remove food plants that other species depend on.
• Intensive livestock farming produces more meat from less land, but raises welfare concerns and concentrates waste.

Habitat destruction

Habitats are destroyed to make room for housing, farming, industry, roads and quarries, and by pollution and by taking too much from the environment.

The main effects:

• Reduced biodiversity, as species lose the conditions they need.
• Extinction, when a species loses its habitat entirely.
• Loss of food chains and webs, so the effect spreads to species whose own habitat is untouched.

Deforestation deserves separate treatment because so many effects follow from it:

• Less photosynthesis, so less carbon dioxide is removed from the air.
• More carbon dioxide if the trees are burned, adding to the enhanced greenhouse effect.
• Soil erosion, because roots no longer bind the soil and rain washes it away.
• Flooding, since trees no longer intercept rainfall and the soil holds less water.
• Loss of habitat and of the species that depend on it.
• Leaching of minerals from the exposed soil, leaving it infertile.

Eutrophication

A sequence question, and the marks come from getting the order right.

1. Excess fertiliser runs off farmland into rivers and lakes, or untreated sewage enters them.
2. Nitrates and phosphates cause rapid growth of algae, an algal bloom.
3. The algae at the surface block the light reaching plants below, so those plants die.
4. Decomposing bacteria feed on the dead plants and algae and multiply rapidly.
5. Those bacteria respire aerobically and use up the dissolved oxygen.
6. Fish and other aerobic organisms die from lack of oxygen.

The commonest error is stopping at step 2 and saying the algae use up the oxygen. It is the decomposers, not the algae, that deoxygenate the water.

Other pollution

• Untreated sewage spreads pathogens such as cholera, as well as causing eutrophication.
• Non-biodegradable plastics persist for centuries. Animals become entangled in them or eat them and starve with a full stomach.
• Acid rain forms when sulfur dioxide and nitrogen oxides from burning fossil fuels dissolve in rainwater. It damages leaves, acidifies lakes so that fish die, and releases toxic aluminium ions from the soil.
• Insecticides can pass along food chains and become concentrated in top predators.

The enhanced greenhouse effect

Greenhouse gases, mainly carbon dioxide and methane, absorb heat radiated from the Earth's surface and re-radiate some of it back, keeping the planet warm. That natural effect makes the Earth habitable.

Human activity has increased those gases: burning fossil fuels and deforestation add carbon dioxide, while cattle farming, rice paddies and landfill add methane. More gas means more heat retained, which is enhanced greenhouse effect and leads to climate change.

Consequences include rising sea levels as ice melts and water expands, more extreme weather, shifting rainfall patterns affecting food production, and species being unable to move or adapt fast enough.

Conservation

Reasons to conserve species and habitats:

• Maintaining biodiversity, so ecosystems remain stable.
• Protecting a source of future medicines and useful genes for crop breeding.
• Keeping a gene pool wide enough for species to adapt to change.
• Ethical and aesthetic reasons, and value for tourism.

Methods:

• Protected areas such as national parks and nature reserves.
• Captive breeding programmes in zoos, and release back into the wild.
• Seed banks, which store seeds cheaply and in great numbers for a long time.
• Monitoring and protecting endangered species, and controlling trade.
• Education, so that support continues.

Sustainable resources

A resource is used sustainably when it is used in a way that allows it to be replaced, so it remains available for future generations.

Fish stocks can be managed by imposing quotas on the mass caught, restricting net mesh size so young fish escape and can breed, setting closed seasons during breeding, and protecting areas entirely.

Forests can be managed by replanting at least as many trees as are felled, taking only mature trees, and coppicing so trees regrow from the stump. Timber is renewable if managed this way, and non-renewable in practice if it is not.

The general test is worth remembering: a resource is being used sustainably only if the rate of use does not exceed the rate at which it is replaced. Fossil fuels fail that test absolutely, because they form over millions of years.

Recycling paper, plastic, glass and metal reduces the raw materials taken from the environment, reduces landfill, and usually uses less energy than making the material from scratch.