AQA GCSE Biology: Land Use, Deforestation and Global Warming

Humans change the land in ways that harm ecosystems and warm the planet. This guide covers how we use land, the destruction of peat bogs, deforestation, and the causes and consequences of global warming. The connecting thread is carbon dioxide and the loss of habitats.

Land use and peat bogs

Humans reduce the land available for other organisms by building, quarrying, dumping waste and farming.

A particular problem is the destruction of peat bogs. Peat is partially decayed plant material, formed where a lack of oxygen slows decay. It is dug up to make cheap garden compost and to burn as a fuel. Destroying peat bogs reduces the area of habitat, so fewer species can live there, and the decay or burning of peat releases carbon dioxide, adding to global warming.

Peat exists because decay stopped halfway. A waterlogged bog holds almost no oxygen, so the decomposers that would normally respire aerobically and break plant material down to carbon dioxide cannot work properly. The plant material piles up instead, and the carbon it contains stays locked in the ground, in some bogs for thousands of years.

That is why digging a bog up does damage in two directions at once. Drain or dig it and the oxygen returns, decay restarts, and carbon that had been held out of the atmosphere for millennia is released. Burn the peat and the same carbon is released faster. Either way a long-term carbon store becomes a carbon source, and the habitat on top of it is gone as well.

Deforestation

Large-scale deforestation in tropical areas is carried out to provide land for cattle and rice fields, and to grow crops for biofuels. It has serious consequences.

• More carbon dioxide in the atmosphere: burning the wood, and its decay by respiring microorganisms, both release carbon dioxide, and there are fewer trees left to take carbon dioxide in for photosynthesis.
• Loss of biodiversity: clearing forests destroys habitats and food sources, so fewer species survive.

Global warming

Global warming is caused by increasing levels of greenhouse gases, such as carbon dioxide (from burning fossil fuels) and methane (from cattle and rice fields). These gases enhance the greenhouse effect, trapping more heat.

Two points that answers routinely get wrong. The greenhouse effect is natural and necessary: without it the Earth would be far too cold for life. The problem is that we have enhanced it by adding more greenhouse gas, so more heat is trapped than before. Saying the greenhouse effect is bad, or that it is something humans created, is marked down.

And global warming is not caused by a hole in the ozone layer. That is a separate problem with a separate cause, and the two get mixed up constantly.

Deforestation is the clearest example of the double effect that makes carbon dioxide so hard to control. Burning or rotting the wood releases carbon dioxide, and removing the trees also removes the photosynthesis that would have taken it back in. Only naming one of the two halves is the usual way to drop a mark on this question.

Its biological consequences include:

• melting polar ice caps and rising sea levels, causing habitat loss and flooding of low-lying land;
• climate change, such as changing rainfall patterns and more droughts and storms, causing habitat loss and crop failure;
• changes in migration patterns, such as birds migrating further north;
• changes in species distribution, such as malaria-carrying mosquitoes reaching more temperate regions.

All of these can reduce biodiversity.

The last two are worth being able to explain rather than just name, because they are the ones that sound like the organisms are choosing something. Birds are not deciding to fly further north. The temperatures and food supplies they need have moved north, and the birds that follow them survive better than the ones that do not. Mosquitoes are not spreading deliberately either. Regions that were once too cold for them are now warm enough, so the area in which they can live has grown, and malaria has grown with it.

Each consequence in that list reduces biodiversity by the same route in the end: a habitat is lost or changed, and the species that cannot move or cannot adapt fast enough die out.

How this fits together

Human impact on the environment completes the core of Topic 7. In Triple award biology, the topic continues with how energy and biomass flow through food chains, in the trophic levels and biomass guide.