Habitat loss and pollution

A delicate balance

Food webs balance themselves. If a good season grows more grass, kangaroos have more food and breed more. More kangaroos then eat the grass down, and more dingoes can survive by eating kangaroos. Numbers go up and down, but each population is held in check by the others. People can tip this balance further and faster than any natural event, because there are so many of us and we can reshape whole landscapes.

Land clearing and urban sprawl

Cities keep spreading outwards into farmland and bush. This is called urban sprawl. Western Sydney is a good example: suburbs now cover land that was Cumberland Plain Woodland, and less than 10% of that woodland is left. Farms clear even more land. Australia is one of the world's top ten land-clearing nations, and hundreds of thousands of hectares of native vegetation are still being cleared each year, mostly in Queensland and NSW, mostly for grazing.

Clearing removes the producers that a whole food web depends on. When woodland becomes a grassy paddock:

• animals that need trees for food or shelter (koalas, gliders, many birds) lose their habitat and must compete for whatever trees are left
• animals that eat grass do well. Eastern grey kangaroos have become far more common since European settlement because there is much more grassland and more water for livestock. They now compete with sheep and cattle, and are culled under licence in parts of NSW.
• animals that live with people, such as noisy miners, rainbow lorikeets, possums and ibis, often thrive in cities

Land degradation

Land degradation means the land becomes less healthy and less productive. In a forest, a thick mat of roots holds the soil together, and a layer of leaf litter shields it from rain and wind. Rain soaks in slowly, and roots and soil filter out mud and some pollutants on the way to the groundwater.

When the plants are cleared, several things go wrong:

• Soil erosion. With no roots or litter, wind and rain carry the topsoil away. Muddy run-off ends up in creeks and on coral reefs.
• Compaction. Hard hooves of cattle and sheep pack the soil down, so water can't soak in and runs off even faster.
• Dryland salinity. Deep-rooted trees no longer use the rain, so the water table rises and brings salt to the surface (see 1J).

People also degrade ecosystems by taking more than they can replace: overgrazing (too many animals on a paddock), overfishing and over-harvesting. Pollution from factories, sewage, rubbish and too much fertiliser does lasting damage too.

Poisons that build up

Some chemicals don't break down in the environment and can't be passed out of the body, because they dissolve in fat instead of water. The pesticide DDT was one of these. It was sprayed widely on crops and to kill mosquitoes from the 1940s.

When DDT washes into a lake, tiny amounts are taken up by plankton. A small fish eats thousands of plankton in its life and stores the DDT from all of them in its fat. A large fish eats hundreds of small fish, and a fish-eating bird eats many large fish. At each step the poison becomes more concentrated.

• Bioaccumulation is the build-up of a chemical inside one organism over its lifetime.
• Biomagnification is the increase in concentration from one trophic level to the next.

Top predators are hit hardest. DDT made birds of prey lay eggs with thin shells that cracked under the parent's weight. After the American biologist Rachel Carson warned about it in her 1962 book Silent Spring, DDT was banned for farm use in many countries, including Australia (in the 1980s). Ospreys, pelicans and peregrine falcons have since recovered in many areas.

The same thing happens today with mercury. That's why Food Standards Australia New Zealand advises pregnant women and young children to limit how often they eat shark (flake), swordfish and other large predatory fish.