Biodiversity is the variety of living organisms in a habitat or on Earth. Human activity can reduce it. Land use is a major cause: clearing forest for farming, building roads and houses, quarrying and draining wetlands all destroy a habitat. The animals and plants that lived there lose food and shelter, so their numbers fall and some species may become extinct. Hunting is another threat. If animals are killed faster than they can breed, as happens with poaching, the numbers of that species fall and it can become extinct.
Pollution also changes ecosystems. Acid rain is made when gases such as sulfur dioxide dissolve in rain water. It can slow the germination of seeds, and stronger acid has a bigger effect. Lichens are very sensitive to sulfur dioxide, so few lichens on trees suggests the air is polluted, while many lichens suggests the air is clean. In the lab, seeds can be watered with different strengths of acid and the percentage that germinate is counted.
Humans can also help. Nature reserves protect habitats, laws can ban hunting and trade in endangered species, and captive breeding programmes raise young animals that are later released into the wild. Seed banks store seeds of rare plants, and replanting hedgerows gives wildlife shelter and food. These conservation methods protect individual species and selected habitats.