Human activity can help or harm ecosystems. Fish farming means rearing fish in cages or tanks to be sold as food. It gives a reliable supply of protein and reduces overfishing, because fewer fish are taken from wild populations. It can also harm local ecosystems. Waste and uneaten food add nutrients to the water and can cause eutrophication. Fish kept close together spread disease and parasites such as sea lice, which can infect wild fish. Farmed fish that escape can compete with wild fish or breed with them.
A non-indigenous species is a species that has been introduced to an area where it does not naturally live. It may arrive by accident or be brought deliberately. It often has no natural predators in its new habitat, so it reproduces quickly and competes with native species for food and space. The grey squirrel, introduced to Britain from North America, outcompetes the native red squirrel and carries a virus that kills red squirrels.
Both fish farming and introduced species can reduce biodiversity, the variety of different species in an ecosystem. A few introductions are useful, for example new food crops, but they always carry a risk because the species can spread out of control.