Humans affect ecosystems in positive and negative ways. These changes alter biodiversity, which is the variety of living organisms in an ecosystem.
In fish farming, fish are raised in cages or tanks. The positive effects are a reliable supply of protein for people and less overfishing of wild stocks, so wild populations have a chance to recover. The negative effects are that waste food and faeces pollute the water around the farm, and crowded fish spread disease and parasites to wild fish. Farmed fish that escape may breed with wild fish and change their genes.
A non-indigenous species is one that has been introduced to an area where it does not naturally live. It often has no natural predators there. It may outcompete native species for food and space, eat them, or carry diseases that harm them, so native species decline and biodiversity falls. In the UK, the grey squirrel carries a virus that does not harm it but kills red squirrels.
Eutrophication is another harmful effect. Fertiliser washed off fields into a river or lake adds nitrates to the water. Algae grow very quickly and block light, so plants beneath them die. Bacteria that decompose the dead plants use up the oxygen, and fish and other animals suffocate.