Monoculture
Monoculture is growing a single crop in a large area.  Farmers decide what is the best crop, and grow only that. This is very common in modern agriculture, as it simplifies the farm and allows relatively few people to harvest large amounts.  Often the crop of a large area is not merely a single species, but a single variety.  Monocultures have problems, however: Pests and diseases that attack only one kind of plant can spread more easily when they only find that kind.