Hummingbird photo: Kevin Schafer

Intricate connections exist not only among the levels of biodiversity, but also among species. For example, some plants and the animals that pollinate them have evolved to be dependent upon one another: A long-billed bird pollinates a flower with a long tube and a night-roving moth pollinates a night-blooming cactus. And some insect parasites have evolved to live in only one host, such as a species of wasp that lays its eggs on only one type of caterpillar. Such close partnerships and interrelationships between species mean that if one species becomes extinct, the other could too.