Wild animals have always been a critical resource for human beings. Historically, food, fur, and leather were key to human survival — more recently, wildlife has assumed high economic and cultural significance. Wild animals provide entertainment in circuses, zoos, and wildlife parks, they form a central attraction in international tourism, and they are key members of ecosystems on which humans rely for vital services. Equally, wild animals can be seen as threatening to human beings; for instance, they can be sources of new human diseases (zoonotics), and they can damage or consume human crops. What matters here, whether as resource or threat, is how useful — or otherwise — wildlife is to human beings. Environmental ethicists often call this instrumental value.