Since animals do not put explicit prices on ecosystems they use, but do behave as if they are valuable, e.g. by defending turf or access to water, it is mostly a matter of definition whether ecology should include valuation as an issue. It may be anthropocentric to do so, since "valuation" more clearly refers to a human perception rather than being an "objective" attribute of the system perceived. Ecology itself is also human perception, and such related concepts as a food chain are constructed by humans to help them understand ecosystems. In many cases by those who hold that markets and pricing exist independently of any individual human observers and "users", and especially those who deem markets to be "out of control", ecosystem valuation is considered a (marginal, ignored) part of economics. 