The concept of “sustainability” comes from forestry and refers primarily to resource management. The forest should be used in such a way that it can continue to be used for a long period of time. To cut down all the trees, something that has happened many times already in history with catastrophic consequences, would not be sustainable. 

In ecology, the concept of sustainability refers to the relations of human beings to their natural environment, that is, to the “ecosystem”. What is important in this context is the stability of the ecosystem, which should not be endangered by reckless exploitation of resources. This definition of sustainability is based in systems theory. An organism is a closed system that should interact with its enviornment in such a way that it can remain viable and ensure its survival. The implication, much as in forestry, is that the resources in the environment which the organism needs in order to live are to be used  in such a way that they are not depleted. The environment must remain the stabile and not change too much. Otherwise, the viablity of the organism is threatened. Here again, the emphasis is on stabililty and preventing changes. Whether an organism intends to act in such a way that the einvironment remains stabile, or does so by accident, or doesn’t do it all and eventually dies off is another matter. To say that sustainability is a value and a norm means that the environment should not change, or that it is somehow “wrong” to change the environment or even to let the environment be changed by other factors than one’s own actions. The value that sustainabilty signifies is stability. To act sustainably is therefore to do everything one can in order to ensure that the ecosystem remains stabile and does not change.