A concept I've only recently become aware of is Ecosystem Services. This is the idea that all ecosystems and the creatures in them provide a service which has a value whether economic or not.
For instance, honey bees living in a field near an apple orchard provide free pollination that allows the apples to grow. If the field was knocked down, there would be no more bees to pollinate the apple orchard. The farmer would have to pay someone to bring bees in from another location. For this reason, the bees in the field have a value equal to the cost of bringing in new bees.
Other examples include trees that absorb harmful greenhouse gases that contribute to global warming. Also, wetlands provide flood water absorption, wildlife habitat and filtration services. We can't forget that many natural products are used for medicines and immunizations.
In a world where resources are becoming increasingly scarce, it may become necessary to catalog these services and attach an economic value to them. A group called the Natural Capital Project is attemptig to do just that. You can read more about ecosystem services and the Natural Capital Project here...http://news-service.stanford.edu/news/2006/november8/natcap-110806.html
It is important to begin incorporating ecosystems services into a multitude of policy decisions. A local zoning board may not ordinarily think of the value of a swamp before permitting its destruction for a new office building. We are learning, however, that the swamp may be more valuable. Why? The cost of man synthetically creating the same services that swamp offers for free would be very high. Here is a Forest Service reference to ecosystem services on their website...http://www.fs.fed.us/ecosystemservices/
Please take some time to learn more about ecosystems services. You'll never look at a lake the same way again.