I think that probably the most important point to make about taking the precautionary approach to things is that even if we don't know anything at a higher level, we understand the importance of an ecosystem or an area for its representativeness of a region or for the fact that it is relatively untouched but could be disturbed in the future.
There again, we should not be limited by our knowledge of the detail of these areas before making decisions about boundaries, but at the same time, we should not be excessively using the precautionary principle just to put boxes around things.