Is it valid to "offset" destruction an area of habitat by declaring another equal area legally protected? I don't believe so.

The reasoning for it seems to be based on the assumption that every area of forest that isn't protected now will certainly be destroyed later. Only thus would it follow that protecting one square mile now increases by one square mile the area of habitat that will ultimately survive.

That assumption is terribly pessimistic. If it were valid, disaster would be assured. But it clearly can't be valid in general, because any forest that isn't protected this year will probably still exist next year and could be protected then.

There is a questionable assumption on the other side, too. Declaring a square mile of forest "protected" won't assure it survives. It could be burnt in a wildfire 300 years from now, or next week.

I think the only valid way to offset the destruction of some habitat is to create a new area of substitute habitat -- and that more easily said than done.