Algae processes more CO2 than trees. You could convert the forests into logging land and just set up massive platforms on the ocean that manage algae. Maybe not even that.
Additionally, I'll mention that I agree with blaabjerg's response: it's worth maintaining biodiversity for the fun of it.
See, for example: http://en.wikipedia.org/wiki/Trophic_cascade
You can use "incredible complexity" on literally anything. Everything is complex. Humans manage.
Corollary: I don't see how a broadly conservative approach to managing biodiversity can be discounted because it isn't specific enough.