These are conservation orgs suggested to us for partners in "I Mother Nature". We'll check them out tonight.
The Wildlife Conservation Society based at the Bronx Zoo in Bronx, NY, Robert Cook, Exec. VP (718) 220-5100
They have work all over, including focuses to protect wildlife and wild places in US including Klamath forests, Adirondack, and Yellowstone.
Conservation International based in Arlington, VA, Russ Mittermeier, Pres (703) 341-2400. Focused on sustainability and interactions of people and environment. [***Cheryl, I think that they are also larger than what you are seeking*** W.]
Bat Conservation International based in Austin, TX (don't have contact there) (512) 327-9721. Very small but well conceived wildlife/conservation organization that addresses wildlife, land use, and sustainability interactions.