Ra's Al Ghul often cites the preservation of the environment as one of the reasons behind why the human population should be culled. Not exactly a popular environmentalist position, but it counts, I suppose... And then there's Swamp Thing. :)

Now this is a cool list.

Both D.C.'s Swamp Thing and Marvel's Man-Thing have been portrayed as environmentalists. A parody of those characters, The Toxic Avenger also at times had an environmental agenda especially after he was a cartoon character. Around the same time there was a lot of interest in environmentalism, and another cartoon hero appeared, Captain Planet, which was licensed by Marvel. In a more general way, jungle heroes like Tarzan and water heroes like Aquaman and Namor have frequently been protectors of their environments.

Ra's Al Ghul, Swamp Thing, and Man-Thing must be added.