Has anyone else seen Studio Ghibli's "Pom Poko"? I wouldn't call it a "sad" movie, but it is quite sombre in places. If cmoon hasn't seen it, I highly recommend it because I think you could draw some interesting parallels between it and Tezuka's take on environmentalism/conservationism.
Basically the traditional "shape-shifting" magical racoons of Japanese lore decide to declare war on humanity in order to try and preserve their mountain home in the face of urban expansion. One of my favourite parts of the movie is when the "undercover" Racoons in Human-form realize that they're negotiating with "undercover" shape-shifting Foxes in Human-form (another mythical Japanese animal). I found it quite interesting that the foxes have adopted a "if you can't beat 'em, join 'em" attitude.
Pom Poko is a bit off the Ghibli beaten path.. and I didn't really pay it much heed because it looked rather goofy, but it's become one of my favourites.