Endangered Species No, not owls or elephants. Humans who fight to save the planet are putting their lives on the line.

FOR MOST PEOPLE INTERESTED IN protecting the environment, the costs of activism are measured by a little time and a little money. Attend an Earth Day rally, write a check, recycle the trash -- that's about all it takes. But for some of the most determined defenders, the commitment -- and the costs -- can be much higher. Around the world, more and more ecoactivists are paying with pain and peril, and sometimes their lives.

Chico Mendes, the Brazilian organizer who was assassinated in 1988 by ranchers for trying to preserve the Amazon forests for small-scale rubber tappers, remains the best...