environmentalist

Short definition from Wiktionary

Noun

1. 1
   One who advocates for the protection of the environment and biosphere from misuse from human activity through such measures as ecosystem protection, waste reduction and pollution prevention.
2. 2
   One who holds the view that environment, rather than heredity or culture, is the primary factor in the development of an individual or group.

Adjective

1. 1
   Of, or relating to environmentalism.