ARCATA, Calif. — It took the death of a small, rare member of the weasel family to focus the attention of Northern California’s marijuana growers on the impact that their huge and expanding activities were having on the environment.

The animal, a Pacific fisher, had been poisoned by an anticoagulant in rat poisons like d-Con. Since then, six other poisoned fishers have been found. Two endangered spotted owls tested positive. Mourad W. Gabriel, a scientist at the University of California, Davis, concluded that the contamination began when marijuana growers in deep forests spread d-Con to protect their plants from wood rats.

That news has helped growers acknowledge, reluctantly, what their antagonists in law enforcement have long maintained: like industrial logging before it, the booming business of marijuana is a threat to forests whose looming dark redwoods preside over vibrant ecosystems.

Hilltops have been leveled to make room for the crop. Bulldozers start landslides on erosion-prone mountainsides. Road and dam construction clogs some streams with dislodged soil. Others are bled dry by diversions. Little water is left for salmon whose populations have been decimated by logging.