India, Oct. 23 -- In 1959, two New Brunswick fishermen came up with a simple plan for 12 snowshoe hares. Wesley Ingalls and his nephew, Junior, carried the animals by boat from Grand Manan Island to nearby Hay Island in the Bay of Fundy. They hoped the hares would reproduce and give them something to trap during the winter, when they were not fishing.

Instead, the experiment transformed the islands' forests. The hares multiplied rapidly and crossed from Hay Island to neighboring Kent Island. Research published in The Canadian Field-Naturalist found that tree recruitment on Kent Island plummeted after the 1959 introduction and remained low, while trees continued to establish on nearby hare-free Outer Wood Island. It took repeated attempts...