Rats

For millions of years, birds lived nearly predator free in the Aleutian Islands. The volcanic archipelago stretches westward for 1,200 miles from the Alaska Peninsula, dotting a border between the North Pacific Ocean and Bering Sea. Hundreds of bird species thrived here. But then came the rats. When a Japanese boat sank in the Western Aleutians around 1780, stowaway rats jumped ship and made it to one of the islands, wreaking havoc on the ecosystem.