After habitat destruction, invasive species are the second leading cause of extinctions worldwide. When organisms are moved outside their native ranges, they devastate ecosystems that evolved without defenses against them.

The brown tree snake introduced to Guam wiped out most of the island's native forest birds. In Australia, introduced cats kill billions of native animals every year.

Global trade speeds this up by opening transport pathways to new continents. Ballast water in cargo ships carries marine organisms across oceans, and imported plants bring insects that native species have never met.