On many occasions, humans have transferred species from one continent or island to another -- sometimes intentionally and on other occasions by accident. Sometimes the newcomers rapidly outcompete the species that already live there and drive them to extinction, thereby reducing biodiversity of an area. The brown treesnake, for example, was accidentally introduced to the island of Guam during WWII and, ever since then, the number of bird and reptile species on the island has rapidly plummeted.