Bullfrogs compete with other frogs for food, and can even kill their own kind if food is no where to be found. Bullfrogs are considered one of the worst alien invasive species (AIS). The AIS are animals who are able to decrease other species and even make them extinct worldwide. Bullfrogs are responsible for the decline of garter snakes in Mexico. In a laboratory experiment by Alford in 1989, bullfrogs decreased the population of Cope's gray treefrog. The study introduced the bullfrogs with Cope's gray treefrog in a controlled environment.