A species introduced by human activity or other means into a region where it did not originally live. Those that outcompete or prey on native species and damage ecosystems and agriculture are specifically called invasive.
In 1859, a British settler in Australia released 24 rabbits into the wild. With no predators and mild winters, they multiplied into billions. Australia tried continent-spanning fences, foxes, poison, disease, and the military—all failed. The rabbit war continues to this day.