Until recently, humans released non-native animals pretty much whenever they felt like it.
Need foxes for hunting? Import them from Europe and set them free in California. Bought some European deer for the zoo? Throw a few of them out in nature too (seriously, this happened in the 1940s).
Without regard for delicate ecosystem of California, humans for centuries have been introducing strangers into the environment.
But ubiquity can be a bad thing — non-native species compete with locals for food and habitat, sometimes to a fatal degree.
Red foxes, brought to the Bay Area for game hunting, have since become the single biggest contributor to the decline of the clapper rail.
The endangered wetland birds have been “decimated” by foxes, who kill adults, babies and also eat eggs.
In the 1980s, red foxes killed so many clapper rails in Seal Beach National Wildlife Refuge that only five mating pairs remained.