Cats Out of Bag Department, or Pandora's Box (on humans introducing harmful species)
Given that it is always more dangerous to put something into an environment than to take something out, consider this. For all their 10,000-plus years in the Western Hemisphere, the Indians lived by “taking things out” and whatever they took was immediately replaced, with the possible exception of the “Pleistocene Megafauna” (assuming these big animals went extinct from overhunting). What they put into the environment was just what they should have—almost nothing. Everything recycles. They didn’t even make garbage of the modern kind (that is, the kind that doesn’t immediately recycle). What they brought in with them from Asia, apart from themselves, was limited to a few organisms at most, all of which were directly dependent on humans: presumably they brought dogs, and presumably they brought a few parasites (lice, fleas) and possibly a few diseases, although this is open to question. They may have entered North America several times, each time bringing no more than the few organisms listed above. In other words, they allowed the locally-evolved flora and fauna of California—and the whole hemisphere—to continue doing its thing. They lived by hunting and gathering (and outside California, by a small-scale agriculture that required the importation of several food crops from Mexico), and used fire and limited surface disturbance and other small-scale technologies to increase the productivity of seeds and game.
After 1492 everything changed. Not for a while yet in California, but at an accelerating pace that has yet to stabilize. Then began the era of humans putting things into the environment on a massive scale (as well, of course, as taking things out on a vastly increased, industrial/ commercial, unsustainable scale).
Every time that humans in their various conveyances arrive here from distant shores, they can and often do, whether intentionally or quite unawares, bring other living things with them, in the form of spores, seeds, microorganisms, and sometimes larger animals and plants, sometimes on their persons or in their luggage as often on ballast or in cargo or clinging to the hulls of ships. Thus have arrived every one of the serpents in the California Garden. And between them they (and we) have devoured the place.
And every new plane that lands or ship that docks or truck that rumbles into the woods can carry the next plague. And there is no going back.
We are living, as one paleontological wag put it, in the Homogecene, and only over geologic time will all these nasty newcomers—including ourselves I fear—evolve into something you could legitimately call “native”. Meanwhile a lot of eggs will (have to) be broken.
There is, however, one little thing you can do to at least put off the inevitable on a small scale. And that is, if you propose to make a home for yourself in some out-of-the-way place that hasn’t yet been provided with all the foreign annoyances—like snails and Argentine ants and apple worms, for example—please do what you can to minimize the unintentional imports, and that means don’t bring in potted plants that might have snail eggs or a baby ant colony in the dirt, and when that very first French broom plant or Cape ivy pops up—as it no doubt will sooner or later—send it to heaven before it makes seeds. And please, please, please don’t start buying and planting a bunch of so-called “natives” Best of all, of course, don’t move into that out-of-the-way place at all, but such requests are to whistle in the wind.
Some credit is due to our pioneering forebears, who introduced into their wilderness homesteads only a limited number of foreign plants, and all of them useful, pretty, and non-problematic ones at that, like fruit trees and lilacs. There is a place for diversity, but there is also a place for simplicity.
