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Why Alien Species Thrive
planux writes "The Sacramento Bee has an interesting article about why invasive animal species thrive, pushing out native species -- sometimes to the point of extinction. Kevin Lafferty, a U.S. Geological Survey marine ecologist at the Western Ecological Research Center in Santa Barbara says "Invasive species end up with about half the parasites, or diseases, they had at home." Animals with an average of 16 parasites on their home turf typically bring about three of the parasites with them to new locations. And only about four new parasites will typically adapt to attack the invading species. Net gain: 9 fewer parasites!"
Most species are going to have predators pleased to greet them, as most plants and animals are edible. What's important is the edge, a relative rather than absolute advantage over native species.
A couple of examples are kudzu and zebra mussels -- they don't complain about much. Our local favorite is the Asian "tiger" mosquito. But there are serious bad effects from monoculture -- one disease can destroy everything in its path. Think Windows desktops and a nasty virus.
Nature will tend to reach an equilibrium, or oscillate wildly, or the newcomers either exterminate the old, or vice versa. There are equatios for estimating equilibrium. Only in a really catastrophic situation does shortage of food supply rein in the invader. But evolution, as opposed to natural selection, is slow, so an alien species can easily exterminate native species before they have a chance to develop defenses, even to the point of suicide. Hawaii is suffering significant losses of species diversity because of newcomers. It takes thousands or millions of years for a new species to develop, yet perhaps the blink of an eye to perish.
I hate seeing natural selection described as some sort of moral quest for the "best," when it's just a way of explaining natural phenomena (I'm not saying you're doing that, but lots of people do). Darwin didn't judge was was "best," just tried to predict which species would do better under given circumstances. Nature doesn't care about the outcome, or become improved as a result. Species diversity is often the preferable situation.
Maybe I'm being clueless, but I'm hazy on your relevance to "devil's weed," slang for pot (which does grow quite nicely in the wild).