What marks an invasive species, really? We all hear the term and those of us who bother to notice can see the effects of an invasive species on a native ecosystem.
Is the definition actually for a species that is currently in place where it does not belong? If so, Homo sapiens is highest on the list, since we likely should only be where we evolved: Africa. But that's not going to change until we go extinct, so back to the real issue.
There is no doubt--to use an example--that pythons are a harmful invasive species in south Florida and in the Everglades National Park. The ecosystem of that part of Earth slowly evolved over millions of years without the presence of a vast, aggressive constrictor snake in its midst. The sudden introduction of such an animal is resulting in the decimation of the native animals that did evolve there and which do belong there. I have no problem labeling pythons in Florida as an invasive species.
Further, there are invasive animals that are present in a foreign ecosystem which are not--apparently--harmful within that environment. When I go kayaking on the Silver River in Florida I generally encounter bands of feral Rhesus monkeys. I won't go in to how they got there (humans brought them, of course), but I often used to wonder if they were harmful to the system in which they found themselves living. I did some reading about it, and learned that apparently they are not considered particularly harmful. General browsers, they don't seem to eat enough of any particular food item in the forests to hinder the lives of the native animals and plants around them.
So. Not all invasive species are harmful.
Some of the worst invasive species are not mammals, reptiles, mollusks, or fish. Many of the most destructive of these things are plants, insects, and pathogens (think of the American chestnut blight). Again, to take Florida as an example, the list of plants that are changing and destroying native ecosystems is off the scales. Quite actually every waterway in Florida is home to any number of invasive plant species. Once established, how do you even begin to remove plants that are fast-growing and endlessly clogging rivers, lakes, ponds, and streams? It's probably an impossible task. The thing is to try to keep it from happening in the first place. But where humans are concerned, the problem seems to be ongoing and never-ending. People are going to acquire and dispose of exotic plants where they should never have been allowed to have them in the first place.
But in the case of plants, what constitutes a harmful invasive species? I actually do have a definition of that which meets popular consensus:
multiple reproductive methods
wide dispersal and survival
broad environmental tolerance
and resistance to management. (I'll get back to this later.)
And the problems they cause are:
loss of recreation
severe oxygen depletion
stunted fish populations, fish kills
water-flow restrictions, flooding
loss of biodiversity
reduction in property values.
What strikes me about the list of problems is that they are so centered on the troubles presented to human recreation and human economics. The concern for the local animals and plants is really just something that sits there and is only noticed for its tendency for causing an inconvenience to humans.
These are problems that beset the web of life in which we all live. Economic concerns are moot. Humans are at the base of the situation, sitting on the web of life that supports everything and methodically plucking out the strands until the whole thing collapses. This constant degradation of Mother Earth is not just going to result in the extinction of some species of big animals, or even of some entire environments that have been present for tens of millions of years. Currently we are in danger of witnessing the total collapse of everything that gives us the air we breathe, the water we need, and the food that sustains us. No more rhinos? No one cares. Hemlock trees extinct? Big deal. Invasive carp eating and out-competing every fish species they encounter? Feh.
We keep plucking out the bits of the web, one string at a time.
Implosion will arrive. And it's not as if we didn't see it coming.
In reference to that list--humans can't manage the Earth. We either live within the limitations of not causing the planet irreparable harm, or we kill it and die with it. You can't manage something with trillions of moving parts. You flow with it, or perish while breaking it.
We have chosen to ignore every warning.