"Big Bill" said:
"And further, letting foreign people plants and animals in always increases diversity. Sure, some native peoples, plants, and animals will die out, but it’s not like they have any right to the land. There is no God-given title. If the native peoples, plants and animals cannot compete and survive, that is their fault. It’s Darwin in action."
Bill's statement does capture a basic fact about the biological effects of invasive species: if the invasive species outcompetes the natives, resulting in the extinction of the native species, it is simply a case of natural selection. I cannot argue with that. There are some who might claim that situations involving invasives do not count, because the invasive arrived as the result of human intervention rather than "naturally". I dislike that argument, both because it ignores the fact that the effects would probably have been the same regardless of the mode of arrival and because it implies that humans aren't really part of nature.
In this case, there isn't any direct competition involved - the gall wasps aren't eating the food of some other species of insect. In fact, they aren't even eating the wiliwili trees to death. They are depositing their larvae inside the leaves. This damages the leaves that are used in this way, and the wasps are currently so successful at reproducing that they are damaging trees beyond repair. But no matter - the gall wasps are a new factor in the wiliwili's environment, and the wiliwili do not seem to be adapting quickly enough to adjust to this new environment. The same general pattern of events has probably occurred with different sets of species countless times over the last few billion years, and such events have likely been a contributing factor in the extinction of many, many species. The loss of the wiliwili would hardly be a unique or even uncommon occurance in the history of life. I wouldn't call it "Darwin in action", as the old man isn't known to have moved much since they stuck him under that old church floor, but the loss of the wiliwili would (will, if nothing changes) be the product of the harsh reality of natural selection.
How do I, as a person who accepts the obvious reality of evolution, justify taking action to preserve the wiliwili - or any other species - when it is failing the test of natural selection? For that matter, how does anyone who understands and accepts evolution justify aiding organisms that do not seem to be fit to survive without our help? Why is there such a field as conservation biology, particularly if it is filled with "Darwinist" scientists?
The answer is simple, it is clear, it is obvious to anyone who understands science, and it is apparently incomprehensible to any number of creationists. Science can provide us with information about the world that we can use to inform moral judgements, but it does not provide moral guidance. Scientific knowledge can tell us what the situation was, what it is now, and what it may or may not be in the future. It does not tell us what we should or should not do.
The question of whether or not we should try to preserve species requires us to make a moral judgement. There may be some people who think that a world that is populated by far fewer species than we see today might be a good place to live. I am not one of them, and I don't know anyone who is. Like many other people, when I look at nature I see something that is wonderful, even awesome - and I mean that in a very literal way: nature often evokes awe and wonder. I believe that a world in which entire communities of diverse species have been destroyed would be a far poorer place to live. I believe that we have a duty to pass on to our children a world that is better than the one we inherited, and I believe that doing our best to preserve the diversity of nature is part of doing that.
My understanding of science allows me to understant that the actions of the gall wasps are likely to lead to the extinction of the wiliwili unless something is done. My moral principles tell me that something should be done. My understanding of science does not change my moral principles, and my moral principles do not change my understanding or acceptance of science. And that is as it should be.