I think I have not made my arguments clearly enough. I merely intended to summarize my moral case for suppressing invasives as part of my summary of the off-forum conversation. My numbered paragraphs were intended to address the claim that there is no ecological difference between native and exotic species, and the claim that there is no ecological difference between human-mediated dispersal and dispersal by any other agent. My responses to Matt's responses to those paragraphs are below:
JC(1) Exotic species, on average, interact with fewer species than native > species, and their interactions are weaker, on average. In particular, > they have fewer parasites, pathogens, and predators, counted in either > individuals or species. This is especially true of plants, and especially > non-crop plants. I suspect, but have not heard, that exotic plants also > have fewer mycorrhizal associates than native ones, but I doubt that they > have significantly fewer pollinators or dispersers. Meanwhile, back in > their native ranges, the same species have the same number of associations > as any other native species. > MC(1) Natural selection only produces interactions good enough to persist > under prevailing conditions; there is no gold standard. By definition, 50% > of all species interact with fewer species than average, and 50% of all > interactions are weaker than average. Preferring stronger, more complex > interactions means preferring more tightly-coupled (and therefore) > 'riskier' > systems with a higher likelihood of failure. > JC (1b) The argument about how many species interact with fewer species than average misses my point. I'm saying that, if you counted the biological interactions for each native species and each exotic species in some area (could be a square meter, could be the world), I believe you would find that the average number for exotic species would be significantly lower than the average for exotic species. Thus, exotic species are ecologically different from native species. Actually, having more interactions may mean greater stability, on average, since some of those interactions are functionally redundant. I would have to brush up on my community ecology to be sure I'm not being overly simplistic, but I know this is true in pollination systems; pollinators that interact with more angiosperm species have greater population stability, on average, and angiosperms with more pollinator species have greater reproductive stability, on average (though I don't know if this leads to greater population stability for long-lived species). I'm not sure what you mean by "systems with a higher likelihood of failure." It seems to me that failure is a matter of human values not being realized. If, by "failure," you mean "rapid change," well, that hardly seems to be a problem for you. I would have to agree that systems managed to promote natives at the expense of exotics are more prone to failure than those where any and all ecological outcomes are deemed acceptable, but that's only because "failure" in the former group means invasion and domination by exotic species, while there is no such thing as "failure" in the latter group. > > JC(2) Very-long-distance dispersal by humans confers a fitness advantage > over very-long-distance dispersal by other agents, on average, for two > reasons. First, humans often disperse organisms in groups, such > as containers of seeds, shipments of mature plants and animals, or large > populations contained in ballast water, allowing them to overcome the Allee > effects (lack of mates, inbreeding depression) their populations would face > if introduced as one or a few individuals. We also often take pains to > maximize the establishment success of organisms we disperse, by shipping > healthy, mature plants and animals and propogating them when they arrive, > while non-human dispersal agents usually introduce small numbers of > organisms, often nowhere near their peak fitness potential (e.g., seeds, > spores, starving and dehydrated animals). > MC(2). JC appears to be arguing that once rare occurrences are no longer > rare. I agree. But I draw the opposite conclusion, because he is arguing > that to generate such changes is morally wrong, while I am just saying: > when > these conditions prevail, long distance dispersal becomes normal. > JC(2b) I'm not saying anything (here) about whether the recent commonness of previously-rare dispersal events is morally wrong. I'm countering the argument that human-mediated dispersal confers no fitness advantage over dispersal by any other agent. Others may be aware of an invasive exotic species that was not imported by humans in far greater numbers than we could reasonably expect from any other agent, even if it had 100,000 years to work, but I am not. Furthermore, most invasive species were carefully planted and tended across large areas. Others may know of a dispersal agent that takes such care of the species it disperses AND has any realistic potential of dispersing something over 1,000 miles, but I do not. Human-mediated dispersal is unlike dispersal mediated by any other agent. > JC(3) Although the population dynamics of invasive species do not differ by > what agent introduced them (whether humans brought them, some other agent > did, or they evolved in situ), it is ecologically consequential that human > activities are generating so many more invasive species than natural > processes usually do. Aside from maybe continents or oceans merging > through > plate tectonics, nothing non-human introduces such a flood of new species > to > new environments as we humans have in the last several centuries. > MC(3). See MC(2). What was once normal is no longer normal. 'Ecologically > consequential' in this context is standing in for 'morally consequential'. > Ecologically, change is change. > JC(3b) By this logic, ice ages are ecologically inconsequential. I'm not making a moral judgement (here). I'm only pointing out that humans have accelerated the frequency of invasion by new species by many orders of magnitude, and that this is having the sort of dramatic ecological effects you would, in theory, expect it to have. Again, this goes toward countering the claim that there is no ecological difference between human-mediated dispersal and dispersal by other agents. If we are increasing the probability of intercontinental dispersal by many orders of magnitude over what you find for all other agents combined, there is an ecologically important difference between human-mediated dispersal and dispersal by other agents. > > JC(4) To arrive at the conclusion that the terms "native" and "exotic" (or > "alien") are ecologically meaningless, you must approach the issue this > way: if there is no set of criteria by which one can reliably categorize > an > organism as native or exotic in the absence of historical evidence, the > distinction is meaningless. I think the valid approach is this: if there > is no set of criteria by which one can reliably distinguish the category > "native species" from the category "exotic species" (*after* the > categorization is done based on geographic history), the distinction is > meaningless. By analogy, the first approach is like saying that there is > no > difference in height between men and women because one cannot reliably > identify the height of a person by their sex, while the second approach is > like saying that there is a difference in height between men and women > because men are, on average, significantly taller than women. > MC(4). JC undermines his argument here by trying to make the difference > between natives and aliens morally inconsequential. I think we can assume > that he sees no moral imperative emerging from the statistical likelihood > that men are (and have been) taller than women. But we know he believes a > moral imperative emerges from the claims he makes in (1-3). So his analogy > isn't really an analogy. A better analogy would be a claim that men are, > on average, more politically powerful than women, evaluated in light of a > moral claim that no such difference should exist. But even that analogy > would only recommend equalizing average fitness; leveling the playing > field. > And it flies in the face of the "past = desired future" formula inherent in > anti-alien sentiment. Finally, if Williamson's legendary '10s' rule is > even > remotely accurate, the aliens are already disadvantaged by multiple orders > of magnitude. Long distance transport is now vastly more likely, but > establishment at the other end is still a long shot. > JC(4b) Again, I'm not making a moral argument at all. You (and others) have said that native and exotic species cannot be distinguished ecologically. One way I've seen other people arrive at this conclusion is by observing that there are no ecological criteria that can perfectly predict whether a species is native or exotic (e.g., there are invasive natives, there are exotic plants with more insect herbivores than related natives, etc.). I'm saying that this approach is like trying to predict people's sex based on their height, noting that you often guess wrong this way, and concluding that sex is not relevant to height.
