Hi all. I think the responses to Teresa's question have been pretty informative. There have been a dozen responses giving logical and/or anecdotal evidence of invasive species causing extirpations...and one citation. The published examples are very rare (I'm not saying there is none, there is just surprisingly little considering how many people are studying invasive species) and the kind of evidence is similar to the evidence that we find in the Conservation Biology paper on Phragmites. We are presented with evidence that species richness is higher in marshes without Phragmites than marshes with Phragmites, but we also find out that sites with high Phragmites cover also have higher nutrient levels, lower salinity, and higher surrounding development. All of these things quite reasonably could be expected to cause declines in species richness without help from Phragmites (a removal experiment might actually get at this question). But let's take this a bit further - the aut! hors are not saying that there has been a local extirpation, only that in a particular zone of the marsh, species richness is lower. Is there any evidence that Phragmites invasion is causing extirpations in marshes? I don't see any. What prediction would we make if invasive species cause local extirpations of native species? Well, I think a crude first prediction would be that if I look in a bunch of quadrats in a homogeneous system there would be a negative relationship between the number of native and the number of invasive species. Any literature I have seen on this (and my own analyses) have shown a positive relationship between native and exotic species richness. Now, people will argue that invasive species have an effect but it is lost in the 'noise'. That may be so but at a minimum these positive relationships suggest that the effects of invasive species are not very important - certainly not important enough to provide a measurable signal. Further, there is all kinds of theory suggesting that competition is unlikely to cause local extirpations. Tilman suggested that as long as there is a tradeoff between dispersal and competitive ability the poorer competitor would persist in a community indefinitely. I don't follow this literature very closely but I suspect there is some evidence that simple stochasticity (even without the tradeoff between dispersal and competitive ability) can allow persistence. Lastly, I think a great point has been made about using extirpations or extinctions as the bar by which we measure the effects of invasive species. But we, the conservation minded ecologists, have set that bar by writing blurbs on every website about purple loosestrife, describing the terrible effect that loosestrife has had on the diversity of wetlands, when there is a pile of empirical evidence to the contrary. If we are going to state that the problem with exotic species is that they cause extirpations/extinctions we can't be annoyed when people ask to see the evidence. If we really mean that invasive species cause declines in the abundance of native species, let's say so (and make sure that that statement is true and at what spatial scale it is true). Best.
Jeff Houlahan
