Jeff and Teresa, You make an excellent point about the need for us to back up our statements with evidence. Although many have provided very believable and plausible arguments for why invasive species have caused native plant extinctions, I find it interesting that hardly anyone is offering clear cut published examples.
I'd like to see these examples, because in my experience, documenting even local losses of plant species is very difficult (as Andy Dyer alluded to). I have personally seen cogongrass (Imperata cylindrica) displace about 2/3 of the native longleaf pine savanna species in those portions of the site it has invaded. How do I know this? It produces discrete patches that expand radially. So, you can literally witness the "disappearance" of natives as each patch expands via clonal growth (much the same way as in Phragmites). However, to truly document their local "loss", you also have to look at the seed bank, especially if the species that are disproportionately lost are annuals with persistent seed banks. In their case, the invader may simply be preventing seedling emergence in these species (for the time being). I've looked at the seed bank in infested areas, and it turns out that it is depauperate as well. However, one only has to walk a few meters out of the patch to find the "displaced" species. So, no extirpation, just reduced numbers. All this brings back the original question that Teresa raised, are there documented instances in which an invasive plant has extirpated native species? I'm not aware of ANY (but I haven't read Davis' article yet). Although I have certainly seen cogongrass (considered one of the worst weeds in the southeastern US) dramatically reduce species richness in longleaf pine savannas, I cannot say that I have seen it extirpate any species. I believe this is simply because its competitive effects operate at too small a spatial scale to cause such losses in the short term. I recognize that reduced numbers mean reduced population viability, but even then, given the potential for rescue effects, I would feel more confident saying that invasive species caused extinctions if there were actual direct evidence, as opposed to inferences from stochastic simulations on a computer or negative correlations. Turning the population viability question around, I have just witnessed 100% mortality of woody goldenrod (Solidago pauciflosculosa) on three barrier islands as a result of the storm surge associated with Hurricane Katrina (though there may be a few viable seeds left in its short-lived seed bank, assuming they weren't washed away). Would anyone suggest that I need not bother monitoring this species on these islands anymore? I'm inclined to think that this species won't recover on these islands, but I wouldn't bet more than $50. Also, I think we should be careful about assuming extinction when we document that an invasive species has altered the ecosystem (e.g., fire regime, nutrient regime) in a way that is "incompatible" with the persistence of some native species. I think it is sufficient to say that the invasive species has altered the ecosystem in a way that is harmful to native species. Period. I agree with Jeff that perhaps we can be confident in saying that some invasive species have reduced the numbers and have threatened the viability of some native species. I fear that skeptics will brand all of us who are genuinely concerned about the impacts of invasive species as chicken littles prone to hyperbole if we make claims we can't back up with empirical evidence. Furthermore, I believe that if make overly pessimistic assumptions about extinctions, this will lead some to conclude that, once an invasive species has "taken over" an area, there is nothing we can do. Anyway, as far as documentation is concerned related to cogongrass, Teresa, you might check out: Lippincott, C. L. 1997. Ecological consequences of Imperata cylindrica (cogongrass) invasion in Florida sandhill. Ph.D. dissertation, University of Florida, Gainesville, Florida, USA. Brewer, J. S. and S. P. Cralle. 2003. Phosphorus addition reduces invasion of a longleaf pine savanna (southeastern USA) by a non-indigenous grass (Imperata cylindrica). Plant Ecology 167:237-245. The first one deals more directly with negative effects of cogongrass on natives. The second study indirectly shows displacement (i.e., more native species in adjacent to patch than within a cogongrass-dominated patch). The other evidence I alluded to has not been published. Steve Brewer I think Teresa is raising a very good point and to the best of my knowledge the answer to her question is no, there is little in the way of empirical evidence that invasive species cause extinctions or even local extirpations. The empirical evidence on loosestrife for example, is that although we see it everywhere it has not caused extinctions. Best. jeff Houlahan -----Original Message----- From: Teresa Woods <[EMAIL PROTECTED]> To: [email protected] Date: Tue, 4 Apr 2006 13:49:11 -0500 Subject: invasive plants A couple articles to consider: Gurevitch, J. and D.K. Padilla. 2004. Are invasive species a major cause of extinctions? TREE 19:470-474. Davis, M. 2003. Biotic globalization: does competition from introduced species threaten biodiversity? BioScience 53:481-489. I couldn't believe the conclusions of these articles when I read them last year, but they did bring home the message that if indeed invasives are causing extinctions, even community level ones, we need to be documenting them in ways other than anecdotally. Hence, my question about relevant publications. I'd love to see them. As I said, even ones that show a correlation, as causation is justifiably hard to show. I am not as familiar with aquatic invasives except to know that some like zebra mussels are extremely aggressive and destructive. But regarding plants, invasives are often linked with disturbed habitats, and it may be that habitat destruction is the primary cause of local extinctions, and the invasives follow as the "final nail in the coffin (Gurevitch and Padilla 2002)." It is also likely that the extinction trajectories are definitely occuring but are longer-term, and just haven't reached the end yet. The link to the NYTimes article was, as I said, meant to fan the flames here -- not that I agree in any way with him, but to put on the table what landscape designers and architects are surely also being influenced by -- even if from, as has been correctly pointed out, a very biased point of view. So my query still stands -- is there empirical evidence supporting the sense that most of us have that invasives are causing native extinctions? I'd love to have evidence to contradict the NYTimes author's view. Teresa Teresa Woods Graduate Assistant Division of Biology 232 Ackert Hall Kansas State University Manhattan, KS 66506 785-532-9834 [EMAIL PROTECTED]
