QUESTIONS AND ANSWERS

Theoretical and empirical approaches to understanding invasion by Carduss thistles.  Katriona Shea, Pennsylvania State Univ.

Q:    What community-level processes do you believe were affecting your results from New Zealand and Australia, and how might they change the life history processes?

A:    I think differences in soil and climate (i.e., nutrient and rainfall-type resources) play a part, as well as differences in ground cover of other species in the two locations.

Q:    Competition treatment in USA experiment showed little/no thistles. Does the species composition of these competitors matter? Are some species better than others at regulating thistle germination and survival?

A:    That is something that a new grad student of mine may look at, but at this time we have too little data to be sure.

Q:    Do you think genetic differences could be contributing to the different strategies that the thistles use to survive well in different populations?

A:    Possibly, but the thistle invaded Australia from New Zealand, so I think it less likely in this situation. Differences between the US and NZ, however, may well be because of genetic issues. It's something I hope to get to eventually, but I'd need to find a collaborator with more expertise in plant invasion genetics to work with me.

Q:    How many thistle congeners are present in NZ, NA, and Australia? Could differences be the result of native natural enemies differentially affecting the life histories of invasive thistles by host switching? What is driving invasion success in the US, where there are lots of native and nonnative congeners?

A:    There are no native thistles in Australia and New Zealand. There are lots in NA, and the person who works most on that is Svata Louda at Nebraska--she's working on some questions like this at the moment.

Q:    How do your results from New Zealand and Australia differ from the population dynamics of this species in its native habitat?

A:    It's not such a big problem in its native range, but I've never had data to look at this before. However, I now have some new data from a colleague in France and my lab is starting to explore this question. So, no answer for you yet, but I hope to get one soon.

Q:    Are the seed dispersion vectors different in NZ and Australia? Have you looked at the interaction between the type of dispersal and which life stages are most important to their success?

A:    The main form of dispersal that we can study is wind dispersal, and that is similar in all these locations to the best of our knowledge.

Q:    How do the thistle invasions affect animal communities? Are thistles used by native insects/birds?

A:    We don't really know, except that it badly reduces vegetation quality for livestock, so presumably for other grazers also.

Q:    In explaining the paradox of the number of exotics to the number of natives, do organisms from different trophic levels influence this pattern? Does this relationship hold up in aquatic systems where trophic interactions are very strong?

A:    Yes, the figure I presented really is focused on a single trophic level. I expect the situation to be more complicated if we explicitly thought about multiple tropic levels. At this time I think the same story might apply in aquatic systems, but I've yet to find anyone with the data to test this idea. The only tests I know of are for plant communities.

Katriona Shea