Invasive species constitute a significant potential threat to habitat viability. However, not all non-native species become invasive. The Jack-of-all-trades hypothesis posits that those exotic species that do become invasive owe much of their success to their ability to thrive under a broad range of conditions. Canada thistle (Cirsium arvense) is a long-lived, asexually and sexually reproducing, dioecious Eurasian plant which is naturalized across much of North America. Canada thistle is considered a noxious weed in 33 states because it can reduce rangeland quality and cause significant losses in agricultural productivity. Current management options include chemical, mechanical, and biological control strategies. Biological control, using non-native insects, has only been successful in a few cases. It is possible that the effect of insect herbivores in reducing Canada thistle growth and reproduction depends upon abiotic resource availability to the plant. We examined potential interactive effects between nitrogen availability and herbivory by indigenous insects on growth and reproductive success (number of flowers and viable seeds) of Canada thistle populations in western tallgrass prairie in Nebraska. We did a 3-by-2 factorial field experiment where we manipulated soil nitrogen level (reduced, ambient, and enriched) and insect herbivory (ambient and reduced). We maintained the manipulations for two successive years.We measured plant growth parameters and reproduction (number of flowers, viable seeds) at the end of each growing season.

Results/Conclusions

We found no significant change in either plant growth or reproduction with a change in soil nitrogen. This finding is consistent with results of our observational study of plant growth and reproduction across a range of soil nitrogen values. Significant variation in plant performance occurred among sites, but this variation was not correlated with standard environmental variables. We also found no significant effect of herbivory by native insects on plant growth and reproduction. This is likely due to the low ambient level of herbivory recorded in southeastern Nebraska. In conclusion, the results suggest that Canada thistle acts as a Jack-of-all-trades plant with respect to nitrogen and herbivory in its new North American environment, allowing similar levels of plant growth and reproduction across a range of levels of resources (soil nitrogen) and of consumption (insect herbivory), leading to its success as an invasive species.