Background/Question/Methods

The European Cirsium vulgare is an invasive species worldwide, but in Nebraska it has remained at very low densities for over 200 years. The leading hypothesis is that pre-adapted, specialized floral insect herbivores from a similar, synchronously flowering, native congener (C. altissimum, tall thistle), have included the introduced thistle in their food range; documented reductions in C. vulgare seed production exceed 90%. Since the native thistle is relatively abundant in suitable habitats, there is no enemy free space for the introduced thistle to thrive. However, in a previous demographic study we concluded that even though floral herbivory was the most important factor controlling the introduced thistle, on its own floral herbivory is unlikely to be sufficient to explain the low abundance of the introduced thistle. Furthermore, the effect of floral herbivory on the native thistle is similarly severe. This begs the question, why is the density of the native thistle not as low as that of the introduced thistle? Here we expand our research to incorporate experimental data on insect foliage herbivory and plant competition. We use integral projection models (IPMs) to estimate population growth rate, ?, for both thistle species under different herbivory and competition treatments.

Results/Conclusions

Under reduced competition and reduced insect herbivory the introduced thistle populations are predicted to increase 15-fold annually. This growth potential is reduced by insect herbivory to a 3-fold increase. Then the additional impact of competition with surrounding vegetation produces a drastically shrinking population. Compared to the introduced thistle, the native thistle suffered more from herbivory, but was less impacted by competition. The model suggests a crucial interaction between competition and herbivory. For the native thistle, a reduction in competition increased ? by 10% under ambient herbivory and by 30% under reduced herbivory. In contrast, for the introduced thistle, a reduction in competition increased ? 6-fold in both herbivory treatments. Thus, the differences in population growth between the native and introduced thistles depends upon the particular combination of competition and herbivory. If competition is low, then populations of the introduced thistle will grow much faster than those of the native thistle. If competition is medium to high then ? of both thistle species is similar under medium to high herbivory. However, if competition is medium to high and herbivory is low then native thistle populations will grow faster. Thus, herbivory combined with competition strongly limits population growth of the introduced species.