Successful exotic species often outcompete native plants, to the detriment of local diversity and ecosystem processes. Yet our understanding of what drives exotic success is still poor. In this study, we pursued research that assessed the relative support for two leading invasion hypotheses, namely the ideal weed and the enemy release hypotheses. We tested these hypotheses using sulfur cinquefoil (Potentilla recta) and spotted knapweed (Centaurea stoebe), which are two common invaders in the Intermountain west. These invaders are native to Eurasia and were introduced to the United States over 100 years ago. We compared growth rates and the potential for enemy release of the two invaders to two closely related native species; sticky cinquefoil (Potentilla glandulosa) and yellow hairy aster (Heterotheca villosa). We grew seedlings of these focal species in soils that had previously contained these species to test how culturing of soil biota might influence subsequent seedling establishment. To experimentally suppress soil fungi, of which some are expected to be pathogenic, we sprayed half of the plots with fungicides. If invader success is due to superior traits as predicted by the ideal weed hypothesis, then the invasive plants were expected to grow larger than the native plants in all plots under all conditions. However, if invader success is due to enemy release, native plants would benefit from fungicide treatments whereas invasive plants would not respond, because fungicides reduce pathogens that are more likely to afflict native plants.
Results/Conclusions
Regardless of soil treatment, the two invaders grew larger than their native counterparts (P<0.001). This supports the ideal weed hypothesis because the two invaders were able to grow much larger than the natives over the same time and given the same resources. Native and invasive plants did not respond differently to fungicide, however, nor did native and invasive plants grow differently in conspecific and heterospecific- trained soils. These findings do not support the enemy release hypothesis. In fact, leaves of the invasive sulfur cinquefoil were afflicted by a fungal pathogen (likely Phragmidium ivesiae) that did not attack the native cinquefoil, contrary to the enemy release hypothesis. From these preliminary findings, we conclude that superior traits rather than a release from specific enemies may help explain the success of spotted knapweed and sulfur cinquefoil in the Intermountain west.