Apparent competition between native and exotic genotypes of Phragmites australis in North America

Background/Question/Methods

Biological invasions have been one of the major drivers of ecological alterations in the natural ecosystems. Although the ecological and economic consequences of these invasions are well understood little is known about the mechanisms supporting them. The importance of indirect biotic interactions has mostly been overlooked in these processes. Invasive plants may facilitate their invasion success by increasing densities of herbivores which have greater negative impact on their neighbors, a process known as apparent competition. However, empirical evidences to support this hypothesis are still rare.

Phragmites australis offers an ideal system to examine the importance of biotic interactions on invasion process. It has been a member of wetland communities of North America for millennia but an introduced Eurasian genotype has recently spread throughout North America and overlaps broadly in distribution with various native genotypes. In this study, we examined the hypothesis that the exotic genotype enhances its invasion success by apparent competition. Using co-occurring native and exotic stands of P. australis, we performed reciprocal transplantation experiments in the wetlands of five coastal states (Maine to Louisiana) and analyzed damage caused by arthropod herbivores on the transplanted plants for all major feeding guilds.

Results/Conclusions

Native plants received greater herbivore damage than the exotic plants for all major feeding guilds (sucking, chewing, mining, and galling). Native plants transplanted in exotic stands received greater damage by sucking (aphids), leaf mining, and stem-feeding (galling and boring combined) insects than those transplanted in the original native stand. Proportion of chewed leaves did not differ between native plants growing in the stands of different genotype. Moreover, exotic plants growing in native stand produced significantly higher stem density than all other plants. These results suggest that the exotic genotype of P. australis may enhance its competitive ability by apparent competition.