Effects of an invasive plant transcend ecosystem boundaries through a dragonfly-mediated trophic pathway

Background/Question/Methods

Trophic interactions can strongly influence the structure and function of terrestrial and aquatic communities through top-down and bottom-up processes.  Although most studies of trophic cascades traditionally focus within an ecosystem (e.g., terrestrial or aquatic), trophic cascades are not necessarily restricted from crossing ecosystem boundaries.  In fact, species with life stages in both terrestrial and aquatic systems may be particularly likely to link the effects of trophic interactions across ecosystem boundaries.  Using experimental wetlands planted with purple loosestrife (Lythrum salicaria), we tested the degree to which the bottom-up effects of floral density of this invasive plant could trigger a chain of interactions, changing the behavior of terrestrial flying insect prey and predators and ultimately cascading through top-down interactions to alter lower trophic levels in the aquatic community.  

Results/Conclusions

The results of our experiment support the linkage of terrestrial and aquatic food webs through this hypothesized pathway, with high loosestrife floral density treatments attracting high levels of visiting insect pollinators and predatory adult dragonflies.  High floral densities were also associated with increased adult dragonfly oviposition and subsequently high larval dragonfly abundance in the aquatic community.  Finally, high flower treatments were coupled with changes in zooplankton species richness.  Through changes in animal behavior and trophic interactions in terrestrial and aquatic systems, this work illustrates the broad and potentially cryptic effects of invasive species and provides additional compelling motivation for ecologists to collaborate across traditional ecosystem boundaries.