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

Abstract

Trophic interactions can strongly influence the structure and function of terrestrial and aquatic communities through top-down and bottom-up processes. 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. 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 and shifts in the composition of zooplankton communities. 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 conduct investigations that cross traditional ecosystem boundaries.

Fig. 1

Conceptual diagram of the hypothesized trophic pathway in this study. We predicted that introductions of the prolifically blooming invasive plant purple loosestrife (L. salicaria) would attract insect pollinators and, subsequently, adult dragonflies, which prey upon the smaller insects. Through increased dragonfly oviposition and greater abundance of predaceous larval dragonflies in the wetland, these bottom-up terrestrial trophic interactions could then translate to top-down effects on the aquatic zooplankton community

Fig. 2

The abundances of a pollinating insects, b adult dragonflies, c dragonfly oviposition events, and d dragonfly larvae in wetlands increased with purple loosestrife (L. salicaria) flower abundance. Abundances were summed over equal sampling effort for each tank. The data points in each panel were fitted with a linear regression (solid line)

Fig. 3

Zooplankton species richness increased with loosestrife flower abundance. Zooplankton richness was based on counts from 15 L of water from each tank. The data points were fitted with a linear regression (solid line)