Pollinator-mediated matrix effects on multi-trophic interactions of flower-visiting insects

Traditionally, landscape ecological studies have focused on direct effects of landscape structure on individual species or pairwise species interactions.  Indirect effects of landscape structure on multiple interacting species have rarely been demonstrated.  In a factorial field experiment, we explored the direct and indirect effects of habitat area, habitat fragmentation, and matrix composition on a community of flower-visiting insects in red clover.  Pollinator visitation of clover inflorescences was 1.6 times higher in clover patches embedded within bare-ground than in patches surrounded by grass.  These matrix-dependent changes in pollinator visitation propagated across a tri-trophic system: higher visitation rates positively affected seed set, which in turn resulted in higher abundances of seed predators and their parasitoids.  Therefore, this study suggests that landscape structure can strongly influence the structure of species interaction webs through indirect effects, and also emphasizes that these effects can be propagated through mutualistic as well as trophic interactions.