In arid environments, dominant plants such as shrubs or trees typically facilitate other plant species within their understory. These direct positive effects are relatively well studied; however, the indirect effects arising from this interaction are not. Here, the magnet hypothesis for pollination was tested using the dominant plant Caesalpinia spinosa and its associated understory species Grindelia glutinosa. We predicted that visitation rate (a proxy of pollination), abundance, and diversity of arthropod pollinators should increase for the target understory species when associated with C. spinosa. To test this hypothesis, pollination visitation to the understory target was recorded using iPod Nanos and pan traps both under C. spinosa and in open adjacent microsites. The experiment was replicated twice – when C. spinosa was flowering, and when it was in vegetative state (but with the understory species in flower in both instances).
Results/Conclusions
We found higher abundance and diversity of visitors in microsites associated with the dominant plant, and this facilitation was greater when the dominant was also in flower. Arthropod communities occurring in open microsites were subsamples of the arthropod communities visiting understory microsites. The most dominant taxa in both microsites were Hymenoptera and Diptera. Overall, this study demonstrates that dominant plants increase pollination visitation for understory species, which provides support for the magnet hypothesis. Moreover, this study also provides evidence of indirect effects mediated by dominant plants that go beyond plant-plant interactions. Finally, we show that dominant plants are important for understory species because they enhance pollinator services locally.