An invasive shrub facilitates pollination and reproductive success of a native herb over a small spatial scale

Background/Question/Methods

Determining the spatial scale over which invasive plants affect pollinator services to native plants will help to resolve the scope of effects on native plant reproduction and population persistence. While interactions for resources such as water, soil nutrients, and shade are unlikely to extend beyond the zone of rhizoshpere overlap, interactions for pollinators may occur over larger distances because pollinators are mobile. Pollinators forage from 150m up through kilometers away from nesting sites, meaning that invasive plants potentially influence the pollination dynamics of native plants over hundreds to thousands of square kilometers. Because little is known about the scale of such interactions, we determined the spatial scale over which an invasive shrub, Lonicera maackii, and a native herb, Geranium maculatum, interact for pollinators. We compared pollinator visitation rate, pollen deposition, size of bee visitors, reproductive success, and pollen limitation of reproduction of G. maculatum plants in rows arranged parallel to the edge of an invaded forest up to 140m away (in 20m increments) into an adjacent old field habitat.

Results/Conclusions

Contrary to our expectations, we detected facilitation of pollination over a small spatial scale (<1m from edge of L. maackii canopy); G. maculatum pollen deposition, fruit set, and seed set were all significantly higher when adjacent to L. maackii plants compared to plants 20-140m away from L. maackii. However, visitation rate to G. maculatum was significantly lower in adjacent plants.  The discrepancy between visitation and pollen deposition was likely due to differences in pollinator size among distance classes. Bees, the most common visitors to G. maculatum, were represented by larger species nearer to L. maackii. Because larger bees are often more efficient pollinators, we hypothesize that they deposited more pollen despite making fewer visits. Due to variation in pollinator size across space, G. maculatum pollen deposition and female reproductive success were facilitated over a small spatial scale. These results are consistent with two other studies that suggest the spatial scale of pollinator-mediated interactions is larger than interactions for other resources but considerably smaller than foraging ranges of pollinators.