Plant invasions affect native plant reproductive mutualisms, such as biotic pollination, in negative and positive directions. Whether increases or decreases in pollination occur in response to plant invasions should depend on environmental context, and mechanisms are poorly understood. I examined how shade, phenology, spatial scale, and floral density influence interactions between a non-native invasive shrub in the USA, Lonicera maackii, and two native herbaceous species, Geranium maculatum and Hydrophyllum macrophyllum.
I designed an experiment to investigate direct (via shading) and indirect pathways (via pollinators) by which L. maackii may interfere in native plant reproduction in an invaded forest. Potted G. maculatum plants in treatments containing L. maackii shrubs (with and without flowers) received fewer pollinator visits and conspecific pollen grains than plots in which L. maackii was removed. Hydrophyllum macrophyllum did not co-flower with L. maackii and also received fewer visits in the presence of L. maackii foliage. Thus, invasive plants can decrease pollination of native plants (via shade), regardless of whether they co-flower or share pollinators. Although potted G. maculatum and H. macrophyllum also produced fewer seeds in plots containing L. maackii, hand pollen treatments suggested that light limited seed set in both native plants, not pollen receipt. Therefore, the mechanism of impact on native plant reproduction was increased understory shade. At a different site where H. macrophyllum and L. maackii co-flowered, pollinator visitation was higher and the magnitude of pollen limitation lower in the presence of L. maackii compared to plots in which L. maackii was naturally absent. Comparing H. macrophyllum results across these two sites in which flowering phenology was asynchronous or synchronous with L. maackii suggests that effects may vary depending on flowering phenology.
Because pollinators are mobile, interactions between plants for pollinators may occur beyond the local neighborhood in which interactions for abiotic resources occur. In an old field habitat, G. maculatum pollen receipt and seed set of potted plants decreased, and magnitude of pollen limitation of seed set increased from 0-40m from forest edge habitat invaded by L. maackii, suggesting pollinator-mediated facilitation of reproduction. Presence of L. maackii was confounded with presence of forest edge habitat, but pollinator composition data and pollinator nesting biology support the interpretation that L. maackii affected pollination and subsequent seed production of G. maculatum over a 40m spatial scale.
Finally, relative floral density may influence whether plants compete for pollinators. In an array experiment in an old field habitat, G. maculatum received more visits in arrays containing a low density of L. maackii flowers compared to control arrays in which L. maackii was absent. The duration of pollinator visits was lower when L. maackii was present at a high density compared to controls. However, the number of G. maculatum seeds per flower did not differ among array treatments, suggesting that altered pollinator foraging behavior is unlikely to lead to population-level impacts on G. maculatum reproduction. In summary, light availability, floral density, and perhaps flowering phenology all influence whether invasive plants have positive or negative effects on native plant pollination.