The dominance of invasive plant species can result in the decline of native plant species, through a wide range of mechanisms. Lonicera maackii, or Amur honeysuckle, is a successful invader of Eastern US forests, and possesses allelochemicals to inhibit seed germination of other species. In addition, L. maackii exhibits extended leaf phenology to outcompete native understory plants for light in spring, and fix carbon in autumn long after the leaves of native plants have senesced. Furthermore, extended leaf phenology and complex branching architecture provide protection for seed consumers, which may suppress the regeneration of plant species that have dispersed to a patch. Here, we investigated the impact of L. maackii on the seed predation of two co-fruiting native plant species, Vitis labrusca and Parthenocissus cinquefolia. We conducted our experiment in late autumn, after leaf abscission for V. labrusca and P. cinquefolia, but while L. maackii still had leaves. We placed seeds of all three species in L. maackii and V. labrusca patches. We hypothesized that seed predation would be most intense in L. maackii patches for all three species because of the maintained complex habitat, and highest for V. labrusca in V. labrusca patches because of Janzen-Connell effects (density-dependent mortality).
Results/Conclusions
We found that a greater proportion of seeds were depredated in native V. labrusca patches compared to in L. maackii patches (p=0.0002), with no differences between species (p=0.21). For only P. cinquefolia, survival time of seeds was longer in L. maackii patches (p=0.048), with a higher (1.18x) risk of being depredated in native V. labrusca patches. We attribute our surprising results to the most frequently observed seed predator at our site: Prenolepis imparis (winter ants). In late autumn, the activity of invertebrate seed predators is likely to be higher in warmer open areas, such as in V. labrusca patches, than under the dense canopy of L. maackii. Although the leaf phenology of L. maackii may extend the duration of seed predation by small mammals, it may simultaneously diminish the risk of seed predation by invertebrates under its canopy. A possible consequence of invasion by L. maackii is that in late autumn, invertebrate seed predators concentrate their activity in patches of native, leafless plants, and constrain the regeneration of native plants in these patches.