Ecosystems worldwide are increasingly being invaded by plants from exotic origins. It has been stressed that these invaders perform better than similar native species in the invaded communities. Although some plant invasions have taken place for more than a century, the mechanisms explaining the success of these invaders are not well understood yet. The release from aboveground natural enemies has been widely stressed as one of the most important factors accounting for the ability of invaders to become successful. Novel biotic interactions, if favorable, enable many invasive plants to become dominant in their new community, thereby displacing native species. Currently, the consequences of climate warming are being noticed in range shifts of plants and animals to higher elevations and latitudes. Although biotic interactions strongly affect responses to warming, they are generally not included in climate studies. Also, top-down effects mediated by natural enemies in the third trophic level have not yet been taken into account in these studies. Hence, when plants spread faster than their natural enemies or than the enemies of their enemies, multi-trophic interactions can become, at least temporarily, disrupted. These changes in relations may create invasion opportunities for species within geographical regions.

Results/Conclusions

In order to test whether release from enemies may be applicable to the natural situation of range-expanding plants we studied the arthropod community on 2 range-expanding plant species (exotic to the Netherlands) and their native congeners. All species co-occur in riverine habitat and insects were sampled in 3 different populations in 3 consecutive periods in the growing season (early, mid and late summer). Insects were classified to different herbivorous guilds providing insight on the specificity of enemy release. Furthermore, we discriminated between phytophagous and carnivorous insects to determine the control potential of the third trophic level. As expected, we found higher aggregate loads of herbivorous insects on both range-expanding plants than on the native species. Depending on the species pair, either the chewing and sap-sucking insects or the galling insects showed the strongest differences, which in some cases depended on the temporal context. Surprisingly, we found higher predator pressure for the range-expanding plants, indicating an additional advantage over their native congeners. These results suggest that climate range-expanding plants are, at least partially, released from natural enemies, and that they experienced indirect positive feedback from carnivorous arthropods in the third trophic level.