The role of insect herbivores in the interaction between Tanacetum vulgare and Solidago canadensis in a cross continental experiment

Background/Question/Methods

Mechanisms underlying successful species invasion remain poorly understood. But some invasive species may suffer less from co-evolved specialist herbivores and show increased vigour due to changed resource allocation in the invaded range. Solidago canadensis (Canada goldenrod; hereafter goldenrod) is native to North America and invasive in Europe while Tanacetum vulgare(common tansy; hereafter tansy) is native to Europe and invasive in North America. Both species often co-occur and appear to suffer less from herbivory in their non-native range. The broad objective of this study was to determine if this mutual invasion leads to a reversal of competitive hierarchies in the two areas, Europe and North America. We investigated the role of insect herbivory for goldenrod and tansy in Germany, Hungary, and in Montana (USA) in a cross-site field experiment where species were grown in monoculture or in competition with the other species. In addition, seeds of both species from the different origins were planted in a common greenhouse.

Results/Conclusions

Both tansy and goldenrod showed greater biomass and height in their non-native ranges compared to their native ranges. In mixed plots, tansy showed higher biomass and more shoots than in monocultures, while the response of goldenrod depended on site (increase in Germany, decrease in Montana. Herbivory on both tansy and goldenrod was higher in their native ranges. For tansy, insecticide application increased biomass, height and flowering in its native range of Europe and decreased these traits in the Montana. For goldenrod, insecticide treatment did not affect performance despite experiencing high herbivory in its native range in the Montana. In a common greenhouse experiment, plants derived from populations in the invaded range were larger than plants from populations in the native range, for both species. As there was no competitive suppression of native species, our experiment could not confirm the enemy release hypothesis. Instead there was an indication of genotypic differences between plants from the native and invaded ranges suggesting adaptation in the non-native range.