Competitive superiority of non-native invaders becomes weaker when plant diversity increases – a case study with Solidago species

Two popular hypotheses in invasion biology are the “inherent superiority hypothesis”, stating that non-native species generally perform better than native species, and the “biotic resistance hypothesis”, stating that at local scale invasibility decreases with resident species richness. To test if plant community diversity reduces the performance of invasive non-native species more strongly than that of native ones, individuals of three Solidago species, S. virgaurea (native), S. canadensis (non-native, frequent in the local flora) and S. gigantea (non-native, non-frequent in the local flora), were planted into 80 experimental grassland communities varying in plant species richness (1, 2, 4, 8, 16, and 60 species) and functional group number (1 to 4 functional groups) (Jena Experiment, Germany). Survival was monitored and growth-related traits were studied as performance measures over two growing seasons. Both non-native invasive Solidago species grew faster than the native one. At low species richness, the non-native invasive S. canadensis and S. gigantea produced more biomass and had taller shoots with more leaves than the native S. virgaurea . This was presumably due to their better capability to take advantage of unused resources, supporting the inherent superiority hypothesis. With increasing sown plant species richness associated with a denser and taller canopy of the grasslands, survival and individual plant performance decreased in all three Solidago species, which is in line with the biotic resistance hypothesis. The decline in growth performance with increasing species richness was particularly pronounced in the non-native S. canadensis and S. gigantea so that their inherent superiority was more weakened compared to the native S. virgaurea with increasing plant diversity. These results emphasize the important role of native plant diversity for invasion resistance at local scale.