Restoring functionally diverse communities enhances invasion resistance in a freshwater wetland

Abstract

Understanding how plant communities reassemble is particularly relevant when invaders are part of the species pool and can compromise restoration or conservation goals. If initial species composition has a lasting impact on community dynamics, it may be possible to enhance biotic resistance to invasion, particularly through diversity effects, by manipulating early community composition. Diversity effects have been investigated much less in wetlands than in uplands. We conducted a large plant community restoration experiment in a freshwater wetland to investigate species and functional diversity effects on biotic resistance to invasion during early community assembly. The wetland was under propagule pressure mostly from Phragmites australis (common reed), but all species not sown in this experiment that established in the plots were treated as ‘invaders’, whether exotic or native. We used a two-step model selection approach and a diversity-interaction model framework to test hypotheses about the contribution to invasion success of abiotic conditions, species richness, as well as functional group richness, identity and pairwise interactions, providing a rare test of diversity effects against multiple invaders in wetlands. In all, 20 species invaded the plots. They were grouped for analysis to evaluate the overall resistance of resident communities. The three main invaders, P. australis and two native taxa (Salix spp., Populus deltoides), were also analysed separately. Diversity-interaction models revealed invader-specific responses. Functional group identity determined resistance to P. australis, whereas pairwise interactions between functional groups explained resistance to Salix spp. Both effects were retained when grouping all invaders. Resistance to P. deltoides was determined by abiotic conditions. The contribution of specific functional groups varied with invader. Annuals and clonal perennials explained resistance to P. australis, suggesting priority and limiting similarity effects. Interactions, when significant, mostly included fast-growing annuals. When considering all invaders, identity effects were detected for all functional groups. Synthesis. Diversity effects on invasion resistance vary with invaders in early assembly of restored wetland communities. Restoring functionally diverse communities provides resistance against multiple invaders. Consistent responses between this field experiment and previous pot experiments with the same species suggest that results are robust enough to inform management.