Non-native invasive plants can greatly alter community properties and ecosystem processes, however efforts to predict which invasive non-native plant species have major impacts on ecosystem properties have been generally unsuccessful. Two of the best hypotheses to predict the effects of invasive non-native plant species are the mass ratio hypothesis and the plant traits that drive ecosystem processes hypothesis. We tested these hypotheses using data on plant community structure, soil nutrient status and the abundance of soil biota in different trophic levels from an exclusion experiment in which the presence of two dominant shrubs and subordinate plant species were manipulated in factorial combinations over 4 yr in a primary successional floodplain system.

Results/Conclusions

In contrast to expectations of the mass ratio hypothesis, our experiment showed that low-biomass non-native plant species comprising c. 3% of plant mass exerted disproportionate effects on ecosystem properties, and belowground communities in particular, compared to dominant shrubs comprising >90% of plant mass. Low-biomass non-native species increased soil C by 17%, increased the abundance of microbial-feeding nematodes by at least three-fold, and caused significant shifts in soil microbial community composition measured as phospholipid fatty acid profiles. Our experiment provided stronger support for the plant traits hypothesis; low-biomass non-native plant species had distinct life history and quantitative foliar traits compared to native species, or either dominant shrub species (i.e., relatively high foliar N concentrations and high leaf area per unit mass). The key finding of our exclusion experiment is that low-biomass non-native species ‘punch above their weight’, and do this through functional distinctiveness, has several implications. First, high-biomass, dominant species do not necessarily exert the largest impacts on community or ecosystem properties. Second, low-biomass, and sometimes inconspicuous species, can have important, persistent effects in ecosystems. Finally, non-native species can influence community composition and have important trophic consequences belowground without having obvious effects on soil nutrient status or resource availability to soil biota. These findings support the principle of seeking simplicity, i.e., by invoking biomass and traits as drivers of plant species’ effects, but to distrust it because simple drivers can have complicated outcomes or require more complex models for understanding species’ effects in ecosystems.