Multi-Species Plant Invasions in Temperate Successional Grasslands Drive Changes in Community Functional Composition

Abstract

Questions: Biological invasions can alter community composition and ecosystem processes. However, most research has focused on the consequences of a few abundant invaders rather than the net effects of whole non-native communities. This limits our understanding of how increasingly widespread multiple non-native species invasions (e.g., higher non-native richness) affect community structure and functioning. Location: New Zealand. Methods: We compiled data from a nationwide vegetation inventory for deforested grassland communities that contained non-native species (n = 48). For each species present, we compiled leaf and root traits reflecting plant growth strategies and calculated community-weighted mean (CWM) traits. We determined how total non-native species cover is associated with community functional composition and assessed how the most abundant non-native species cover and non-native species richness influence total non-native cover. We also tested how non-native richness is associated with the most dominant non-native species cover. Finally, we determined whether the most abundant non-native species drive changes in community functional composition by comparing their traits to values for the native and remaining non-native species present. Results: Non-native species cover was positively correlated with traits related to rapid resource capture and fast nutrient return strategies (e.g., relatively high CWM specific leaf area, leaf nitrogen concentration, specific root length). Non-native cover was positively correlated with non-native richness, and both were positively correlated with the cover of the most abundant non-native species. Non-native species overall were more closely linked to traits associated with rapid resource use than native species, whereas the most abundant non-native species showed weaker relationships with these traits. Conclusions: Multi-species invasions are associated with greater invader abundance, significantly impacting the functional composition of invaded communities toward more open nutrient cycling, a pattern less pronounced with dominant invaders alone. Our findings underscore the importance of considering the collective effects of multiple non-native species, rather than solely focusing on dominant invaders, when predicting the ecosystem impacts of non-native plant invasions.