Disentangling by additive partitioning the effects of invasive species on the functional structure of communities

QUESTIONS: Eragrostis plana is the most invasive species in grasslands of southern Brazil. We ask how it affects the functional structure of communities considering leaf traits related to forage palatability for grazers. As consensus is lacking about whether to include or omit the invasive species in the analysis, we propose a method for partitioning functional diversity (FD) and community‐weighted means (CWMs) of invaded communities into invasive and resident species as additive components. LOCATION: Southern Brazil. METHODS: Permanent plots were located on a natural grassland that was partially invaded by E. plana. Invader removal was performed by four different treatments (n = 10): (a) no removal, (b) clipping, (c) herbicide, and (d) hand‐pulling. Additionally, we located plots (n = 10) (e) in an adjacent non‐invaded patch. With data from annual vegetation surveys and leaf traits (mass per area, LMA; dry matter content, LDMC; and force to tear [toughness], Ft), we assessed temporal changes (2012–2016) in FD and CWMs, partitioned into resident and invasive components. Partitioning was based on 0 or 1 weights attributed to invasive and resident species. Temporal trajectories of communities by fuzzy‐weighted species composition were also evaluated. RESULTS: Invasion modified the resident community in terms of CWMs of LDMC and Ft and fuzzy‐weighted composition, but FD was not affected. Resident species of invaded plots contributed less to CWM values of LDMC and Ft than non‐invaded communities. However, E. plana´s higher values of Ft increased the total community toughness. Removals of the invasive were not enough to make the functional structure of resident communities similar to that of non‐invaded ones. CONCLUSIONS: Species loss associated to invasion is not random, as E. plana displaces species with higher contribution to CWM of LDMC and Ft. The contribution of invasives to the total community increases toughness, decreasing the forage palatability of invaded grasslands. Our partition method helps to disentangle the additive components of such effects on resident and invasive species.