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

Abstract

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.

Biology of the invasive species Eragrostis plana in Southern Brazil: What have we learned and how may this help us manage it?

“…Grazing has been practiced since the 17th century; as such, plant species have co‐evolved in response to grazing, exhibiting adaptations such as meristem protection and high regrowth capacity (Baggio et al, 2018). A community invaded by E. plana increases leaf hardness, which means less consumable forage for cattle (Guido et al, 2021). This causes overgrazing of native species, thereby promoting E. plana invasion.…”

Desempeño y habilidad competitiva de la gramínea invasora Eragrostis plana en condiciones de estrés hídrico

“…En particular, la invasión por Eragrostis plana Nees, una gramínea perenne-estival originaria del sur de África, está causando impactos significativos en los pastizales del sur de Brasil y Uruguay al afectar la diversidad de especies y la producción ganadera por su baja palatabilidad (Medeiros et al 2009;Favaretto et al 2015a;Dresseno et al 2018;Guido et al 2021). Según un estudio experimental en el sur brasileño, E. plana presenta mayor habilidad competitiva en comparación con gramíneas nativas de la región (Guido et al 2019).…”

“…Generalmente, E. plana posee características ventajosas para competir por recursos, como, por ejemplo, rápido crecimiento (Sheffer-Basso et al 2019), gran producción de semillas de rápida germinación (Favaretto et al 2011(Favaretto et al , 2015bBittencourt et al 2018;Guido et al 2020). Además, tiene alto contenido de materia seca foliar (Guido et al 2021) y un sistema radicular denso con espacios aireados en la pared cortical (Favaretto et al 2015a), lo que le podría brindar mayor tolerancia a eventos de déficit hídrico. Sin embargo, no existen evidencias empíricas que demuestren su tolerancia y superioridad competitiva en situaciones de estrés hídrico, como posible explicación de su invasividad en los pastizales del Río de la Plata.…”

“…El período de floración de estas tres especies se extiende desde octubre a abril. A nivel productivo, las especies nativas son consideradas de mediana a baja palatabilidad (Rosengurtt 1979), mientras que la especie exótica invasora es considerada una especie poco palatable, consumida principalmente en períodos de escasez forrajera (Medeiros and Focht 2007;Guido et al 2021).…”