The invasive tropical tanner grass decreases diversity of the native aquatic macrophyte community at two scales in a subtropical tidal river

Abstract

The tropical tanner grass, Urochloa arrecta, is one of the worst aquatic plant invaders in Brazilian freshwater ecosystems. This study aimed to compare beds of macrophytes dominated and without U. arrecta in the Guaraguaçu River, a subtropical tidal river in South Brazil, to analyze effects that this invasive grass has on the biodiversity of native macrophytes at two spatial scales. We compared macrophyte beds dominated by tanner grass to macrophyte beds without, considering species richness, composition, and beta diversity at two scales of variation: among-beds and within-beds. We expected that beds dominated by tanner grass would have lower biodiversity, thus promoting macrophyte biotic homogenization. Our results confirmed our hypotheses, highlighting the lower species richness in beds dominated by tanner grass. Species composition differed among beds. In general, nestedness among beds dominated by U. arrecta was more related to variation in species composition. Beds dominated by tanner grass had lower turnover and higher nestedness within beds. This study indicates a prominent sign of biotic homogenization promoted by U. arrecta, and highlights the degree of biotic homogenization among and within beds.

Community stability and seasonal biotic homogenisation emphasize the effect of the invasive tropical tanner grass on macrophytes from a highly dynamic neotropical tidal river

“…Our results corroborate many previous studies that reported a negative impact of U. arrecta on the diversity of plant communities (Thomaz and Cunha 2010 ; Fernandes et al 2013 ; Bao et al 2015 ; Sato et al 2021 ), and show that even the beds dominated by U. arrecta are subjected to some seasonal variation. Even so, it is important to mention that the main homogenising effect of U. arrecta was probably observed in the first years after its establishment in the river (see e.g.…”

“…This component of spatial structure makes it more complicated to disentangle which are the main factors promoting the higher dissimilarities among the sites without U. arrecta : if it is its absence or other factors (e.g., physical and chemical conditions) that we did not measure. Even so, a previous study comparing neighbouring macrophyte beds have also demonstrated that U. arrecta decreased biodiversity and promoted compositional changes at different spatial scales (Sato et al 2021 ). The analyses on our monitoring data reinforce this conclusion and add that U. arrecta effects extend to the entire river length.…”

“…This opposite direction of increase and decrease between sites where U. arrecta is dominant and where it is absent, probably explains why the significant differences in distance to centroids among these categories occurred in winter and not summer. This indicates that more than affecting spatial community structuring (Sato et al 2021 ) the invasive tanner grass can also change the temporal dynamics of macrophyte communities. We hypothesized two main factors, acting in conjunction, for the enhanced similarity among sites where U. arrecta dominates in winter.…”

“…This species was introduced in many countries as a robust and resilient pasture grass, a former common practice in coastal areas of South Brazil (Araújo et al 2021 ). In Brazil, the sad history of Urochloa species invasion is also associated to accidental introduction given its use as mattress for slaves in slave ships (Sato et al 2021 ). The high growth rate, wide physiological tolerance and rapid evolutionary responses of this species result in a very aggressive invader in aquatic macrophyte communities (Michelan et al 2010 ; Bora et al 2020 ).…”

Temporal stability in beta diversity does not guarantee surrogacy or compositional stability in a micro-phytoplankton metacommunity

“…Indeed, it has been reported in the Guaraguaçu River an overwhelming impact of an invasive ecological-engineer aquatic tanner grass -the Urochloa arrecta (Hack. ex T. Durand & Schinz) Morrone & Zuloaga in decreasing biological diversity (Araújo et al, 2021;Sato et al, 2021;Galvanese et al, 2022). Therefore, the temporal stability in phytoplankton metacommunity beta diversity only reinforces that spatial ecological filtering of species due to several factors prevail over temporal changes in the Guaraguaçu River.…”