Aquatic Plant Co-occurrence Patterns Associated with the Invasive African Signalgrass in a neotropical Heritage Site

Abstract

Urochloa arrecta, commonly known as African signal grass, is an aquatic plant native to Africa that has become highly invasive in Brazil. This species poses a particular threat in a neotropical coastal river of high conservation value. We conducted a co-occurrence analysis to assess patterns between U. arrecta and native macrophytes. We hypothesized positive associations with free-floating species due to potential anchoring effects, and negative associations with other life forms due to competitive interactions. For that, we used classic null-model co-occurrence models and repeated analyses for different sampling periods to evaluate consistency in patterns. Negative associations occurred only among erect emergent and creeping emergent species, possibly due to resource competition. Positive associations were more frequent, mainly with free-floating species, as the dense stands formed by U. arrecta may facilitate their persistence. However, some creeping emergent species in some periods were also positively associated, likely because the invader expanded the suitable habitat for these species beyond the margins. Results highlight the complexity of interactions and reinforce that either positive or negative associations can occur, depending on the context.