Euphorbiaceae responses to chronic anthropogenic disturbances in Caatinga vegetation: from species proliferation to biotic homogenization

Chronic anthropogenic disturbances (CAD) have posed tangible threats to biodiversity-relevant tropical biotas, but community- and ecosystem-level impacts still remain poorly understood. Here we address a 152 km² Caatinga landscape in northeast Brazil in order to investigate how Euphorbiaceae species and these seasonally dry tropical plant assemblages respond to a CAD gradient. Woody plant species were recorded across 26 0.06 ha spatially independent plots exposed to CAD. Euphorbiaceae species accounted for 78.9% of all plants and 21.5% of all species, with some species reaching up to 283 individuals per 0.06 ha or 92% of all recorded plants. Despite such contributions, Euphorbiaceae total and relative abundance, as well as total and relative richness, did not correlate with disturbance intensity at plot scale. At species level, some Euphorbiaceae species responded positively to disturbance, while others declined or did not exhibit a consistence response (i.e., positive, negative, and neutral responses). CAD intensity affected patterns of community similarity considering the whole plant assemblage as follow: first, plot-level disturbance correlated positively with NMDS scores; second, C. sonderianus experienced a 100% increment in terms of relative abundance along the disturbance gradient, and its plot-level abundance correlated positivity with NMDS scores; and finally, cross-plot similarity correlated positively with cross-plot geographical distance. Among others, our results suggest that CAD may favor some particular species (i.e., proliferating taxa), leading to community-level changes, including biotic homogenization as disturbance increases.