High pasture land cover reduces modularity of trophic networks from Brazilian stream fish assemblages

Humans have driven extensive land-use changes, posing serious threats to ecosystems. Despite recent advances, the impact of these activities on ecological interactions and food web metrics in freshwater ecosystems remains poorly understood at large spatial scales. We hypothesize that land use intensity simplifies and homogenizes trophic networks by increasing nestedness and decreasing modularity, connectance, trophic specialization, and the number of links, due to the loss of specialists and stream homogenization. We compiled studies on Brazilian stream fish trophic networks (1982–2021) and quantified land-use influence using the percentage of land cover in buffers around sampling regions to assess how anthropogenic impacts shape trophic structure. We quantified nestedness, modularity, trophic specialization, connectance, mean number of links per species, link density, and number of fish species nodes. The trophic networks were more nested than modular, but we observed no overall effect of land use on the metrics. However, when discriminating between the two primary land cover categories, pasture percentage reduced modularity, while cropland showed no effect. At a large scale, different land uses produce subtle changes in food web structure, but increasing pasture cover simplifies aquatic habitats, favoring generalists and reducing specialists.

Keywords:
Cropland cover; Fish diet; Food items; Nestedness; Pasture cover