Metacommunity structure of stream invertebrates is contingent on complex interplays between species dispersal ability, spatial extent and watershed environmental specificities. Previous studies showed that high diversity of Chironomidae (Insecta, Diptera) is observed in southern Brazilian montane streams, although the knowledge of the processes driving such high diversity is poorly comprehended. In this study, we evaluated the relative contribution of environmental (in-stream and landscape) and spatial drivers to the metacommunity structure of larval Chironomidae in a watershed (N = 40 stream reaches) running across a forest–grassland transition landscape in southern Brazil. Overall, 51 taxa were recorded in the study region. We found similar contributions of the environmental (in-stream and landscape) and spatial (broad- and fine-scale spatial drivers) fractions to the metacommunity structure of larval Chironomidae. Our results suggest that environmental processes likely associated with the elevation and stream longitudinal gradients predominated in the assembly of the metacommunity structure of larval Chironomidae. Additionally, mass effects (exchange of individuals via dispersal) possibly associated with the short-range drift or flight and the contact zone between the fauna from each biome also accounted for the observed pattern. Finally, dispersal limitation associated with the spatial extent and the topographic heterogeneity of the study region likely played a minor role in the assembly of Chironomidae metacommunity structure.

溪流无脊椎动物的元社区结构取决于物种扩散能力，空间范围和分水岭环境特异性之间的复杂相互作用。先前的研究表明，在巴西山地南部河流中观察到了猫科（Insecta，Diptera）的高度多样性，尽管对导致这种高度多样性的过程的知识知之甚少。在这项研究中，我们评估了环境（河流和景观）和空间驱动因素对流域幼虫Chironomidae的亚群落结构的相对贡献（N = 40条小溪）穿过巴西南部的森林-草原过渡景观。研究区域总共记录了51个分类单元。我们发现环境（溪流和景观）和空间（广域和精细尺度的空间驱动因素）部分对幼体Chironomidae的亚群落结构具有相似的贡献。我们的结果表明，可能与海拔和溪流纵向梯度有关的环境过程在幼体Chironomidae的亚群落结构的装配中占主导地位。另外，可能与短程漂移或飞行以及来自每个生物群落的动物区系之间的接触区有关的质量效应（通过分散交换个体）也解释了观察到的模式。最后，