Directional spatial processes override non-directional ones in structuring communities of lotic macroinvertebrates differing in dispersal ability.

Dispersal is an essential natural process that influences community assembly, yet directional dispersal through wind and water may have distinctive effects. Environmental and spatial factors jointly influence community structure, but their relative importance is anticipated to vary with spatial distance, dispersal mode, and season. Accordingly, a systemic survey was conducted in subtropical Chinese mountain lotic systems to distinguish the relative contributions of environmental control and spatial structuring upon communities of macroinvertebrates with different dispersal ability. Macroinvertebrate samples were collected from the upper reaches and five tributaries of the Hanjiang River in October 2017 (autumn) and April 2018 (spring). These macroinvertebrates were identified and classified into three dispersal groups: aquatic passive (AqPa), terrestrial passive (TePa), and terrestrial active (TeAc). Variation partitioning analyses were performed on environmental factors and different sets of spatial factors (overland dispersal: Overland, directional downwind dispersal: AEM_Wind, along watercourse dispersal: Watercourse, and directional downstream dispersal: AEM_Water). Findings showed that both environmental filtering and spatial structuring influenced the structure of macroinvertebrate metacommunities. For AqPa and TePa groups, pure environmental effects were stronger than pure spatial effects based on most distance matrices; however, in AEM_Water, the effects of spatial processes surpassed those of environmental filtering. For TeAc group, the role of environmental control and spatial structuring varied depending on different spatial models. The results also highlighted seasonal shifts in metacommunity structuring processes. Spatial structures featuring direction, especially AEM_Water, were predominant in explaining the construction of macroinvertebrate communities. This work suggests that directional dispersal should be explicitly considered when examining the structure of ecological communities.