Integral to many models of ecological communities is the notion that dispersal connects communities in disparate locations. Significant dispersal entails permanent movement of individuals from natal to reproductive locations, and on ecologically relevant scales. Robust tests of these models for aquatic insect communities are scarce. Some approaches are flawed, "snakes", whereas others show promise, "ladders". Snakes include inferring dispersal from spatial patterns of juveniles, using traits that lack empirical evidence, or ignoring critical life history traits. Ladders typically involve examining dispersal stages directly (winged adults), combined with elegant sampling designs that provide unambiguous evidence of dispersal and meaningful insights.
We applied a novel survey design to test whether caddisflies disperse between major catchments (rivers with separate outflows into the sea), where the shortest route is to fly up one stream valley to the catchment boundary, and then down a valley into the adjacent catchment. If caddisflies fly along the topographic features of stream valleys, then we expect to find adults on the ridge between catchments and, if dispersal is successful, high similarity between stream communities in adjacent catchments. In five pairs of streams that each share a catchment boundary, we trapped adult caddisflies at the streams margins ("residents") and up on the boundary ridge, where there was no running water. Thus, caddisflies on the boundary must have dispersed. From a pool of >130 species, approximately half occurred on the boundary. Similarity in species composition was significantly higher between resident assemblages within stream pairs than between streams in the same catchment. This suggests that movement of diverse caddisflies over catchment boundaries is extensive and potentially greater than between streams within the same catchment, and therefore, the branching structure of streams may not dominate metacommunity structure. Importantly, such trans-catchment dispersal could ensure community connectivity and facilitate recovery after catchment-wide extinctions.