Invertebrate drift and colonization processes in a tropical andean stream

We studied invertebrate drift (intensity, propensity, and diel changes) and the recolo -nization process of stones in a tropical high-altitude stream in Ecuador for a period of 7 to 25 d during the wet and dry seasons. Our aim was to identify whether seasonal differences in flow produce differences in drift parameters and in the recolonization process, and to examine the role of micro-habitat (fast-flow vs. slow-flow areas) in this process. Invertebrate community metrics in drift did not differ between seasons at base flow (<100 l s ^-1), but they were highly correlated with high flow events. As a result, drift propensity values for several taxa were higher during the wet season, when higher flows occurred, indicating that some taxa suffer catastrophic drift (e.g. Podonominae, Simuliidae). In the short-term experiment, taxa richness, density, and diversity of the colonizers increased with time (up to Day 7). On a longer scale (Days 7 to 25; mid-term experiment), time did not affect community metrics. Microhabitat had strong effects on the composition of colonized stones, and was related to the abundance of taxa in the drift. While some taxa that are scarce in drift were only found in slow-flow areas (e.g. Nectopsyche), suggesting that these species actively search for refuges in the stream, copious species in the drift (e.g. Baetodes) were very abundant on the recolonizing stones, especially during the wet season. The present study suggests that flow, the variability of flow (within and between seasons), and its consequences on the drift of several species play a pivotal role in invertebrate colonization and in the structuring of invertebrate communities in different microhabitats of high-altitude tropical streams.