Many species have life cycles where a life history stage (e.g. winged insects) undertakes dispersal. Recruitment effected by successful dispersers from source communities can have profound effects on the species diversity of receiving communities. Previously, we showed that a large number of caddisfly species are able to fly upstream to catchment boundaries and link rivers on either side (“boundary species”), whereas others do not (“non-boundary species”) (Lancaster et al. 2024). This raises a question: how do boundary species navigate successfully up to catchment boundaries and then fly down-slope to reach rivers?
We explored whether caddisflies use thermally driven slope winds to fly from catchment boundaries and locate streams in the valleys below. Down-slope ("katabatic") winds develop at sunset. As air near the ground cools, it becomes denser than surrounding air and consequently slides down valleys under the power of gravity. Katabatic winds can reach ~20 km/h on nights when skies are clear and prevailing winds are light. We sourced data on air temperature, maximum wind speed and direction, and daily global solar exposure (DGSE, which measures whether skies were clear) during trapping periods in our earlier study. We examined relations between numbers of adults trapped and each of these variables for (i) boundary and (ii) non-boundary species. We found that boundary species respond differently to temperature, wind direction and DGSE compared to non-boundary species. The nature of these differences suggest that dispersing individuals of boundary species ride katabatic winds from ridgelines down into river valleys after sunset.
These findings challenge current models. The Isolated Headwaters model proposed that species at the tops of catchments are isolated from those in areas downstream. Far from being isolated, our results suggest that headwater areas can act as conduits for dispersal between river systems.