Climate change in south-western Australia (SWA) has caused streams to switch from perennial to seasonal flows. Flow regime change (FRC) is expected to profoundly impact streams, however empirical evidence is rare. Fortuitously, Stuart Bunn examined macroinvertebrate life histories and assemblages of SWA streams prior to FRC, providing a baseline for measuring change. My research aims to quantify the impacts of FRC on species distributions, life histories and functional organisation in formerly-perennial streams, through comparison with Bunn (1985).
Macroinvertebrates were sampled from 6 perennial and 8 seasonal reaches along 5 streams within the Wungong catchment, at 6-weekly intervals between October 2016 and December 2017, sampling the same streams as Bunn. Only one stream remains entirely perennial, allowing for comparison with present flow regimes. Here I present the results of sampling from October 2016 (peak flow) and May 2017 (peak dry). Invertebrate assemblages were compared between seasons, flow regimes, streams and site levels. Taxon composition (presence/absence) was compared with data from Bunn (1985).
Invertebrate assemblages differed between each level, owing to changes in abundance and occurrence of some Chironomidae and amphipods (Uroctena sp.). Some functionally important taxa (e.g. shredding caddisflies) are now rare or absent in formerly-perennial streams, as are many locally endemic dragonflies. Apparent arrivals of a few new species appear to be replacing some previously abundant (cf. Bunn) taxa. Drying pools were dominated by predaceous Coleoptera and Hemiptera, likely preventing survival to emergence of species whose life histories require surface water during the dry months.
FRC has altered invertebrate assemblages in formerly-perennial streams, which exhibit lower diversity and different species assemblages. As SWA is further along the trajectory of climatic drying than many other regions, this research identifies the risks to stream ecosystems in regions where climates are expected to dry in the future.