Impacts of Indian waterfern (Ceratopteris thalictroides (L.) Brongn.) infestation and removal on macroinvertebrate biodiversity and conservation in spring‐fed streams in the Australian arid zone

Removal of invasive macrophytes is a priority for river managers. However, the ecological effects of macrophyte removal on macroinvertebrate diversity are rarely examined but may be of particular significance in conservation reserves and when threatened species are present. This study investigated the macroinvertebrate fauna inhabiting invasive and native macrophytes in spring-fed channels in the Millstream-Chichester National Park, Australia. The effects of waterfern management (periodic hand-weeding) were examined by comparing assemblages at weeded and unweeded reaches on three occasions. Ceratopteris thalictroides harboured a diverse, insect-dominated macroinvertebrate assemblage, including the endangered damselfly Nososticta pilbara. Total taxon richness was similar between waterfern and native macrophytes, but macroinvertebrate assemblages differed in the dry season. Damselflies (including N. pilbara) were associated with waterfern-dominated reaches, whereas dragonfly nymphs were more common among native macrophytes. Weeding altered macroinvertebrate assemblage composition. Some weeded reaches developed assemblages indistinguishable from those in native-dominated reaches, but others did not. Weeded reaches often supported taxa that were rare or absent from waterfern-dominated reaches, including suspension feeders, found also in native-dominated reaches. Odonata are particularly diverse at Millstream, with 18 species recorded. Odonate species richness was significantly lower at weeded reaches than unweeded reaches. Nososticta pilbara and other short-range endemic species were absent from weeded reaches. As most odonates are univoltine, these adverse effects on local population size may affect species persistence. Invasive macrophyte species may support a high diversity of native invertebrates, including endangered and short-range endemic species. Furthermore, although hand-weeding appeared to enable a greater diversity of species to co-exist, the removal of a large biomass of macrophytes appeared to remove whole cohorts of insect populations from stream reaches, including endangered species. Removal of invasive macrophytes should not be implemented without understanding their effects on invertebrate assemblage composition and life-cycles.