Investigating whether structural in-stream habitat additions are an effective tool for restoring benthic communities in lowland Canterbury waterways (2018)
AuthorsGreen, Timothy Charlesshow all
Land-use change is occurring at an unprecedented rate in New Zealand and globally. Freshwater ecosystems reflect the characteristics of their surrounding landscape, and consequentially, in-stream habitat quality is often poor. The loss of in-stream habitat is widely recognised amongst the greatest threats to benthic macroinvertebrates. However, how habitat should be most effectively restored remains a contentious topic, as landscapes are not akin, and thus the overarching effect that landscape scale processes have on the efficacy of local restoration projects is highly variable. My project aimed to identify to what extent the local restoration of structural habitats (i.e. boulders and large wood) can restore benthic macroinvertebrate communities in lowland Canterbury. I focussed on two life stages common to these fauna, benthic larvae and winged egg-laying adults. I conducted a survey of 11 forested South Island streams to determine whether benthic larvae discriminated between in-stream habitat types. This was followed by an experiment where I added cobble-boulders and large wood into three lowland Canterbury waterways with homogenous in-stream habitat to test whether these additions would improve the benthic macroinvertebrate fauna. Next, I conducted a survey of eight forested streams in Banks Peninsula, to determine whether winged adult insects preferred/avoided ovipositing on emergent cobble-boulder substrate based on microhabitat-scale physico-chemical/habitat cues. This was followed by a small-scale oviposition habitat addition experiment in a single urban waterway to test whether a more abundant and rich composition of adults would oviposit on complex surfaced substrate (i.e. rough vs. smooth surfaces). I found that microhabitat-scale benthic macroinvertebrate communities varied significantly between in-stream habitat types in forested streams. However, although cobble-boulder and wood additions improved invertebrate abundances in lowland waterways, they did not significantly improve taxonomic richness. I also identified significant generic and family level preferences for oviposition habitat by adults in forested streams. These taxa (mostly hydrobiosids and simuliids) preferentially oviposited on larger emergent substrates in areas with high dissolved oxygen availability yet avoided substrate with less surface area availability (i.e. from embeddedness or moss cover). My small-scale oviposition habitat addition experiment was limited, as the ovipositing adult insects at my experiment site consisted of a single family-level group (Hydrobiosidae). However, I found that hydrobiosids did not oviposit in significantly higher abundances on substrate with complex surfaces. From my results, I determined that structural in-stream habitat availability may provide refugia and oviposition habitat for some taxa, but this was not the factor most limiting the restoration of benthic macroinvertebrate communities in lowland Canterbury, and should therefore not be prioritised as a restoration tool. Instead, practitioners should prioritise wider-scale factors that limit insect dispersal, such as poor landscape connectivity associated with extensive deforestation, and constrained insect oviposition due to sedimentation.