In-stream habitat unit additions: if you build it, will they stay?

Abstract

River restoration is often focussed on riparian planting, hoping water quality improvements will improve overall ecosystem health. While these interventions are important, successful community recovery requires additional steps to improve habitat. In-stream restoration can be challenging to implement, particularly where restoration efforts are undertaken by not-for-profit and community organisations with limited funds, expertise and equipment. We trialled the addition of simple, in-stream habitat units as an opportunity to investigate barriers to community recovery. Habitat units were designed to be simple to construct using sustainable, biodegradable, readily-available materials, and optimised to create heterogeneous habitat and refugia for macroinvertebrates. Community establishment within habitat units likely depends on the existing community within a reach, and colonisation. Stream invertebrates predominantly disperse via downstream drift, with limited upstream movement except for species with terrestrial life stages. Invertebrates arriving at a reach with undesirable conditions will continue to drift until a more suitable environment is found. Therefore, by comparing drifting invertebrates to those in existing habitat and those which established in added habitat units, we can identify mechanisms behind community recovery. Potential mechanisms include insufficient habitat quality and lack of available niche space, but also biotic drivers. Habitat units were installed in Glenariffe Stream on the Upper Rakaia River, which forms a system of channels across a high-country wetland system. The surrounding land has recently been retired from agriculture, and the area is targeted for restoration. Water quality is good, however in-stream habitat is homogenous with limited invertebrate refugia. Six weeks post-installation, invertebrate communities occupying habitat units were more diverse and included more sensitive species than communities from existing habitat, indicating a positive impact of habitat unit addition. Furthermore, drifting invertebrate communities were more similar to communities within the installed habitat units than in the existing habitat, suggesting habitat addition facilitated the establishment of additional taxa drift. This trial highlights the importance of considering in-stream restoration within wider river restoration projects, and that the addition of habitat units is a promising first step to aiding community recovery.