Factors limiting invertebrate recovery during stream restoration
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
Many stream restoration projects, as part of returning a degraded ecosystem to a healthier state, aim to restore aquatic invertebrate populations. Unfortunately, many attempts only „beautify‟ streams without achieving improvements in biodiversity. Lack of connectivity of a restoration site to a regional species pool may explain some failures. I tested this by collecting larval and adult aquatic insects from an agriculturally impacted Canterbury high country stream to evaluate connectivity of the regional species pool. The stream was surrounded by high-quality habitat in an adjacent National Park. Surrounding streams contained diverse assemblages of aquatic insects, but processes in the environment and limitations of in-stream habitat meant their adults did not always arrive at the target. In addition, oviposition habitat for hydrobiosid caddisflies was added to sections of stream and compared to un-manipulated control sections to test oviposition site limitation. The addition of oviposition habitat led to more hydrobiosid egg masses in comparison to control reaches. However, oviposition was also limited by in-stream habitat conditions, particularly the abundance of fine sediments. Sedimentation is a common pollutant in streams and is linked to decreases in habitat, food resources, and invertebrate populations. Moreover, common restoration methods, such as riparian management, have little success at reversing already high sediment levels, and are therefore insufficient to bring improvements to in-stream communities or sought-after habitat conditions. Therefore, after determining sediment was restricting sensitive invertebrate recovery at Riversdale Stream, by adding patches of high quality habitat I experimentally compared the factorial effects of sediment flushing and channel narrowing on sediment removal. Treatments improved habitat and prompted recovery of sensitive invertebrates, but an interactive effect where both flushing and channel narrowing combined created the most improved habitat conditions and the greatest improvements of invertebrate communities. Thus, while habitat improvements are an important part of restoration, features limiting species recovery such as connectivity and sedimentation, are particularly important.