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Effects of emergent structure on the abundance and size distribution of Trichoptera in Bay of Plenty hill country streams

Restoration of streams is not limited to recreating structural habitat complexity, but also requires recolonisation of biota that had been excluded prior to restoration. Aquatic insects are a key component of a streams ecosystem, but often do not recolonise restored sites at expected rates. It is hypothesised that this is due to a lack of suitable habitat for the oviposition of certain groups, particularly the Trichopteran family Hydrobiosidae which is known to use emergent stones to access the stream bed for oviposition. I investigated associations between Trichoptera groups with different oviposition strategies and emergent structure in the form of boulders in six Bay of Plenty stream sites. I compared abundances and size class distribution of Hydrobiosidae, Conoesucidae and Hydropsychidae at sites with and without emergent boulders. Results from that study suggest that the presence of oviposition structure was not linked to either the abundance or size distribution of Hydrobiosidae within the sites we examined where other factors such as degree of shade, water velocity and substrate size were key determinant of larval abundance. This first suggested that Hydrobiosidae may arrive in sites lacking emergent structure via alternative means. Two potential pathways for the colonisation of Hydrobiosidae in reaches without emergent structure were subsequently identified and investigated: (i) access via the banks or emergent vegetation for oviposition; and/or (ii) Hydrobiosidae drift from upstream areas with suitable oviposition habitat. A study on longitudinal patterns of Hydrobiosidae drift showed inconclusive results, with no significant changes in drift densities or larval size with distance downstream of emergent structure. A study on the lateral distribution of Hydrobiosidae in a reach lacking emergent structure provided evidence that adults were utilising stream banks to access submerged oviposition habitat with higher numbers of smaller Hydrobiosidae found near the stream edges compared to within the channel. Findings from my study suggest that there should not be any constraints on the recolonisation of Hydrobiosidae within a restored reach as long as: (i) there is suitable oviposition habitat <2km upstream of a restored site; (ii) there is a source population of adults capable of reaching oviposition habitat within the targeted stream; (iii) the water quality is suitable for colonisation; and (iv) instream conditions such as shade, substrate size and water velocities are within the preferred ranges.
Type of thesis
Cooper, J. A. (2018). Effects of emergent structure on the abundance and size distribution of Trichoptera in Bay of Plenty hill country streams (Thesis, Master of Science (Research) (MSc(Research))). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/11942
The University of Waikato
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