The effects of clear-cut logging and variable retention on emergent Dipteran biodiversity in headwater streams in the Tsitika Valley on Vancouver Island

Headwater streams are ubiquitous in coastal British Columbia and comprise a large proportion of our watershed ecosystems. These small streams provide the tightest coupling between aquatic and terrestrial environments, yet they often receive the least protection under current forestry management guidelines. One management strategy that may minimize the impact of logging on headwater streams is variable retention, a silvicultural technique that involves leaving single, or patches of standing trees after harvest. Many aquatic true flies (Order: Diptera) utilize headwater streams during their larval stages, and then emerge as flying adults to disperse and reproduce. A primary objective of this study was to determine the short-term impact of clear-cut logging on aquatic Dipteran communities from headwater streams. The second objective was to determine the effectiveness of variable retention as a method to minimize the impact of logging on aquatic insects. Dipteran communities were described from two streams within Weyerhaeuser’s TFL39 experimental block, located in the Tsitika Valley, on Northwestern Vancouver Island. Emergent Diptera were sampled using emergence traps within uncut, clear-cut, and 20% patch retention treatments along two streams. Total abundance and taxa richness was similar for all three treatment sites. Uncut sites were found to have lower diversity and a different community structure than clear-cut and variable retention sites. Clear-cut and variable retention sites were found to have a larger abundance of predators than uncut sites. Also, uncut sites had a higher average abundance of fungus gnats than clear-cut sites, while the variable retention site did not differ from the clear-cut or uncut sites. This study demonstrated that clear-cut logging impacted aquatic Dipteran communities one and two years after harvest by altering the community structure, and increasing diversity and predator abundance.