Multi-year larval life-history of Cordulegaster dorsalis and microhabitat use in intermittent streams of coastal British Columbia.

L.B. Miller and J.S. Richardson. Department of Forest Sciences, University of British Columbia, Vancouver, British Columbia, Canada

Cordulegaster dorsalis, is a common dragonfly in western North America, but little is known about its life history. We sampled C. dorsalis repeatedly over about a 1-year period in three small headwater streams on the southwest coast of British Columbia. This species appears to have at least two and more likely three overlapping cohorts of larvae, indicating three years to complete larval development. Many other species of Cordulegaster are semivoltine. Despite the long larval period, larvae of this species were regularly found in the nearly dried up pools of the intermittent reaches of small, headwater streams during late summer and early autumn. This was a surprising habitat for a larva that requires multiple years to develop. We found a weak positive association between pool size and larval size in these drying stream channels, suggesting some degree of assortative distribution. In addition, larval headwidth was positively correlated with average substrate sediment size. This suggests that larval size classes utilise different microhabitats within pools. The impacts of seasonal drying on these populations are unknown, we plan to use mark-recapture to determine how survival rate is affected by seasonal flow conditions.