Effect of tree species and decay stage on saproxylic Diptera diversity in an eastern Canadian deciduous forest

We examined the effect of tree species and decay stage on the abundance, species richness and community composition of saproxylic Diptera associated with decaying logs in southern Quebec. Emergence traps were placed over Sugar Maple and American Beech logs in two decay stages (two and six years after tree death) and insects were collected from May to September 2004. Based on 200 species identified to date, species richness and diversity were highest in young maple and old beech, and did not differ significantly between young maple and old beech, or between old maple and young beech. There were significant effects of tree species and decay stage on species richness, but only decay stage had a significant effect on abundance. Community composition in young maple differed significantly from old maple and old beech. The number of specimens collected between replicates within young logs was significantly different and outliers in Correspondence Analysis were all young logs, suggesting that assemblages in young logs are more variable, but become similar in older logs. This may be dictated by ecological interactions through the decay cycle, or by random colonization of young logs. Most Diptera do not appear to differentiate between host species; decay stage appears to have a stronger influence. The quantity and age of coarse woody debris may be more important than tree species in maintaining saproxylic Diptera diversity.