Insect community composition and trophic guild structure in decaying logs from eastern Canadian pine-dominated forests

Abstract

Knowledge of the insect communities inhabiting different types of dead wood, and their responses to variation in its availability, is important in assessing the potential ecological impacts of changes in the quality and quantity of coarse woody debris (CWD) that may result from forest management activities. We collected all Diptera, Hymenoptera, and Coleoptera emerging from four decay classes of pine ( Pinus strobus, P. resinosa, P. banksiana) logs collected from 22 sites within Algonquin Park, Ontario, Canada, in order to analyze variation in community composition and trophic structure as a function of log decay class, and to determine whether the abundance of wood-inhabiting insects in the logs was correlated with the amount of downed CWD in the surrounding area. Multivariate analyses revealed that community composition varied continuously across the four decay classes, and that our system of decay classes explained decomposition-related variation in insect communities relatively well compared to individual characteristics of the logs. Of 27 families that were classified as “common” (present in >25% of the logs from at least one decay class), 19 were found to vary significantly in rank-abundance across decay classes, with nearly all of these favouring logs that were either in an early (class I) or late (classes III–IV) stage of decomposition. These associations also reflected functional differences in trophic guild structure: xylophages and predators tended to be most abundant in fresh logs, whereas saprophages, fungivores, predators, and parasitoids tended to be most abundant in more highly decayed logs. When the community was considered as a whole, insect abundance generally did not appear to correlate with the volume of downed CWD in the surrounding 20 or 79 ha area, either within or across decay classes. When trophic guilds were considered separately though, we found that the average rank-abundance of fungivores was correlated with the surrounding volume of downed CWD. We conclude that wood-inhabiting insect biodiversity, and its associated ecological functions, may best be maintained through forest management practices that do not reduce overall amounts of CWD, and that ensure it is available in both early and late stages of decomposition.