Cyclically outbreaking geometrid moths in sub-arctic mountain birch forest: the organization and impacts of their interactions with animal communities

by Ole Petter Vindstad

Institution: Universitetet i Tromsø
Year: 2015
Keywords: VDP::Matematikk og Naturvitenskap: 400::Zoologiske og botaniske fag: 480::Økologi: 488 ; VDP::Mathematics and natural science: 400::Zoology and botany: 480::Ecology: 488
Record ID: 1284792
Full text PDF: http://hdl.handle.net/10037/7004


In sub-arctic mountain birch forest in northern Fennoscandia, the 2 geometrid moth species Epirrita autumnata (autumnal moth) and Operophtera brumata (winter moth) show high-amplitude population cycles with regular 10-year periodicity. During some population peaks, moth populations attain outbreak densities and cause region-wide defoliation and mortality of mountain birch. The severity and duration of moth outbreaks presently appears to be increasing, owing to climate-driven range-expansions of both native and novel (see below) moth species in the system. The causal mechanisms of moth population cycles have been widely studied, with research focusing on the role of parasitoids during the last decade. This research has focused on total parasitism rates and has paid little attention to parasitoid community organization and its consequences for the functionality of parasitoid communities. Study I – III of this PhD project addressed this knowledge gap for larval parasitoids, which have received more attention than other parasitoid guilds in the research on parasitism the birch-moth system. Study I explored the possibility of stochastic extinction-recolonization dynamics – induced by fluctuations in moth host populations – as a driver of the spatial distribution patterns of different larval parasitoid species. The study documented large-scale spatial segregation in the prevalence of different parasitoid species in O. brumata, which may have resulted from stochastic extinction-recolonization processes. However, the alternative explanation that the observed patterns were caused by spatial gradients in habitat characteristics could not be rejected. Further, the study found that the magnitude of total larval parasitism rates at a given location was independent of which parasitoid species was locally dominant. Study II mapped out the phenology of attack of the larval parasitoid species of O. brumata. The study showed that the attacks of different parasitoid species followed each other in a successional manner throughout the larval season, so that all larval instars were attacked by at least 1 parasitoid species. The study argued that this phenological diversity within the larval parasitoid guild would reduce the probability of climate-induced phenological mismatches between larvae and many parasitoid species within a single season, hence buffering total larval parasitism rates against stochastic climatic variation. Study III compared larval parasitoid species richness and prevalence rates among E. autumnata, O. brumata and Agriopis aurantiaria (scarce umber moth). E. autumnata is native species to the mountain birch forest, while O. brumata and A. aurantiaria invaded this system by range-expansion approximately a century and 15 years ago, respectively. The study found that E. autumnata and O. brumata hosted similar numbers of larval parasitoid species in the mountain birch system, while the larval parasitoid guild of A. aurantiaria was strongly species-impoverished compared to the 2 other moth species. Based…