Spatially patterned guild structure in larval parasitoids of cyclically outbreaking winter moth populations

Abstract

1. Wide temporal fluctuations in host abundance are a potential source of instability and stochasticity in the spatiotemporal population dynamics of associated parasitoid species. Within parasitoid guilds (i.e. parasitoids with similar modes of host utilisation), a conceivable outcome is guild organisation according to a lottery model, in which guild members attain local dominance by colonising previously emptied habitats during increasing host density, before other guild members. In the spatial dimension, an expected manifestation of such dynamics is variable guild structure even across homogeneous habitats.2. We examined the extent of large‐scale spatial patterning of guild characteristics in larval parasitoid wasps associated with cyclically outbreaking populations of the geometrid mothOperophtera brumatain northern Fennoscandia. The study was performed at the onset of the crash‐phase of the geometrid's outbreak cycle, along a 70‐km transect in costal northern Norway, characterised by largely homogeneous environmental conditions, except for a small climatic gradient.3. There was a distinct large‐scale spatial turnover in dominance among the major parasitoid groups (i.e. guild structure) inO.brumataalong the transect, whereas the total prevalence rate of the guild and its diversity showed no consistent variation. Guild structure was unrelated to host density.4. Although group‐specific responses to a slight spatial climatic gradient cannot be rejected as a causal mechanism, we conclude that our results are consistent with the expectation from large‐scale stochastic extinction‐recolonisation dynamics among functionally equivalent parasitoids relying on a host with strongly cyclic population fluctuations.