High temporal consistency in quantitative food web structure in the face of extreme species turnover

Food webs encapsulate the functional makeup of natural communities by describing both the number and identity of species present, and how they interact with each other. Yet, how different aspects of food web organisation vary in time is still poorly known. In this study, we used multiple complementary descriptors of food web composition and structure to examine the temporal consistency of 22 local food webs as followed over two consecutive years. The focal food webs consisted of specialist herbivore hosts (gallers and miners) and their natural enemies (parasitic and inquiline wasps) on individual pedunculate oaks, Quercus robur. In the changes detected among years, we searched for effects of species- specific ecological traits (abundance, trophic rank and feeding guild) on the persistence of species within local communities. Local food webs showed high consistency between years in terms of species richness and quantitative structure. Nonetheless, the species composition and the qualitative link structure of the webs changed dramatically over time. While different aspects of food web structure revealed different levels of consistency in time, species turnover did leave a detectable imprint on quantitative and qualitative descriptors of food web structure. Overall, a mean 44% of all species present in a community in 2006 were no longer detected in 2007, whereas 31% of the species encountered in 2007 were new to the site. Species turnover was unevenly distributed among feeding guilds, and increased with a decreasing abundance of the species. The patterns uncovered suggest that quantitative, emergent characteristics of food web structure will exhibit less temporal variation than patterns at the level of individual food web parts. However, as these features reflect different, complementary aspects of how food webs are built and how communities are likely to function, patterns at both levels should be considered in assessing the temporal stability of natural communities.