Abstract
Agricultural intensification has been shown to result in a decline in biodiversity across many taxa, but the changes in community structure and species interactions remain little understood. We have analysed and compared the structure of feeding interactions for cereal aphids and their primary and secondary parasitoids in organically and conventionally managed winter wheat fields using quantitative food web metrics (interaction evenness, generality, vulnerability, link density). Despite little variation in the richness of each trophic group, food web structures between the two farming systems differed remarkably. In contrast to common expectations, aphids and primary parasitoids were characterized by (1) a higher evenness of interaction frequencies (interaction evenness) in conventional fields, which cascaded to interactions at the next trophic level, with (2) a higher interaction evenness, (3) a higher ratio of primary parasitoid taxa per secondary parasitoid (generality) and (4) a higher link density. Aphid communities in the organically managed fields almost exclusively consisted of a single ear-colonizing species, Sitobion avenae, while highly fertilized conventional fields were mainly infested by leaf-colonizing aphids that benefit from the nutritional status of winter wheat. In conclusion, agricultural intensification appears to foster the complexity of aphid–parasitoid food webs, thereby not supporting the general expectation on the importance of organic farming practices for species richness and food web complexity.Electronic supplementary materialThe online version of this article (doi:10.1007/s00442-012-2387-8) contains supplementary material, which is available to authorized users.
Highlights
The structure of ecological communities is characterized by trophic interactions, by the existence of feeding links and the strength of interactions among species
The relative abundance of Aphidius increased from 55 % at flowering to 80 % at peak ripening in organically managed fields, but decreased from 84 % at the flowering period to 61 % at peak ripening in conventional fields
Our analyses of quantitative food webs revealed marked changes in the interaction structure of cereal aphids and their primary and secondary parasitoids that were related to farming practices
Summary
The structure of ecological communities is characterized by trophic interactions, by the existence of feeding links and the strength of interactions among species Such networks of feeding interactions may be of elemental importance for the functioning of ecological processes, such as biological control (van Veen et al 2006; Tylianakis et al 2010). The insurance hypothesis predicts that a high diversity of natural enemies ensures the functioning of biological control because the larger number of species provide a greater guarantee that some species will maintain functioning if others fail in situations of environmental fluctuations (Yachi and Loreau 1999) The mechanisms of such effects, Oecologia (2013) 171:249–259 may be determined by diversity loss per se, and by the identity of species that are becoming extinct (Cardinale et al 2006)
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