Weeds, insects and food chains in organic cereal fields

Summary

Generally, it is expected to find food webs and food chains in all ecosystems, also highly disturbed ones such as agricultural fields. However, in a preliminary study in Danish conventionally managed fields it was not possible to identify food web nodes between weeds and herbivores, and it was hypothesized that the weed density in conventionally managed Danish agricultural fields were too low to support a more or less specialised herbivorous arthropod fauna.
This hypothesis was investigated through an analysis of data on weeds and arthropods from conventionally and organically managed fields in Denmark. Sampling of arthropods was done by aid of D-vac and the sampling of weeds and crop was done at exactly the same site within two days. Most weeds and some arthropod groups were identified to species level, but some of the arthropod groups were only identified to higher taxonomic levels.
The data were analysed by aid of the statistical package PRIMER and showed no correlations between the weed flora and the arthropod community structure in conventionally managed fields, but a correlation was in some cases found in organically managed fields. Correlations were especially strong in the data from the last year of the investigation, when sampling was confined to “islands” having relatively high weed densities. In the data sets showing correlations between weed flora and arthropod community structure it was analysed which species/groups were most responsible for the correlation, and the results of these analyses were combined with knowledge on the food preferences of the arthropods to identify some of the food web nodes (arthropod – weed dependencies on the species level) in the investigated fields.