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RESEARCH PRODUCT

The role of biotic interactions in a prey-predator system : the case of predation and regulation of weed seeds by carabids.

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For the transition towards agricultural production systems that are less dependent on herbicides, the use of weed regulation by seed-eating carabids is of great interest. Weed seed predation by carabids is variable and occurs within a complex context of many biotic interactions. This complexity partly explains our current inability to predict levels of in-field weed regulation. In this thesis, I analysed variations in the levels of predation and biological regulation of weeds and determined how they respond to two classes of biotic interactions: 1) the availability of alternative prey; and, 2) intra- and inter-specific interactions between the carabids themselves. Field measurements carried out on a pan-European field experiment showed a signal of weed seedbank regulation by carabids. The experiment also indicated that the responses of carabid communities to the landscape context and the intensity of farm management are partly due to changes in the availability of animal and plant prey. I demonstrated the existence of a trade-off between weed seed and animal predation by omnivorous carabids, whose effectiveness in suppressing seeds is reduced by the availability of alternative prey. Obligatory granivorous species showed no such trade-off with the presence of animal prey. The key carabid species and complementary species complexes that were associated with high levels of seed predation were identified in a multi-experiment field data-set from Burgundy. Using an experiment in semi-controlled conditions, mesocosms, antagonisms between certain carabid species were identified, which may be the result of direct or indirect interactions. Experiments done under controlled and simplified laboratory conditions did not show any major effects of these antagonist interactions on seed predation, however. This thesis confirms the regulatory role of carabid predation on the weed flora and shows that this system is set within a broader context of interactions that can explain some of the variation in the composition of carabid communities and their effectiveness in consuming seeds. This work makes it possible to suggest management to optimise weed regulation by carabids in arable fields.