Exploring multitrophic interactions in oilseed rape fields reveals the prevailing role of Carabidae

In cropped fields, birds are often at the highest position in the food
chain, feeding on pest arthropods and their intermediate predators in a
process known as intraguild predation. The net effects of bird predation
on phytophagous insect populations (feeding on plants) are difficult to
predict without comprehensively describing prey-predator communities and
their complex interplay. We sampled bird and arthropod communities in 30
oilseed rape fields in the spring of 2019 and 2020 in France. To assess
the top-down control of arthropods by birds, we used a vertebrate
exclusion experiment. Using a taxonomic and functional trait-based
approach, we determined the direct and indirect influences of birds on
arthropod predators and phytophagous insect populations in arable crops.
We observed a negative relationship between the abundance of Carabidae and
phytophagous insects but not with the other predator group suggesting the
key role of Carabidae on phytophagous insects in agroecosystem. We found
no statistical evidence of intraguild predation from birds towards
intermediate predators. Despite the lack of overall effect of predator
functional diversity on their prey, we highlighted the negative
relationship between the functional complementarity (through functional
evenness) of Carabidae and the abundance of phytophagous insects. This
result suggests that functional complementarity between Carabidae species
could help to reduce phytophagous insect populations. We analysed the
effect of agricultural practices on these multitrophic interactions,
showing that pesticide intensity only had detrimental effects on Carabidae
abundance, while the frequency of tillage did not affect the studied
communities. Complementary indices used to depict communities are helpful
to better understand the mechanisms underlying trophic relationships.