1. Understanding how resource diversification affects ecological interactions, food web structure and ecosystem functioning is essential in both fundamental and applied ecology. While plant diversification strategies (either in-field or around-field) are often proposed in agricultural landscapes as practices to improve the biological control of herbivores by natural enemies, results remain variable and unsure.2. Here, we studied the effect of an in-field diversification practice (the intercropping of leguminous crops within cereal fields, an increasingly common practice but with inconsistent results on biological control) on cereal aphid control and the structure of a cereal aphid-parasitoid-hyperparasitoid food web for 2 years.3. We report that aphid control was not increased in mixed fields, nor was cereal parasitoid diversity and food web complexity. Nevertheless, the provision of alternative hosts in mixed fields led to a functional community composition shift, favouring generalist parasitoid species over specialist ones. 4. Moreover, we observed a higher hyperparasitism rate in mixed fields, suggesting that secondary parasitoids were favoured by alternative resources, which may have disrupted aphid control by primary parasitoids.
Synthesis and applications.This study demonstrates that parasitoid community composition shift and increased top-down control by the fourth trophic level can impact parasitoid efficiency to control herbivores. These results highlight the necessity to study fine-scale mechanisms within food webs to be able to set up efficient methods to support biodiversity and associated ecosystem services in agricultural landscapes.
Identifying which behavioural strategies maximize individual fitness is a key objective in ecology. Organisms are known to adapt their foraging behaviour to their environment in response to abiotic and biotic constraints, such as the distribution of resources or the presence of competitors. For instance, bees are known to avoid recently visited flowers and thus focus their foraging on more rewarding patches. Whether other flower‐visiting insects adapt their foraging behaviour in response to exploitative competition for floral resources remains unknown. Here, we asked if a predatory hoverfly Episyrphus balteatus and a parasitoid Aphidius colemani 1) are physiologically impacted by flower resource limitation following exploitation of flowers by a competitor (either the bumblebee Bombus terrestris or E. balteatus); 2) have the ability to discriminate flowers that were previously exploited by a competitor; and 3) modify their foraging behaviour accordingly. Episyrphus balteatus and A. colemani individuals foraging on previously exploited flowers were found to be less concentrated in sugar compounds, especially in fructose and glucose, suggesting that previously exploited flowers contained less available sugars. Nevertheless, individuals did not avoid previously exploited patches in the choice experiment. On the contrary, E. balteatus females preferentially landed on inflorescences that had previously been exploited by conspecifics (but not by B. terrestris), while A. colemani did not show preferences between inflorescences. However, female hoverflies spent more time feeding on unexploited patches, suggesting that exploited patches were resource limited. To our knowledge, this study provides the first evidence of the use of social cues among E. balteatus individuals in food foraging strategies. It also shows that even insects with tiny nectar requirements, such as parasitoids, can suffer from heavy exploitative competition. Such results may have applied consequences for the understanding of natural enemy conservation, in particular in agroecosystems where competition with honeybees may be important.
Interspecific interactions are major drivers shaping ecological communities. Nevertheless, understanding how and to what extent they affect ecosystem functioning remains a key challenge for both fundamental and applied ecology. In the context of agricultural biodiversity loss, sustaining populations of organisms that provide essential ecosystem services is crucial but remains challenging. Interactions among beneficial organisms may explain such results, as they impact organisms' fitness. These effects could accumulate when scaled up to the population level, modifying community structure and functioning. Interactions could also affect organisms' foraging behaviour and consequently their efficiency. Here, we review and synthesize the knowledge of the effect of interactions within and between two groups of ecosystem services providers: natural enemies and pollinators, respectively providing biological control of pests and crop pollination services. We show that intra-guild predation, competition and resource partitioning are common interactions structuring natural enemy communities, and may affect their effectiveness to control pests. Nevertheless, actual evidence in complex systems and long-term studies are still missing, and application in agricultural settings is only at its beginning. Similarly, competition and resource partitioning among pollinators is a recent focus of ecological research. Studying facilitative interactions is clearly missing. Finally, while we show that interactions between natural enemies and pollinators can theoretically happen in natura due to the use of similar resources and habitats, this question remains poorly addressed. We discuss future research perspectives to better capture the complexity of biodiversity effects on ecosystem functioning, and propose guidelines to the implementation of more efficient multi-service schemes in agroecological landscapes.
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