While the restoration of plant cover in vineyards is increasingly used as a solution to stabilise the soil, prevent erosion and facilitate machinery use, it can also benefit the community of arthropods by offering a wider diversity of micro-habitats. Here we showed that the abundance of arthropods generally increased in vineyards with low intensity of soil management that favoured the development of inter-row vegetation. Plant species richness in particular was found to be positively correlated to the abundance of most groups of arthropods. Moreover, a decrease in inter-row management intensity not only affected each ecosystem component separately (vegetation, soil and canopy fauna) but also interactions between them.
Differential responses of arthropods to inter-row management and vegetation
Although most of the studied groups showed a positive link to vegetation, three main arthropod responses to inter-row management can be identified. Some arthropods responded linearly to inter-row management intensity, while other showed either a steep decline in periodic mechanically manage vineyards or a steep increase in fully vegetated vineyards.
In our study, the most abundant group was the order of Coleoptera. Interestingly, beetles responded only in the grapevine canopy with higher abundance in fully vegetated vineyards only. In the literature, the response of beetles to vegetation and management is also ambiguous. While some studies revealed a positive effect of inter-row vegetation on beetle abundance 18, others did not find such an effect30. As beetles are highly diverse regarding their diet and ecology, we classified them into four groups. The increased abundance of beetles occurring in the canopy of fully vegetated vineyards was driven by the dominance of families that feed on plants such as Mordellidae (393 individuals) and Buprestidae (176 individuals). Canopy-dwelling predatory beetles, that hunt directly on plants and can use plant resources such as pollen and/or nectar as supplementary food, also benefited from increased plant cover. Carabidae and Staphylinidae, the most abundant families in our samples, are known to benefit from the vegetation on the ground31. These generalist predators are favoured by the diversification of the agroecosystem and the complexity of resources provided by the vegetation22. By their direct and indirect relationships with the vegetation, these species are therefore more likely to benefit from inter-row vegetation32.
On the opposite, detritivorous beetles showed a preference for vineyards with intensive soil management, as observed in previous studies26. Species of the Anthicidae family largely dominated this group. Several non-exclusive hypotheses may explain this response. First, such beetles can be locally abundant and may benefit from habitat disturbance, probably by feeding on vegetation litter that is provided by regular soil management in the vineyards. They may be also favoured by a lower level of competition and/or predation regularly observed in disturbed habitats33,34. Finally, this increase in Anthicidae abundance might also be the result of a higher probability of capture in pitfall traps. It is known that arthropod abundance data can be influenced by habitat structure, with trappability increasing as habitats become more open35. The reduction in habitat complexity in the absence of vegetation may facilitate their movements on the ground, whereas Anthicidae were commonly observed on plants in vineyards with ground cover (pers. obs.).
Hemipterans, largely dominated by leaf hoppers, responded positively to a decrease in soil management both on the ground and in the canopy. Since leaf hoppers are phytophagous, this result indicated the occurrence of a bottom-up effect with primary consumers favoured by an increase of primary producers36 as observed in previous studies25,30,37. The increase of phytophagous species in vegetated vineyards may be problematic if it includes pest insects. So far, neither in our vineyards nor in vineyards of other studies38–40 such an increase in insect pest infestation in response to increasing inter-row cover has been detected. Improved biological control of potential pests by an increase in the number of predators such as spiders favoured by vegetation cover may compensate for potentially positive effects on pest insects17,41.
We recorded a clear positive response of spider abundances to a decrease in soil management intensity and an increase in plant species richness. This latter relationship was also observed for harvestmen, whereas harvestmen remained largely unaffected by inter-row management as previously observed40. Similar to Anthicidae, harvestmen use to move over vegetation, which may reduce their trappability as habitats become more complex. Ground-dwelling predators may use rich inter-row vegetation as a source of prey42. This hypothesis is supported by the indirect relationships that we found between spiders and plant richness via an increase in both detritivorous beetles and phytophagous canopy beetle abundances, groups known to be prey for spiders43. Vegetation cover may also offer better microclimate (temperature and/or humidity) facilitating the development and/or reproduction of predators44.
Inter-row vegetation provides resources for pollinators, particularly for wild bee species. Several studies have already shown a positive impact of reduced soil management on pollinators45. Contrary to Kratschmer et al. (2019), the abundance of wild bee species did not respond to the increase in total plant cover but rather to the increase in flowering plants. A similar pattern has been recently observed between parasitoids and plant cover providing nectar46. The relationship confirms the importance of the plant species composition alongside with plant species richness and functional diversity to increase food availability for arthropods. In contrast, lacewings, that have the potential to improve pest regulation, did not show any response to inter-row management or vegetation variables, as already observed in previous publications47,48. Lacewings change their dietary habits during their life cycle, from generalist predators as larvae to pollinators as adults. Here, we only captured adults, which limits our understanding of the lacewing response to soil management and inter-row vegetation.
The multi-component effect of inter-row management
The path analysis clearly illustrated the key role of inter-row management intensity in vineyard ecosystems and its major effects on their functioning. Inter-row management and vegetation cover do not only have direct effects on ground-dwelling primary consumers such as detritivorous beetles and phytophagous hemipterans but also on ground-dwelling predators, such as spiders, and on almost all studied canopy-dwelling arthropods. Among the analysed vegetation parameters, plant species richness was by far the most important one, vegetation cover being secondary. Plant richness affected ground- and canopy-dwelling arthropods, from primary consumers to predators, while flower cover only influenced wild bee species, and plant cover only phytophagous beetles. This result suggests that plant species richness, more than vegetation cover, increases the diversity of resources for primary consumers and by makes the habitat more complex, thus offering more ecological niches for arthropods44.
This analysis also illustrated the complexity of these agro-ecosystems, with numerous links between different arthropod groups of the grapevine canopy and in the inter-row. Increases in both primary consumers on the ground and in the canopy influenced spider populations in the inter-row. Similarly, predatory beetles in the canopy responded positively to the increase in ground-dwelling hemipterans. These relationships confirm the movement of individuals from the grapevine canopy to the inter-row and vice versa, an important spill-over effect of inter-row vegetation on the functioning of grapevine agro-ecosystems.
Ecosystem restoration in Mediterranean vineyards
Currently, winegrowers are reluctant to maintain or plant vegetation between rows because of competition for water, a particularly important stress factor in water-limited regions such as the Mediterranean49,50. However, several studies have demonstrated that inter-row vegetation also provides important ecosystem services21,51. In a recent study, we showed that inter-row vegetation improves predation by arthropods16. Here, we further highlighted a strong and positive effect of inter-row vegetation on arthropod communities. This positive effect on the abundance of several groups was not limited to ground-dwelling arthropods but was also detected in the grapevine canopy and in trophic relationships between soil surface and canopy. In particular, plant species richness (spiders, harvestmen and canopy beetles) and flower cover (pollinators) had a positive effect on arthropods indicating that highly diverse, flower-rich mixtures are most beneficial to arthropods whereas grass-dominated inter-row vegetation had predominantly negative effects. In a context of growing interest in sustainable and environmentally sound crop management, we showed that the use of plant cover is a key tool to restore arthropod communities in vineyards.
Credit authorship contribution statement
CB: Conceptualization, Validation, Investigation, Formal analysis, Writing - original draft. CM: Conceptualization, Writing - review & editing. AB: Conceptualization, Validation, Writing - review & editing. AM: Conceptualization, Validation, Formal analysis, Writing - review & editing. PP: Conceptualization, Validation, Writing - review & editing. OB: Conceptualization, Validation, Investigation, Formal analysis, Writing - original draft, Writing - review & editing.