Organic farming improves the spatiotemporal stability of pollinator species richness

Abstract

The spatiotemporal stability of the biodiversity of service providers such as insect pollinators is critical to guarantee high and stable levels of ecosystem services in agroecosystems. The proportion of semi-natural habitats (SNH) in the landscapes has been shown to stabilize the species richness of pollinators but the effect of farming intensity has not yet been studied. In this study, we compared the temporal and spatial stability of two groups of pollinating insects (butterflies and bumblebees) between nine conventional and ten organic farms, distributed along a gradient of landscape complexity. We surveyed pollinator communities and local flower cover during the growing season, along three years and in several habitat types per landscape (cereal fields, ley fields and semi-natural grasslands). At the plot scale we found that within-year stability of bumblebee species richness was higher in organic than in conventional ley fields, and that it was due to higher continuity of in-field flower resources. Our results also suggested that late-season flower resources in organic ley fields were critical to maintain a high within-year stability of bumblebee richness by reducing resource bottleneck during that period. The long term (among-years) stability of bumblebee richness was also higher in organic than in conventional cereal fields. The temporal stability of butterfly richness was not influenced by farming system. On the farm scale we also found that the spatial stability of butterfly and bumblebee species richness was higher in organic than in conventional farms, but that it was not explained by a greater spatial continuity of flower resources. Our study therefore suggest that organic farming can help to maintain the short-term stability of bumblebee species richness, due to practices that favor flower resources in ley fields (no mineral fertilizers, late mowing, etc.). Other practices, such as the non-use of insecticides, might be responsible for higher long-term stability of bumblebee richness in cereal fields, or landscape-scale spatial stability of both bumblebee and butterfly richness.