Moderation is best: Effects of grazing intensity on flowers, pollinators, and pollination networks in Mediterranean communities
Pollinating insects provide important ecosystem services and the structure of pollination networks is an important indicator of ecosystem stability and functioning. Livestock grazing is a frequent land use practice that directly affects the abundance and diversity of flowers and pollinators, and therefore, may indirectly affect the structure of pollination networks. Based on the Intermediate Disturbance Hypothesis (IDH), pollinator diversity is expected to peak at intermediate grazing intensities, but this hump-shaped relationship is rarely found. Here we studied how grazing intensity affected flower cover, abundance and richness of different pollinator guilds, and the structure of pollination networks along a wide range of grazing intensities using data from 11 Mediterranean plant–pollinator communities from Lesvos Island (Greece, north-eastern Aegean). Our general hypothesis was that intermediate grazing stress levels increase abundance and diversity of both flower cover and pollinators, as well as the stability of pollination networks as predicted by IDH. Grazing stress was estimated by measuring the biomass in grazed and ungrazed plots within each selected site; pollinators and plant–pollinator interactions were assessed by pantrap-setting and hand-netting during three monthly rounds during in spring.
Grazing intensity in the sites varied from 0.0 to 76.9 %. Between a total of 84 plant species and 239 pollinator species, we recorded 2705 pollination interactions. Flower abundance and richness showed hump-shaped relationships with grazing intensity, although the strength of the effect on flower abundance varied along the season. Overall, pollinator abundance and richness showed hump-shaped relationships with grazing intensity, but strength and direction of the effect varied along the season. In the middle of the season, bee abundance and richness were highest at intermediate grazing intensities, whereas late in the season grazing enhanced bee abundance. Pollination networks at intermediate grazing intensities were larger, more generalized, more compartmentalized, and contained more diverse and even interactions. At intermediate grazing, species in the network were also more functionally redundant in terms of ecological niches, which may confer higher stability to these networks. Our results reveal that the effects of grazing on pollinators vary with the intensity of the disturbance, generally supporting the IDH. Our results highlight the benefit of maintaining moderate levels of livestock grazing to preserve the complexity and biodiversity of the rich Mediterranean communities, which have a long history of grazing.