Portfolio effect and asynchrony as drivers of stability in plant-pollinator communities along a gradient of landscape heterogeneity

Understanding how pollination services can be maintained in increasingly anthropogenic landscapes is a current challenge for basic and applied ecology. The stability of plant-pollinator communities might increase in heterogeneous landscapes with a high diversity of species and alternative habitats, both through larger independent fluctuations of populations (portfolio effect) and increased species asynchrony. However, how the drivers of stability (portfolio effect and synchrony) vary along land-use gradients remains largely unknown. Using independent samplings for plants, pollinators and their interactions in Mediterranean communities along a gradient of landscape heterogeneity, we assessed the relationships between within-year stability and its drivers, and between the drivers of stability, landscape heterogeneity and species diversity. In addition, we evaluated the relationship between the stability of plant-pollinator interactions (temporal mean/SD of pairwise interactions) and the structure of mutualistic networks (modularity, nestedness, connectance). As expected, stability increased with larger portfolio effects and asynchronies. Interaction stability was positively related to pollinator stability, but not to plant stability. Landscape evenness increased the stability of plants, pollinators and their interactions, through increased portfolio effects. However, for plants and pollinators, the effect was detected at a smaller scale (1-km) than for interactions (2-km); and for pollinators and interactions, the effect was only evident from medium-to-high levels of landscape evenness. Temporal synchrony of species/pairwise interactions was an important driver of stability, tightly linked to species/interaction diversity. More asynchronous communities showed a larger portfolio effect and were also those with higher species evenness for all plants, pollinators and their interactions; while synchrony was also weakly positively related to species richness for plants. Interestingly, more modular mutualistic network structures conferred enhanced overall community stability, through higher portfolio effect and asynchrony. Preserving diverse communities within the heterogeneous Mediterranean landscapes may help maintain the stability of pollination services, both through effects on synchrony and the portfolio effect.