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 increased portfolio effect (property of communities
to fluctuate less than the sum of its counterparts) and decreased
synchrony (coincident changes in species abundance). However, how these
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. Besides, we
evaluated the relationships between the drivers of interaction stability
and the structure of mutualistic networks (modularity, nestedness,
connectance). 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
landscape 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 network
structures conferred enhanced overall community stability, through higher
portfolio effect and asynchrony. Preserving diverse communities within the
heterogeneous Mediterranean landscapes helps maintain the stability of
pollination services, both through increased asynchrony and portfolio
effect.