Patterns of pollination interactions at community-level are related to the type and quantity of floral resources

1. A fundamental question in community ecology is how the quantity of
floral resources affects pollinator activity and how this relates to the
structure and robustness of pollination networks. The issue has been
mainly addressed at species level, while at community level several
questions are still open. 2. Using a species-rich semi-natural grassland
as model ecosystem, we explored the relationship between community-level
quantity of floral resources, the number of flower visits at
community-level and descriptors of the structure and robustness of
pollination networks. To assess whether patterns of pollination
interactions were consistent when using different types of floral
resources, we measured three types of floral resources, namely the volume
of nectar, the number of pollen grains and the number of flowers. 3.
Community-level pollinator visitation rate showed a hump-shaped
relationship with the quantity of floral resources. Results were
consistent among models employing different measures of floral resources
as independent variables. The relationship between the quantity of floral
resources and the structure and robustness of the network was not
consistent among different types of floral resources; only intermediate
levels of nectar volume were associated with high levels of network
robustness. 4. The pattern we found is in contrast with some former
studies which reported positive linear relationships between the
community-level visitation rate of pollinators and the quantity of floral
resources. Rather, the observed hump-shaped pattern resembles the
relationship previously found between the quantity of floral resources of
a given plant species and its visitation rate per flower. Our results
highlighted that, by using specific types of floral resources (e.g., the
volume of nectar and the number of pollen grains) it is possible to reveal
ecological processes that would be masked by using the number of flowers
as the sole measure of floral resource.