In the face of global pollinator decline, extensively-managed grasslands play an important role in supporting stable pollinator communities. However, different types of extensive management may promote particular plant species and thus particular functional traits. As the functional traits of flowering plant species (e.g. flower size and shape) in a habitat determine the identity and frequency of pollinator visitors, they can also influence the structures of plant-pollinator interaction networks. The aim of this study was to examine how the type of low-intensity traditional management influences plant and pollinator composition, the structure of plant-pollinator interactions, and their mediation by floral and insect functional traits. Specifically, we compared mown wooded meadows to grazed alvar pastures in western Estonia. We found that both management types fostered equal diversity of plants and pollinators, and overlapping, though still distinct, plant and pollinator compositions. Wooded meadows had significantly higher connectance and specialisation, while alvar pastures achieved higher Shannon diversity at a standardised sampling of interactions. Pollinators with small body sizes and short proboscis lengths were more specialised in their preference for particular plant species and the specialisation of individual pollinators was higher in alvar pastures than in wooded meadows. All in all, the two management types promoted diverse plant and pollinator communities, which enabled the development of equally even and nested pollination networks. The same generalist plants and pollinators were important for the pollination networks of both wooded meadows and alvar pastures; however, they were complemented by management-specific species, which accounted for differences in network structure. Therefore, the implementation of both management types in the same landscape helps to maintain high species and interaction diversity.

We sampled plants, pollinators, and plant-pollinator interactions on three wooded meadows and three alvar pastures in early July 2018. Ten 30 x 2 m transects with a minimum distance of 20 m from each other were representatively positioned at each site. At each transect, a botanist identified all flowering plants to species and estimated the cover of flowering individuals over the total transect area. Each of the ten transects per site were subsequently surveyed for flower-visiting insects for 15 minutes, not including processing time for caught insects, for a total of 150 minutes of observation per site and 450 minutes of observation per management type. During this period, two surveyors slowly walked back and forth along each transect searching for insects (Lepidoptera, Hymenoptera, or Diptera) that were interacting with the reproductive parts of flowers and noted down the interaction. Most Lepidoptera and bumblebees were visually identified in the field, while all other insects were collected and pinned. Among insects, 87.6% of individuals were identified to species level and 88.9% were identified to genus level by an entomological specialist. The remainder, mainly non-syrphid Diptera, were identified to family and morphospecies level.

We selected a suite of insect functional traits known to influence plant-pollinator interactions from the relevant taxonomic literature.  The two insect traits selected, body length and proboscis length, were related to foraging range, flower choice, and efficiency in acquiring floral resources. Insect traits were grouped into qualitative classes (Proboscis: Short: 0-3.99 mm, Medium: 4-7.99 mm, Long >8 mm; Body length: Small: 0-4.99mm, Medium: 5-9.99mm, Large >10mm).

To test for significance differences between the roles of plant and pollinator species in the plant-pollinator network, we used the Kruskal-Wallis test with the sampled species-level network index (species strength, partner diversity, and specialisation d') as the response variable by species as the explanatory variable. Further, we used pairwise Wilcoxon rank sum tests with the Benjamini-Hochberg correction for multiple testing for pair-wise p-values of differences between species. In this case, the null hypothesis states that there are no significant differences between species indices within a network, or that any observed difference is due to chance alone. We assumed a significance threshold of 5%.

Plant_pollinator_interactions_Estonia2018: The results of the plant-pollinator interaction survey at each transect and site.

Plant_survey_Estonia2018: The results of the plant survey at each transect and site (flowering plants only).

Appendix B2: Functional traits of all insect species, including species name, family name, proboscis length (mm), proboscis size class (short, medium, long), proboscis literature reference, body length (mm), body length size class (small, medium, large), and body length literature reference.

Appendix F1: (A) Results of Kruskal-Wallis tests testing for significant differences between species indices (species strength, partner diversity, specialisation d’) of the species within the management type and (B) results of Wilcoxon rank sum tests with the Benjamini-Hochberg correction for multiple testing for pair-wise p-values of differences between species.