Body-size-dependent effects of landscape-level resource energetics on pollinator abundance in woodland remnants

Land use change reduces floral resource availability, thereby driving declines in important pollinators. However, the severity of land use impact varies by species, influenced by factors such as dispersal ability and resource specialization, both of which can correlate with body size. Here we test whether floral resource availability in the surrounding landscape (the ‘matrix’) influences bee species’ abundance in isolated remnant woodlands, and whether this effect varies with body size. We sampled quantitative flower-visitation networks within woodland remnants and quantified floral energy resources (nectar and pollen calories) available to each bee species both within woodland and the matrix. Bee abundance in woodland increased with floral energy resources in the surrounding matrix, with strongest effects on larger-bodied species. Our findings suggest important but size-dependent effects of declining matrix floral resources on the persistence of bees in remnant woodlands, highlighting the need to incorporate landscape-level floral resources in conservation planning for pollinators in threatened natural habitats. 

Methods
Our study focused on bees (Hymenoptera: Apoidea: Anthophila) and their partner plant species. We recorded plant-pollinator interactions within remnants in a biodiversity hotspot in southwest Western Australia to determine the plant species visited by each bee species and assessed bee body size. We estimated floral resource energy (nectar and pollen calories) available to bees at the landscape-level (in remnant woodland patches and their surrounding mosaic landscape matrix) by directly measuring both nectar and pollen resources from flowers of partner plant species. Detailed descriptions of the methodology used in this study can be found in the article and in Appendix S1 in Supplementary Information.