Cichorioideae abundance and local land use drive Osmia leaiana pollen use in semi-natural grasslands

Abstract

Land-use change is a major driver of bee decline, with solitary and specialist bees being particularly vulnerable due to their nesting and foraging behavior. Understanding how local land use and management practices influence solitary bee resource use is therefore critical for informing conservation in fragmented ecosystems. In this study, we investigated brood cell number and pollen resource use of Osmia leaiana, a solitary bee specializing on Asteraceae, across a network of Estonian calcareous alvar grasslands. We hypothesized that due to the short foraging range of O. leaiana and its dependence on Asteraceae, local management variables such as grazing intensity, shrub cover, and tree cover, affect the taxonomic composition of nest-tube pollen provisions through their effects on local Asteraceae community composition. In addition, we predicted that higher Asteraceae abundance corresponds to greater brood cell production by increasing the availability of preferred resources. Nest blocks were deployed to sites that differed in the composition of the surrounding landscape, management, and plant community composition. Pollen metabarcoding of nest contents was used to assess floral resource use. While Asteraceae abundance did not affect brood cell number, higher proportions of cropland and forest had significant negative effects. Of all assessed variables, proportion of semi-natural grassland within 500 m, tree cover, and the abundance of the subfamily Cichorioideae within grassland patches significantly influenced the composition of pollen provisions. These results indicate that O. leaiana prefers a narrow subset of Asteraceae and primarily forages on plants locally available within semi-natural grasslands, while brood cell production decreased with increasing amounts of alternative land-use types in the surrounding landscape.