Local and landscape effects on the reproduction of wild bees and wasps in Mediterranean communities along a gradient of land-use

The transformation of natural landscapes for agricultural proposes may severely affect wild bee and wasp reproduction. In this study, we located trap-nests on 18 natural Mediterranean communities within agricultural areas to study the effects of landscape (% natural areas and heterogeneity) and local flowering communities (flower abundance and richness) on the total abundance and richness of solitary bees and wasps that nest within cavities. We also estimated several reproductive variables (number of nests, number of cells per nest, sex ratio, parasitoidism and survival) of the two most common species in the study system: the bee Osmia caerulescens and the wasp Ancistrocerus longispinosus. We found that total bee abundance increased with flower richness and landscape heterogeneity, while it decreased with flower abundance. Furthermore, our results indicated local and landscape effects on the focal species’ reproductive success, being overall stronger those at the local than at the landscape scale. Floral richness and abundance influenced the number of cells per nest. However, while the effect of flower abundance was positive in both cases, the effect of flower richness differed, being positive for O. caerulescens and negative for A. longispinosus. The percentage of natural habitats in the landscape had a positive additional effect on the number of cells per nest only in A. longispinosus. Besides, flower richness was positively related to the proportion of females in both species, and increased O. caerulescens survival. Our findings stress the importance of considering different spatial scales for an effective conservation management that takes into account Hymenoptera reproduction.

Methods
This study we investigated how landscape and local factors influence wild bee and wasp reproduction on the island of Mallorca, Spain.

Study Sites: We chose 18 natural scrubland areas surrounded by agricultural landscapes. These sites varied in the amount of surrounding natural habitat and overall landscape diversity.

Landscape Characteristics: To assess resource availability at a large scale, we analyzed a 1km radius around each study site, measuring the percentage of natural habitat and landscape diversity within that area.

Flower community: To assess resource availability at a local scale, we placed sampling squares within each study site and counted the number and types of flowering plants throughout the season.

Trap-nest sampling: To assess bee and wasp reproduction, we placed bundles of trap-nests at each site. These nests were collected and examined monthly throughout the breeding season. Then, we  counted the number and types of nests built, the number of cells within each nest, sex ratio of emerged adults, parasitoid abundance, and larval survival rate.

Focus species: While data was collected for all bee and wasp species, we focused our analysis on the two most abundant species: the solitary bee Osmia caerulescens and the wasp Ancistocerus longispinosus.