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.