Effects of grazing intensity, habitat area and connectivity on snail-shell nesting bees

Abstract

The effects of habitat loss and fragmentation on wild bee populations are still not fully understood. Availability of resources, parasitism and habitat connectivity might influence population sizes of bees. Whereas floral resources for wild bees have been considered in research and conservation, nesting resources have been largely neglected as these are challenging to investigate. Snail shells are the exclusive nesting cavities for several Osmia species and provide a good - but so far unused - tool to study factors driving bee population dynamics. We investigated the effects of habitat area, connectivity and management of semi-natural grasslands on populations of snail-shell nesting bees. On 23 calcareous grasslands, we monitored snail shell colonization by providing empty snail shells as nesting resources, and recorded flower-visitor interactions.Five snail-shell nesting Osmia species were found on the grasslands, which made almost a quarter of recorded bee flower visits. Three species colonized the offered snail shells with high variation between study sites, Osmia species, and snail shell size. Habitat area had a positive effect on the population size of the habitat specialist Osmia aurulenta, whereas the more generalist species Osmia bicolor was positively influenced by habitat connectivity. Destruction rates of snail shells increased with sheep grazing intensity, leading to an estimated loss of more than a third of all bee nests. We conclude that large and connected habitats benefit bee populations in fragmented landscapes, while conservation management regimes should take into account potential negative effects of grazing on specific nesting resources of specialized bee species.