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.

中文翻译：

---

放牧强度、栖息地面积和连通性对蜗壳蜂巢蜂的影响

摘要 栖息地丧失和破碎化对野生蜜蜂种群的影响仍未完全了解。资源的可用性、寄生性和栖息地连通性可能会影响蜜蜂的种群规模。尽管在研究和保护中考虑了野生蜜蜂的花卉资源，但筑巢资源在很大程度上被忽视了，因为这些资源很难调查。蜗牛壳是几种 Osmia 物种的专属巢穴，提供了一种很好的——但迄今为止尚未使用的——工具来研究驱动蜜蜂种群动态的因素。我们调查了半天然草原的栖息地面积、连通性和管理对蜗牛壳筑巢蜜蜂种群的影响。在23个钙质草原上，我们通过提供空的蜗牛壳作为筑巢资源来监测蜗牛壳的定植，并记录花客互动。在草原上发现了五种蜗牛壳筑巢的 Osmia 物种，几乎有四分之一的蜜蜂花访问记录。三个物种在提供的蜗牛壳上定居，​​研究地点、Osmia 物种和蜗牛壳大小之间存在很大差异。栖息地面积对栖息地专家 Osmia aurulenta 的种群规模有积极影响，而更通才的物种 Osmia bicolor 受到栖息地连通性的积极影响。蜗牛壳的破坏率随着绵羊放牧强度的增加而增加，估计导致超过三分之一的蜂巢损失。我们得出的结论是，大型且相互连接的栖息地有利于分散景观中的蜜蜂种群，