Griffin, S. R. 2019. Restoring wild bees across fragmented landscapes. Dissertation, Michigan State University, East Lansing, MI.
This dissertation focuses on the effects of ecological restoration on wild bees across human-impacted landscapes, with the goal of informing restoration methods to best encourage rapid bee establishment in these restored habitats. Bees play an important functional role as plant pollinators but are declining globally due in large part to habitat loss and fragmentation. This has created widespread interest in restoring wild bees, especially across agricultural lands. Because bees depend on flowers for all food and nutrition, restoration efforts for bees generally focus on reintroducing flowers into otherwise resource-poor landscapes. However, restoration efforts rarely take other factors into account, including the effects of standard management practices, spatial aspects of restored habitats, distance to existing habitats, and landscape context. An understanding of how these factors structure bee communities and populations would allow the development of conservation methods that better target this important group.
I conducted three related studies to better understand how the design of restored habitats affects the establishment and persistence of bees across fragmented landscapes. My first chapter examines the effects of restoration management on bee abundance and richness, and disentangles the effects of management and landscape context from those of the local floral community. To do this, I monitored bee and flower communities for three years across an actively managed restored tallgrass prairie and used structural equation modeling to examine the relationships between management, landscape context, flowers, and bees. Contrary to expectations, I found that bee abundance and richness were driven by the presence of bison and the landscape context of the restoration, rather than local floral resources. I conclude that restoration management that only focuses on improving local floral resources may not adequately protect bee communities.
My second chapter tests the effects of a common conservation strategy, landscape corridors, on dispersal and colonization by a solitary bee Megachile rotundata. I released bees within experimentally fragmented landscapes that separate two spatial aspects of fragments altered by corridors, connectivity and edge-to-area ratio, allowing study of the relative importance of these factors on the colonization process. I found that the initial occupation of habitat patches was increased by the additive effects of both connectivity and higher edge-to-area ratio, but that fragment colonization was ultimately determined by higher edge-to-area ratio. I conclude that corridors and high-edge habitats can successfully increase bee colonization.
My third chapter tests the interacting effects of land-use and patch size on the process of bee dispersal and colonization across agricultural landscapes. I conducted another managed release of M. rotundata bees across two experimental landscapes that were both composed of patches of high- and low-flowering habitats, but that differed in patch size. I found that bee colonization was not affected by habitat type when patches were small, but bees showed a strong preference and higher reproductive success in high-flowering habitats when patches were large. I conclude that restoration efforts for bees across agricultural landscapes should emphasize increased landscape heterogeneity, smaller crop field sizes, and higher incorporation of high-quality flowering habitats within the agricultural matrix.
These three chapters show that a consideration of habitat configuration and landscape context in restoration is vital to the establishment and success of wild bees. Though floral resources are one necessary component for bees, a more holistic approach to restoration that also incorporates other factors can better protect this important group.
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