COMMUNITIES, POPULATIONS, AND POLLINATION: MEASURING WILD BEES AND THEIR INTERACTIONS ACROSS ECOLOGICAL SCALES

Wild bees have rocketed to the forefront of ecological research in recent years thanks to their enormous diversity, their outsized role as pollinators across agricultural systems and most terrestrial environments, and their potentially declining abundance. Even so, an overwhelming focus on studying and monitoring bees at the community level and in the context of agriculture has left unanswered questions about most bees’ basic population biology and their function as pollinators. These gaps hamper bee conservation efforts and leave us largely without baseline knowledge of how populations operate, making it difficult or impossible to evaluate which species might be in decline, what mechanisms are responsible for such declines, or how changes in bee abundance may be of consequence for plant populations. In this dissertation, I hope to help fill some of these research gaps. In Chapter 1, I examined the relative sampling biases of three frequently-used sampling methods in bee community studies—hand netting, pan traps, and blue vane traps—using a combination of preexisting bee community datasets collected by the Winfree Lab. Through a combination of rarefaction, multivariate statistics, and linear modeling, I compared the number and identity of species captured by these methods and quantified how much they differed in their relative representation of common bee genera. I found that all three methods differed not only in the types of bee species they collected but also in the number of species, as hand netting captured greater species richness than pan or vane traps. Additionally, pan traps had significant biases—both positive and negative—with respect to most bee genera when compared against hand netting and vane traps. A key takeaway from this study was that accounting for differences in the absolute number of individuals collected by different sampling methods is essential for measuring taxonomic biases, i.e. a given sampling method might collect high raw abundances of a variety of genera even if it collects some of those genera in lower relative abundances than another method. In Chapter 2, I studied the basic population ecology and movement of Andrena parnassiae, a rare mining bee species that specializes in collecting pollen from the wildflower Parnassia glauca in highly fragmented wetlands called calcareous fens. I conducted a two-year mark-release-recapture study of this species across a series of fens in northwestern New Jersey where its host plant grows. Using this dataset of individually marked bees, I then examined how bees moved among patches of fen habitat, tested how the number of bees foraging in each fen related to abundance of flowers there, and used mark-recapture models to estimate the abundance of bees across all fens in each of the two populations I studied. I found that populations of this bee were quite small, consisting of fewer than 200 individuals each, and that the number of female bees foraging in each patch was positively related to the number of flowers there, suggesting that floral availability might limit local bee abundance. Bees were highly faithful to the individual fens they foraged in; even though distances between fens were mostly less than a few hundred meters, individuals rarely moved between them but were often resighted in the same fen where they were originally found. Taken in the context of the few other studies of solitary bee populations, my findings support the idea that populations of highly specialized bees like A. parnassiae are smaller than those of some more widespread species and that individuals are not very mobile, such that populations potentially face a heightened risk of local extinction. In Chapter 3, I continued working in the same system I worked in for Chapter 2 to examine the role of pollination as a driver of reproductive variation in Parnassia glauca, the sole host plant of Andrena parnassiae. I conducted an experiment to monitor the flowering phenology and seed production of plants that I provided with supplemental pollination and unmanipulated plants flowering throughout the period when this plant flowered in a single year. In addition to tracking plants’ reproduction, I also examined how biotic conditions relating to pollination changed over this time by measuring the availability of male (pollen-producing) flowers and pollinators. Despite great changes in access to pollen and pollinators over the course of the flowering period, plants’ seed production did not appear to be limited by pollination at any time. Instead, I found signs that individual plants’ reproduction may be constrained by abiotic resources: smaller plants and flowers that opened later than others on the same plant were less likely to produce seeds. Additionally, the role of pollination as a possible driver of variation in reproductive output among individuals was eclipsed by variation in the abundance of a seed-eating moth, which consumed seeds in nearly half of the flowers that opened in the earlier half of the flowering period. Altogether, my thesis work explores multiple different facets of monitoring and measuring bee populations and their significance to the plants they pollinate. By applying this mix of traditional population biology and pollination ecology, informed by natural history, we might continue to expand the horizons of bee ecology to better understand when and how to enact measures to conserve them.