Bees are a diverse group of insects responsible for pollinating plants in agricultural and ecological landscapes. Wild bees are impacted by anthropogenic activity and associated habitat loss. Although 75% of bee species nest underground, this nesting strategy has been overlooked. In my thesis, I explored the nesting habitat requirements of ground-nesting bees by studying the relation between bees and edaphic factors. First, I reviewed existing literature about ground-nesting bee nesting characteristics (Chapter 2). I found that certain abiotic factors (e.g., soil texture, temperature, moisture, compaction, slope, and soil surface features), as well as biotic factors (e.g., floral resources, conspecifics, predators), could be key in nest-site selection. I also discuss possible explanations for the choice of these nesting characteristics, particularly edaphic factors. Secondly, in Chapter 3, I assessed relationships between specific soil factors (soil texture, compaction, slope and ground cover) and the wild local ground-nesting bee communities on 35 farms around Ottawa throughout 2018 and 2019. I found that higher percentages of sand and bare ground were linked to increased total ground-nesting bee abundance and species richness, whilst slope and diversity (Simpson’s index) were negatively correlated. Furthermore, associations with soil factors were found to be species-specific, especially for associations with sand content, which were either positive, negative, or non-significant. Ground-nesting bee community’s composition was influenced by sand content, slope, soil compaction and bare ground in sampled agroecosystems. Finally, in Chapter 4, I experimentally tested nesting preferences of ground-nesting bee species for soil texture. Based on one season of sampling, there was no association between bee abundance or species richness and soil texture. Together, my research sheds light on the influence of soil characteristics on species-specific nesting-habitat associations for ground-nesting bees. Along with floral resources which have received far more study, nesting habitat is vital for the well-being and persistence of bee populations, and therefore for their conservation.

Neonicotinoids are broad spectrum insecticides that are widely used to control many insect pests. In Ontario they are applied directly to the seeds of most corn and soy crops, after which they are incorporated into the tissues of the developing plant. While researchers have investigated how these insecticides impact honeybee health, much less attention has been given to the effects of neonicotinoids on wild pollinators. Ground nesting bees face exposure to neonicotinoids both in the soil, where they nest, as well as through other exposure routes (e.g, nectar and pollen of treated plants). I studied whether a higher concentration of neonicotinoid pesticides in soils within and near corn and soy crops is negatively associated with the abundance and diversity of ground nesting bees. To determine whether an association exists between soil neonicotinoid concentration and the abundance of associated ground nesting bee communities, I surveyed 16 eastern Ontario (mainly corn and soy) farms once per month from May to August, 2019, sampling the bee communities, soil pesticide levels, and floral resources. I found a significant interaction between the sampling period and neonicotinoid soil concentrations on ground nesting bee abundance. However, no similar relationship existed for an analysis with non ground nesting bees only. Specifically, I found that high concentrations of soil neonicotinoids were associated with lower expected bee abundances and low seasonal variation, a low concentration of soil neonicotinoids was associated with a high degree of seasonal variation, including spikes of relatively high expected abundances, and that sites with no neonicotinoids were associated with low seasonal variation and moderately high expected bee abundances. The number of floral units at a site was also positively associated with bee abundance, which is consistent with what has been reported in other studies. Diversity data are currently being processed off-site and unavailable at the time of publication. My results provide evidence that there exists the potential for higher risks of neonicotinoid seed treatments to ground nesting bees compared to the non-ground nesting community.

Increased growth of the mid-continent population of Lesser Snow Geese (LSGO) has led to the degradation of coastal salt marsh and sedge meadow habitats across Canadian Arctic and sub-Arctic ecosystems. It is believed that a human-induced trophic cascade caused by agricultural habitat modification along migratory routes and wintering grounds has contributed to the increase in LSGO numbers, which has resulted in the alteration of habitat quality and connectivity along northern breeding and stopover sites used by various avian species. This habitat degradation has been shown to decrease the presence and temporal persistence of ground-nesting passerine and shorebird species at a local level and may lead to decreases of Arctic / sub-Arctic breeding avian species across landscapes that LSGO utilize and degrade. In 1999, four paired study plots were established, and used in conjunction with a single study plot from 1976, in order to measure the composition of habitat parameters (barren ground extent; graminoid and shrub cover) and to estimate the number of avian nests found in these plots. Using this historical data along with our findings from 2010 and 2011, our main objectives were to: 1) document the change in the aforementioned habitat parameters over time; 2) estimate the local nesting occupancy rates of the common Savannah Sparrow (SAVS), a robust and adaptable ground nester; and 3) determine which habitat variables are indicative of the rates of change and occurrence of nesting by SAVS within the study plots. By using ANOVA, linear mixed effects, and multi-state occupancy models, results suggest that an increase in barren ground, decreases in shrub and graminoid cover, and a loss of connectivity between suitable nesting patches has led to a 10% (λ = 0.90) annual decline in the probability that SAVS nesting occurred across the study plots from 1999 to 2010. These model results may be used to estimate long-term trends in persistence of breeding SAVS and other similar ground-nesting avian species that share habitats with LSGO along Arctic and sub-Arctic ecosystems. (93 pages)

In my thesis, I examined impacts of nesting and floral resources on ground-nesting bee assemblages at nine ranch sites with differing grazing histories. Because ecological disturbances can strongly affect the availability of foraging and nesting resources over time, I also examined the impacts of grazing history on nesting and floral resources for ground-nesting bees. I sampled bee assemblages, a comprehensive array of vegetation and soil measures, and floral abundance and richness. I used these data to determine (1) the importance of different nesting habitat resources and livestock grazing history for ground-nesting bees and (2) how livestock grazing history influenced the availability of nesting and floral resources. I found that sites with sandier soils had greater abundance and richness of ground-nesting bees, and sites with less compacted soils had greater bee abundance. Contrary to many other studies, the availability of bare ground was not important for ground-nesting bee abundance or richness. Grazing history did not affect any measures of nesting or floral habitat, and had negligible effects on ground-nesting bee abundance, richness, and community composition. My results suggest that the availability of sandier or less compacted soils may be limiting for ground-nesting bee abundance or richness, especially in areas with predominately clay soil types. In this context, the availability of such nesting resources may be more limiting than floral resource availability. More research is necessary to tease apart the effects of different nesting and floral resources for bees, and how different grassland management types affect bees through those resources. My findings highlight the importance of considering nesting habitat resources in bee conservation and restoration efforts.