Evaluating the impacts of local and landscape level site characteristics on bumblebee (Bombus spp.) communities along the Flint River riparian zone

Abstract

Flying insect populations, including pollinators such as wild bees, have been declining globally due to anthropogenic changes. Pollinators are important not only for conservation of biodiversity of insects, but also for the conservation of biodiversity of plants. Almost 90% of wild and cultivated plants rely on animal facilitated pollination, including many crops humans rely on for sustenance. Population declines of wild bees has largely been linked to land use changes such as agricultural expansion, but in many cases these insects persist even in intensely industrialized and urban areas. Bees’ habitat requirements include suitable nesting habitat, suitable forage availability, and mate availability. In the case of bees in urban areas, drivers of population declines and community structure is unclear. Possible drivers of bee abundance in urban areas include landscape factors such as greenspace, or site characteristics such as floral abundance, floral diversity, quality of floral resources, availability of nesting habitat, soil texture, and soil pollution. Soil texture and soil contaminants could be particularly impactful on ground nesting bees such as species within the genus Bombus. Bees in urban areas have oftentimes been studied in parks, natural areas, gardens, and greenroofs but oftentimes bees in urban riparian areas have been overlooked. Riparian areas could be uniquely suited to be important conservation areas for wild bees in urban areas, because they are often left as some of the only intact greenspaces due to difficulty of developing land adjacent to a river.

To study the impacts of landscape factors and site characteristics on Bombus communities in a post-industrial urban riparian area, I selected 12 sites within Bombus flight range to study where bees were preferentially selecting to be, and what landscape or site characteristics predict this. I captured bees between May and October 2021 and identified all Bombus specimens to species. I then used a generalized linear model to find that Bombus abundance is predicted by sum of flowering forb cover (p << 0.05), total green cover within a 100m radius (p << 0.05), and total lawn cover within a 100m radius (p << 0.05). For Bombus richness I found that only total green cover of the surrounding landscape at 100 m was a significant predictor (p < 0.05). For Bombus diversity I found that only the Floristic Quality Index was a significant predictor (p < 0.05).

To study the impacts of soil texture and soil contamination by heavy metals on Bombus impatiens, our most abundantly captured Bombus species, I assessed soil texture at each site and heavy metal contamination within soil samples and within individual worker bees and individual worker bee weights. I also assessed heavy metal contamination of Apis mellifera, the western honeybee which is not a ground dwelling species, as a negative control as we would not expect it to come into contact with soil contaminants. Soil textures were analyzed using the jar method and ribbon method, and according to both methods each site had a sandy loam soil texture. Because all sites had the same soil texture, this was not used as a predictor in any analysis. Soil heavy metal contamination and individual worker bee heavy metal loads were analyzed using ICP-MS. Heavy metal analyses found for both soils and bees were correlated with Lead (Pb) loads, we performed statistical analyses for lead. Because lead is also a known contaminant in post-industrial cities such as Flint, we expected it to be in the soils. We found lead loads in soil ranging from 13ppm to 76ppm, which is far lower than in many other post-industrial areas. Because lead loads in study sites had a low, limited range the results of this study cannot conclusively say that soil lead contamination is the cause of low worker weights in Bombus workers in this case, but it lays out the methods for future studies. Lead in the environment has been shown to have sub-lethal negative effects on Bombus, so repetition of this study in more highly polluted areas may show more evidence of this in wild populations than my study.