Land-use and climate drive shifts in Bombus assemblage composition

Pollinators play pivotal roles in maintaining agricultural and natural plant communities, yet some bee populations are declining. The conversion of agricultural and semi-natural lands for urban use has reduced bee abundance and diversity. Additionally, climate change has affected bee distributions and led to disruption of plant-pollinator synchrony, impacting ecosystem processes. However, how these factors concurrently influence bee assemblages is poorly understood. Therefore, we linked differences in bumble bee (Bombus) diversity to landscape composition and climate in agroecosystems to understand their co-occurring effects. Bombus assemblages were evaluated in relation to the proportion of agricultural, semi-natural, and urban landscapes and interannual variation in temperature, precipitation, and relative humidity in Utah agroecosystems from 2014 to 2018. Bombus species richness and diversity were highest in rural agricultural landscapes characterized by low temperatures and high relative humidity during the growing season, and lowest in urbanized agricultural areas with high temperatures and low relative humidity. Ongoing and future land-use and climate change may therefore lead to reduced Bombus diversity in Utah. Although some historically uncommon species, such as B. pensylvanicus, may thrive under future land-use and climate scenarios, others (e.g., B. sylvicola, B. californicus, and B. occidentalis) are at increased risk of extirpation due to loss of suitable habitat. Continually monitoring Bombus populations will help document shifts in assemblages and potential consequential impacts to ecosystem services. These findings emphasize that management strategies should consider the effect of co-occurring factors based on geographic location and local diversity to prevent ecological homogenization and to foster future resiliency of Bombus populations.