Effect of urbanization and its environmental stressors on the intraspecific variation of flight functional traits in two bumblebee species

Abstract

Landscape anthropization replaces natural areas with either agricultural or urban covers, leading to land-use intensification. This worldwide phenomenon affects biodiversity, but little is known about the effects on the intraspecific variation of functional traits in spite of their importance for ecosystem functioning and services.Here we investigated if the functional traits of pollinator insects responded to land-use intensification and environmental stressors, by characterizing the variation of important traits for flight performance that are also indicators of stressors during organism development. The possible impact of different land cover (i.e. semi-natural, urban and agricultural) and of multiple biotic and abiotic stressors (i.e. temperature, resource availability and air pollutants) was investigated.Along a gradient of landscape anthropization, we quantified the impact of biotic and abiotic factors on two European bumblebee species (Bombus terrestris and B. pascuorum) sampled at 37 sites in Northern Italy. Through geometric morphometrics we investigated the variation of multiple morphological traits, namely wing centroid size (as a proxy of body size) and asymmetry of wing shape and size. Data on land-cover, climatic conditions and air pollutants were retrieved through remote sensing, while the estimation of resource availability was conducted directly in the field.Our results highlighted the relevance of the two main land-use alteration trajectories (i.e. urbanization and agriculture intensification), although the two species responded idiosyncratically. Urbanization led to shifts towards smaller body sizes due to increased impervious cover and air temperature in B. pascuorum. Instead, B. terrestris responded to agriculture intensification with larger body size in response to higher farmland cover and floral resource availability. Asymmetry in wing shape was not affected by abiotic factors of land-use intensification, but wing size asymmetry was positively associated with warmer temperatures and increased level of NO2 in B. terrestris.This study describes patterns of how landscape anthropization shapes pollinator insects’ functional traits: urbanization is associated with smaller bees, while agriculture intensification leads to larger bees. In addition, environmental stressors impacted wing size asymmetries and could compromise flight performance, particularly in warmer, more polluted habitats. This study indicates that the land-use intensification of anthropized landscapes determines functional trait variation at the intraspecific level and that these responses are highly taxon-specific even in syntopic related organisms.

Warming up through buildings and roads: what we know and should know about the urban heat island effect on bees

“…This could occur through two different mechanisms: either higher temperatures may accelerate the larval development of the bees (Howe, 1967), or it could be an adaptation to hotter urban landscapes, in accordance with Bergmann's rule. The evidence supporting the body size-shrinking hypothesis in urban areas has been obtained at the intraspecific level in different species and countries: in Europe on different Bombus species (Apidae) (Theodorou et al, 2021;Tommasi et al, 2022), and in central America on two Euglossa species (Apidae) (Garlin et al, 2022). The variation in intertegular distance (proxy of body size) reached up to 5% in an approximate gradient of 10°C (Bombus terrestris, Theodorou et al, 2021).…”

“…The existing studies, however, yield contrasting results. For example, Tommasi et al (2022) found increasing FA in hotter urban areas in only one of the two Bombus species investigated, whereas Banaszak-Cibicka et al (2018) found no variation in wing asymmetry along an urbanization gradient in an Anthophora species. We note again that this contrasting evidence may be the result of the investigated species.…”