How the sensory ecology of the bumblebee affects their functional traits

Abstract: Morphological traits are important for trait-based ecological studies as they allow for the prediction of biodiversity across the biosphere. Currently, the predictive power of trait-based ecology is not often applied to terrestrial arthropods, even though they provide us with important agricultural, ecological, and cultural value. The aim of my PhD was to study how functional morphological traits of bumblebees are linked to different environmental factors.Overlooking intraspecifc variations of functional morphological traits in trait-based ecology is common and might lead to underestimation of adaptive capabilities in a species. In Chapter I, we used allometry to compare the scaling relationships of three different organs (compound eyes, wings, and antennae) among the size polymorphic workers of the buff-tailed bumblebee, Bombus terrestris. We used 12 colonies of which half developed at a sub-optimal elevated temperature (32°C, rather than 25°C). We found that each colony had a specific scaling profile for each organ and these differences could not be attributed to temperature stress. The variability in scaling among colonies suggests an intraspecific flexibility in investment in functional morphological traits.In Chapter II, I developed a method to estimate the size of the dorsal rim area (DRA), a specialized region in the compound eye of B. terrestris. The DRA has been greatly understudied due to the lack of easy and high-throughput methdods to study it. My method encompasses the use of 2D photography and micro-computed tomography (micro-CT) to determine the size of the DRA and the characteristics of its ommatidial structures. Using my method, I performed an allometric study on the scaling of DRA with body size in B.terrestris and found that crystalline cones of the DRA are significantly smaller compared to other regions of the eye.In Chapter III, I compared the visual systems (compound eyes, DRA, ocelli) in the workers of four different bumblebee species (B. jonellus, B. pratorum, B. hypnorum, and B. monticola). These species are found in different habitats and I found that this is reflected in their visual system investment. Species living in forests invested more in their visual systems compared to species found in open or open/forested environments. Our study suggests that habitat preference may indeed shape the visual system of different bumblebee species.Bumblebee species often occur sympatrically and we studied their co-existence in a hemiboreal forest with a limited variety of floral resources. Bumblebee species can avoid competition by foraging at different times of the day. To identify the traits that might allow this coexistence, we combined full-day monitoring of bumblebee communities with detailed functional trait measurements in Chapter IV. We found that temperature affected the presence and absence of species while light intensity affected the abundance of individuals and suggest that this pattern could be linked with visual and thermoregulatory traits.My combined findings show that bumblebees are an excellent model system for studying trait-environment relationships as they exhibit (i) intraspecific differences in traits, (ii) idiosyncrhatic and ubiquitous traits like the DRA, (iii) clear trait-environment relationships, and (iv) an example of how traits aid in the coexistence of similar species.