Resumen de Spatial variability of bee communities: From local assemblages to interaction networks
Organisms are heterogeneously distributed following spatial patterns, which are the consequence of many biotic and abiotic factors. One of the main goals in ecology is to understand how species interactions with other organisms and with the environment determine these observed species distribution patterns. In this thesis, we analyze the factors explaining spatial variability of bee communities and associated interaction networks at local scale. The study was conducted in the Garraf Natural Park, in a 32km2 area homogeneously occupied by Mediterranean scrubland, and lacking strong gradients or ecological barriers. Because bees rely on floral and nesting resources to survive and reproduce, these are expected to influence the local spatial variability of the bee community. In the first chapter, we explore how the spatial distribution of flower and nesting resources determines local spatial variability in the bee community. We found a clear geographical pattern of spatial variability in bee community composition. This pattern was partly driven by floral resources with a negligible contribution of nesting substrate availability, and was strongly related to body size: small bee species (<55mg) displayed strong spatial patterns, while large species (>70mg) tended to be more evenly distributed across the area. Bee spatial distribution may also be affected by perturbation regimes. The study area contains several apiaries, affording an opportunity to explore the potential effects of beekeeping on wild bee communities (Chapter 2). Honey bees (Apis mellifera) are highly efficient foragers and have the ability to recruit other foragers to the most rewarding flower patches, potentially establishing a competitive effect on other pollinators. We found that honey bees were the main contributors to pollen/nectar depletion of the two main flowering plants in the study area. We also found that the bee community was modified in areas close to apiaries, where honey bee densities were higher. Large-sized bee species, with greater energetic requirements, and therefore more likely to be affected by low food resource availability, were less abundant in areas close to apiaries. These spatial changes in community structure are bound to affect the identity and the network structure of the interactions bees establish with other organisms Consequently, in the third chapter, we study the local spatial variability of the community of cavity-nesting bees and wasps together with their nests associates (parasitoids, cleptoparasites, predators and scavengers), as well as their interactions. We analyze the relationship between community and interaction ¿-diversity, and explore the sources of the observed spatial variability. Spatial variability of both communities (hosts and parasites) was high and mainly driven by species turnover, with a very low influence of nestedness, meaning that local communities were highly idiosyncratic. Interaction ¿-diversity was also very high and mostly due to the high species turnover, with a very low contribution of interaction rewiring. In other words, species tended to interact similarly across plots. Overall, this thesis demonstrates that bee communities and their antagonistic interaction networks may vary at a very small scale and are highly conditioned by local factors, and that bee communities may be affected by intensive beekeeping, a widespread human activity usually assumed to be beneficial.
