Structure of a metacommunity of urban bees: Species diversity and spatio-temporal modularity

As the globe gets more urbanised, the question about how natural biodiversity is structured in cities becomes increasingly pertinent. To contribute to an answer, we studied species richness and spatio-temporal structure of bees in a North European metropolitan area. A gradient of 13 sites in the city of Aarhus, Denmark, was censused from April to September 2016. Forty species, i.e. 29 solitary species (40 % of all individuals), ten Bombus species (28 %), and Apis mellifera (32 %), were sampled monthly in pan traps. (i) Information about species traits was extracted from the literature, and trait values were correlated and used to characterize the fauna. Most were soilnesters, pollen generalists, and common. (ii) Habitat diversity within five concentric circles with trap at the centre and radii from 50 m to 1000 m was related to bee alpha diversity. The relationship was significant only within 1,000 m for all bees and for bumblebees. Solitary bee diversity was uncorrelated with habitat diversity at all spatial levels. (iii) Spatio-temporal structure was analysed as two networks, one for bees linked to sites, and one for bees linked to months. Link patterns were analysed for levels of nestedness, modularity, and spatio-temporal beta diversity. The two networks were weakly and non-significantly nested, but strongly modular, being composed of five and four modules of co-occurring bees, respectively. (iv) Finally, we studied total beta diversity, beta TOTAL, being the sum of species turnover, beta TURN, and species loss/gain or nestedness, beta NEST. For both site and season, beta TURN was higher than beta NEST, and site beta TOTAL was higher than season beta TOTAL. One reason for this metacommunity structure may be a high spatio-temporal habitat patchiness, sustaining a rich biodiversity. Thus, a few large areas may not compensate for the loss of several small patches. Consequently, establishment of many green, even small habitats is recommended.