Body-size shifts in aquatic and terrestrial urban communities

Body size is intrinsically linked to metabolic rate and life-history traits, and is a crucial determinant of food webs and community dynamics 1,2. The increased temperatures associated with the urban-heat-island effect result in increased metabolic costs and are expected to drive shifts to smaller body sizes 3. Urban environments are, however, also characterized by substantial habitat fragmentation 4 , which favours mobile species. Here, using a replicated, spatially nested sampling design across ten animal taxonomic groups, we show that urban communities generally consist of smaller species. In addition, although we show urban warming for three habitat types and associated reduced community-weighted mean body sizes for four taxa, three taxa display a shift to larger species along the urbanization gradients. Our results show that the general trend towards smaller-sized species is overruled by filtering for larger species when there is positive covariation between size and dispersal, a process that can mitigate the low connectivity of ecological resources in urban settings 5. We thus demonstrate that the urban-heat-island effect and urban habitat fragmentation are associated with contrasting community-level shifts in body size that critically depend on the association between body size and dispersal. Because body size determines the structure and dynamics of ecological networks 1 , such shifts may affect urban ecosystem function. Body size is a fundamental species trait relating to space use and key life-history features such as longevity and fecundity 6. It also drives interspecific relationships, thus affecting ecological network dynamics 1. Size-biased species loss has profound effects on ecosystem function 7,8. Ectotherms rely on ambient conditions to achieve operational body temperatures 9. Because higher ambient temperature increases metabolic rates and the associated costs for a given body size 2 , global climatic warming is expected to drive shifts to communities consisting of smaller species 3. Our planet is urbanizing quickly 10 , which is a primary example of human-induced rapid environmental change. Cities are urban heat islands characterized by increased temperatures that are decades ahead of global averages 11. Not only are cities warmer than surrounding areas, but they also experience extensive fragmentation of (semi-)natural habitats , and both of these effects increase with percentage built-up cover (BUC; a proxy for urbanization) 12,13. This provides an opportunity to study the opposing effects of size-dependent thermal tolerance and dispersal capacity, as larger body size favours dispersal in some, but not all, taxa. Here we test the hypothesis that urbanization causes shifts in community level body size, and that these shifts are dictated by the community specific association between body size and dispersal. We generally expect the urban-heat-island effect to drive shifts to species with smaller body sizes in communities of ectothermic species, in line with Atkinson's temperature-size rule 14. For taxa characterized by a positive association between body size and dispersal, however, we also expect a filtering in favour of larger-bodied species associated with habitat fragmentation 5,15. Filtering for increased mobility has been demonstrated for urban ground beetle and plant communities 16,17. Hence, for taxa characterized by a positive body-size-dispersal link, we predict that the general community-level pattern of smaller species with increasing urbanization may be neutralized or even reversed. To test our hypothesis, we engaged in an analysis of community-level shifts in body size across a broad range of both terrestrial and aquatic taxa along the same systematically sampled urbanization gradients. We studied the direction of change of community-level body size in ten taxa using a replicated, highly standardized and nested sampling design that covers urbanization gradients at seven spatial scales (50-3,200 m radii; Fig. 1). We sampled each taxon at up to 81 sites, sampling 95,001 individuals from 702 species, with species-specific body size varying by a factor of 400 (0.2-80 mm; Extended Data Table 1). Three of the ten groups are characterized by a positive association between body size and dispersal capacity (see Extended Data Table 1). We show that the local temperature of pond, grassland and woodland habitats significantly increases with urbanization (linear mixed regression models, P <0.002; Extended Data Table 2). The intensity of these urban-heat-island effects is consistently larger during night and summer, in accordance with slower night-time city cooling and higher irradiation levels in summer 18 (Fig. 2, Extended Data Fig. 1, Extended Data Table 2). We also show that increased urbanization is linked to significant declines in habitat amount and the patch size of terrestrial habitats, and significant increases in distances among patches for both terrestrial and aquatic habitats (Pearson's r correlations, P ≤ 0.020; Extended Data Fig. 2). Confirming our metabolism-based prediction that interspecific mean body size decreases with increasing temperature, urban communities for four out of the seven taxa (ground spiders, ground beetles, weevils and cladocerans) that did not have a positive size-dispersal link display reduced community-weighted mean body size (CWMBS). For ostracods, bdelloid rotifers and web spiders, no relationship with urbanization is found. By contrast, all three taxa with positive size-dispersal links display increased CWMBS in response to urbanization