Body size is intrinsically linked to metabolic rate and life-history traits, and is a crucial determinant of food webs and community dynamics1,2. The increased temperatures associated with the urban-heat-island effect result in increased metabolic costs and are expected to drive shifts to smaller body sizes3. Urban environments are, however, also characterized by substantial habitat fragmentation4, 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 settings5. 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 networks1, such shifts may affect urban ecosystem function.

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We thank M. De Cock, J. Dierick, P. Limbourg, E. van den Berg, M. Van Kerckvoorde and P. Vantieghem for help with sampling and identification of species. This is publication BRC419 of the Biodiversity Research Centre (UCL/ELI). This research is part of the SPEEDY-project, funded by the Interuniversity Attraction Poles program of the Belgian Science Policy Office BELSPO (IAP-grant P7/04).

Reviewer information

Nature thanks M. McDonnell and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Author notes

  1. These authors jointly supervised this work: Luc De Meester, Hans Van Dyck.


  1. Behavioural Ecology and Conservation Group, Biodiversity Research Centre, Earth and Life Institute, Université catholique de Louvain, Louvain-la-Neuve, Belgium

    • Thomas Merckx
    • , Aurélien Kaiser
    •  & Hans Van Dyck
  2. Laboratory of Aquatic Ecology, Evolution and Conservation, KU Leuven, Leuven, Belgium

    • Caroline Souffreau
    • , Kristien I. Brans
    • , Jessie M. T. Engelen
    • , Andros T. Gianuca
    • , Lynn Govaert
    •  & Luc De Meester
  3. Evolutionary Ecology Group, University of Antwerp, Antwerp, Belgium

    • Lisa F. Baardsen
    • , Thierry Backeljau
    •  & Erik Matthysen
  4. Directorate Taxonomy and Phylogeny, Royal Belgian Institute of Natural Sciences, Brussels, Belgium

    • Thierry Backeljau
    • , Katrien De Wolf
    • , Frederik Hendrickx
    • , Elena Piano
    •  & Rose Sablon
  5. Terrestrial Ecology Unit, Biology Department, Ghent University, Ghent, Belgium

    • Dries Bonte
    • , Maxime Dahirel
    • , Frederik Hendrickx
    • , Luc Lens
    •  & Hans Matheve
  6. Directorate Natural Environment, Royal Belgian Institute of Natural Sciences, Brussels, Belgium

    • Marie Cours
    • , Koen Martens
    •  & Isa Schön
  7. ECOBIO (Ecosystèmes, biodiversité, évolution), CNRS, Université de Rennes, Rennes, France

    • Maxime Dahirel
  8. Laboratory of Evolutionary Genetics and Ecology, URBE, NAXYS, University of Namur, Namur, Belgium

    • Nicolas Debortoli
    •  & Karine Van Doninck
  9. National Research Council, Institute of Ecosystem Study, Verbania-Pallanza, Italy

    • Diego Fontaneto
  10. German Centre for Integrative Biodiversity Research (iDiv), Halle-Jena-Leipzig, Germany

    • Andros T. Gianuca
  11. Helmholtz Centre for Environmental Research (UFZ), Department of Community Ecology, Halle, Germany

    • Andros T. Gianuca
  12. Centre of Research in Limnology, Ichthyology and Aquaculture/PEA, State University of Maringá, Maringá, Brazil

    • Janet Higuti
  13. Limnology Research Unit, Biology Department, Ghent University, Ghent, Belgium

    • Koen Martens
  14. Department of Life Sciences and Systems Biology, University of Turin, Turin, Italy

    • Elena Piano
  15. Zoology Research Group, University of Hasselt, Hasselt, Belgium

    • Isa Schön


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T.M., L.D.M. and H.V.D. conceived the study. C.S. and L.D.M. coordinated the consortium. T.M., C.S., A.K., L.F.B., T.B., D.B., K.I.B., M.C., M.D., N.D., K.D.W., J.M.T.E., D.F., A.T.G., L.G., F.H., J.H., L.L., K.M., E.M., E.P., R.S., I.S. and K.V.D. contributed to sampling and data collection. T.M. and A.K. performed the analyses. H.M. selected study plots, calculated fragmentation variables and designed the study area map. T.M. wrote the first draft of the manuscript with all authors contributing substantially to revisions.

Competing interests

The authors declare no competing interests.

Corresponding author

Correspondence to Thomas Merckx.

Extended data figures and tables

  1. Extended Data Fig. 1 Micro-climatic urban-heat-island effect strengths.

    a, b, Slopes of the urban-heat-island effects (measured as the increase in temperature (°C) per 1% increase in percentage BUC) as a function of spatial scale (the radius at which urbanization was quantified) with 95% confidence intervals (CI). Separate measurements are shown for summer (red) and winter (blue) using merged readings for pond, grassland and woodland habitats (n = 104 sites). a, Diurnal measurements. b, Nocturnal measurements. Data points are offset from one another horizontally to improve clarity.

  2. Extended Data Fig. 2 Correlations between urbanization and habitat fragmentation.

    Correlations between urbanization (measured as the percentage BUC) and three habitat fragmentation variables: habitat coverage (a, b), mean size of habitat patches (c, d), and mean nearest-neighbour distance among habitat patches (e, f). Separate plots are shown for terrestrial (that is, all types of (semi-)natural habitat, a, c, e) and aquatic (that is, all pond types, b, d, f) habitats (n = 27 landscape-scale sampling plots). Pearson’s r coefficients and P values are indicated; not significant (NS), P > 0.1; *P < 0.05; ***P < 0.001.

  3. Extended Data Table 1 Taxon-specific details of sampling procedures, body-size data and size–dispersal links
  4. Extended Data Table 2 Model output of average temperature in relation to urbanization and habitat type
  5. Extended Data Table 3 Model output of CWMBS in relation to urbanization
  6. Extended Data Table 4 Model output of abundance and diversity measures in relation to urbanization

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