Phenotypic flexibility in the city: A meta-analysis on variation

Among global changes urbanisation is distinctive because it entangles a variety of human-induced rapid environmental changes, such as habitat loss and fragmentation, temperature change, introduction of human food sources, and pollution. Urban environments are assumed to be heterogeneous and variable in space and time. A key feature of animals coping with high environmental variability ought to be phenotypic flexibility, i.e. the capacity of individuals to express reversible variation in labile traits. However, this “phenotypic flexibility hypothesis” has not been tested rigorously. Using a meta-analysis approach, we compiled available raw data of studies directly comparing urban and non-urban populations and estimated fixed and reversible individual variation. Across all taxa, fixed variation did not differ between rural and urban populations, although patterns emerged without birds. Reversible variation was marginally lower in urban compared to non-urban populations. The potential decrease of phenotypic flexibility in urban individuals could result from different responses of individual plasticity and predictability. Overall, the effects of urbanisation on phenotypic variation are not as generalisable as expected and may depend on the taxa, species and traits. Future studies should increase efforts to directly link temporal and spatial environmental variation at the individual level and disentangle plasticity and predictability.