Urban biotic homogenization: approaches and knowledge gaps

Urbanization is restructuring ecosystems at an unprecedented pace, with
complex and profound consequences for life on earth. One of the
hypothesized trajectories of urban ecosystems and species communities is
biotic homogenization, possibly leading to very similar species
assemblages in cities across the globe. Urbanization can, however, also
have the opposite effect: biotic diversification, with cities – at least
on the local scale – becoming biologically more diverse, mainly as a
consequence of high species introduction rates and habitat
diversification. Applying the hierarchy-of-hypotheses approach, we
systematically map and structure the comprehensive body of literature on
the urban biotic homogenization (UBH) hypothesis, comprising 225
individual studies (i.e. tests of the hypothesis) retrieved from 145
publications. The UBH hypothesis is studied at multiple levels with a
multitude of approaches and underlying assumptions. We show that UBH is
generally used with two very different connotations: about half of the
studies investigated a potential increase in community similarity across
cities, whereas the other half investigated biotic homogenization within
cities, the latter being supported more frequently. We also found strong
research biases: (1) a taxonomic bias towards birds and plants; (2) a bias
towards small and medium distances (<5000 km) in comparisons across
cities; (3) a dominance of studies substituting space for time vs. true
temporal studies; (4) a strong focus on terrestrial vs. aquatic systems;
(5) more extraurban (including periurban) areas than natural or rural
ecosystems for comparison to urban systems; (6) a bias towards taxonomic
vs. functional, phylogenetic and temporal homogenization; and (7) more
studies undertaken in Europe and North America than in other continents.
The overall level of empirical support for the UBH hypothesis was mixed,
with 55% of the studies reporting supporting evidence. Results
significantly differed when a natural/nature reserve, an extraurban or
rural/agricultural area served as reference to infer biotic
homogenization, with homogenization being detected least frequently when
urban systems were compared to agricultural, i.e. other anthropogenically
influenced, study sites. We provide an evidence map and a bibliographic
network, and identify key references on UBH with the goal to enhance
accessibility and orientation for future research on this topic.