DOI: 10.5061/dryad.7sqv9s4v5
title: Urban biotic homogenization: approaches and knowledge gaps

description: 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 (&lt;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.
description: We compiled a comprehensive collection of empirical studies
 addressing the urban biotic homogenization (UBH) hypothesis following the
 PRISMA protocol (Moher et al., 2009), see Appendix S1: Fig. S3 for PRISMA
 flowchart. A search within the ISI Web of Science was performed on 9 June
 2020 using the following terms: ‘(urban* OR city OR cities) AND (biotic
 homogenization OR ((biodivers* OR taxonomic diversity OR functional
 diversity OR genetic diversity OR evolutionary diversity OR species
 diversity OR trait diversity OR species composition OR habitat diversity)
 AND homogenization))’.<br><br>This search yielded 560 hits
 which were screened for relevance (abstract and title) and from these, 385
 articles were assessed (full-text) and included in our database if (1) the
 publication addressed the UBH hypothesis or biotic homogenization in
 cities or in response to urbanization, and (2) biotic homogenization was
 studied empirically by either direct testing, or inferential reasoning
 from qualitative evidence. We thus included a wider range of publications
 than for a formal meta-analysis. We excluded publications if (1) neither
 quantitative nor qualitative evidence was provided on the UBH hypothesis,
 or (2) two entries described the same study or presented the same analyses
 (e.g. doctoral dissertation and subsequent publication of results). For
 preprints and theses, we checked in Google Scholar and personal profiles
 of first authors, e.g. on ResearchGate, whether a more recent publication
 of the results in an English journal was available and included it instead
 if available. We also searched manually whether non-English and non-German
 publications that were retained after the first screening had since been
 published in English, or else excluded them from the analysis. 145
 articles fulfilled these criteria and were thus included in our
 dataset.