Global patterns and drivers of urban biotic homogenization: A meta-analysis

Aim: Urbanization is a major driver of biotic homogenization (BH),
reducing biodiversity through species invasions and extinctions. However,
the drivers and mechanisms remain poorly understood, and urban ecosystems
may also exhibit biotic differentiation (BD). This study uses a
meta-analysis to examine BH and BD patterns, underlying processes and key
drivers across regions and taxa. Location: Global. Time Period: 2004–2023.
Major Taxa Studied: Arthropods, amphibians, birds and plants. Methods: We
conducted a global meta-analysis integrating 110 effect sizes from 45
publications to evaluate urban biotic homogenization within and between
cities. We quantified β-diversity and its turnover and nestedness
components across urbanization levels. Within cities, we evaluated changes
in β-diversity along urban levels, whereas between cities we assessed
similarity among urban assemblages. We then examined how environmental,
urban, and biotic factors (e.g., climate, impervious cover, non-native
richness) influenced homogenization. Results: Across studies, β-diversity
responses to urbanization were heterogeneous but revealed clear patterns.
Birds showed the strongest within-city homogenization, particularly in
North America, whereas amphibians, arthropods, and plants exhibited weaker
or inconsistent trends. Turnover was the dominant β-diversity component
(> 65%), yet declined with increasing urbanization, especially in
birds and plants. Nestedness increased with urbanization, reflecting
species loss in some contexts. Among potential drivers, non-native species
richness consistently predicted homogenization, whereas climatic and
geographic variables had context-dependent or negligible effects.
Between-city analyses revealed biotic homogenization only in certain taxa
and regions, whereas cities remained more similar within regions than
among regions. Main Conclusions: Urbanization influences β-diversity
through context-dependent processes, with turnover decline emerging as the
primary mechanism of homogenization. Non-native species richness acts as a
consistent driver, whereas other factors show idiosyncratic effects.
Although homogenization is evident in some taxa and regions, it is not
universal. This highlights an opportunity to conserve the remaining
biogeographic distinctiveness of some urban assemblages and prevent
further homogenization.