Island Biogeography Theory-inspired predictions reveal that urban non-native plant richness is source dependent and defies classical isolation predictions

Abstract

Summary Although cities are increasingly recognized as ecological islands, a unified framework explaining their susceptibility to alien plant invasion remains lacking. Using the most recent and comprehensive global dataset of urban alien plants, we modelled alien richness, mimicking island biogeography theory (IBT). Across all models, neither city size nor geographic isolation independently explained alien richness. Instead, richness was consistently associated with their interaction, supporting the central IBT prediction. However, the strength of this interaction depends on how city size was quantified, with socio-economic dimensions exhibiting stronger positive interactions with geographic isolation than physical measures of city size. Introduction-hub identity further modified these relationships. North America was the only hub for which the interaction between city size and isolation was consistently weakened, indicating that donor regions of alien plants are not ecologically equivalent. Simulations of simultaneous increases in city size and isolation showed that larger, more connected cities generally accumulated more alien plants despite increasing geographic distance, but the magnitude and direction of these responses are hub dependent. Our findings inspire an extension of classical IBT to a mechanistic explanation for global variation in urban alien plant richness in this increasingly urbanized and globally connected world. Graphical abstract (drawn using AI) The Urban Island Biogeography Framework (UIBF) extends classical Island Biogeography Theory to explain global patterns of alien plant richness in cities. Across six measures of urban size, neither city size nor geographic isolation independently predicted alien richness; instead, their interaction consistently explained invasion patterns. Socio-economic dimensions of urbanization (population size and GDP) strengthened this interaction more than physical city attributes, highlighting the importance of anthropogenic connectivity. Introduction-hub identity further modified these relationships, with Washington uniquely weakening the positive interaction between urban size and isolation. Together, these findings provide a mechanistic framework for predicting urban biological invasions in an increasingly connected world.