Urbanization and Spatial Aggregation Impair Multifunctionality in Urban Vacant Lots

Urban shrinkage, driven by population decline rather than expansion, is an emerging concern in many developed countries. This demographic shift increases the prevalence of novel green spaces, such as vacant lots, prompting interest in their potential to enhance urban biodiversity and ecosystem multifunctionality. However, biodiversity-ecosystem multifunctionality relationships in vacant lots remain largely unexamined. We investigated 69 vacant lots in Yokohama, Japan, a city facing potential population decline, by quantifying six environmental factors, five ecosystem functions, and taxonomic and functional diversity and composition of plants, bacteria, and fungi. We used structural equation modelling to analyse the direct and indirect effects of environmental and ecological variables on ecosystem function and multifunctionality. Additionally, to account for trade-offs and synergies among ecosystem functions, we examined the relationships between environmental factors and multifunctionality using a multiple-threshold approach and generalized linear mixed models. Our results indicate that environmental factors exerted a dominant influence on ecosystem functions, although all components (environment, plants, and microbes) played a role. Specifically, soil moisture directly enhanced several ecosystem functions and average multifunctionality. In contrast, spatial aggregation of vacant lots indirectly impaired them, mediated by increased plant richness and altered fungal composition. Moreover, urbanization indirectly affected all ecosystem functions and exerted a direct negative effect on average multifunctionality, with its negative effects intensifying at elevated multifunctionality thresholds. These findings highlight that the multifunctionality of urban vacant lots is intricately shaped by environmental factors mediated by diverse taxa. Given that dispersed vacant lot configurations in less urbanized areas may enhance multifunctionality but reduce plant diversity, future urban planning in shrinking cities should balance biodiversity conservation with the enhancement of ecosystem multifunctionality through strategic spatial configuration.