Biotic interactions and structural decline of exotic urban trees shape biodiversity patterns in a megacity

Urban ecosystems are increasingly shaped by multiple, interacting biotic stressors driven by global change, including habitat fragmentation, climate variability, and the widespread introduction of exotic species. In this study, the structural decline of Eucalyptus camaldulensis was evaluated across 20 sites in Mexico City over five monitoring periods (2019-2024), integrating tree traits and site variables, associated fauna, and biotic stressors within a unified analytical framework. Cumulative link mixed models showed that structural decline progressed consistently through time and was strongly associated with borer beetle galleries, Glycaspis brimblecombei galls, and foliar fungi, while lower vulnerability was observed in larger individuals and trees located in larger plots. No significant effects were detected for mistletoes, epiphytes, or woodpecker activity. Tree structural decline emerges as a dynamic process shaped by shifting biotic stressors, with networks showing structurally declining trees embedded in webs dominated by G. brimblecombei-foliar fungi interactions together with borer beetle galleries. Although no direct associations with associated fauna were detected, indirect connections through biotic stressors suggested that deteriorating eucalyptus can act as transient ecological hubs, concentrating interactions that later collapse or reorganize once trees die or are removed. From a biodiversity perspective, microhabitats were provided and diverse organisms were supported by trees undergoing structural decline, underscoring their dual role as both ecological assets and safety risks in densely populated areas. These findings highlight the ecological implications of exotic trees in urban landscapes and emphasize the need for management strategies that balance safety-driven removals with the preservation of biodiversity and ecosystem functions.