Where the green goes: green-network configuration and the super-additive loss of urban biodiversity under polyuncertainty

Kiatanantha Lounkaew 

Climate assessment for cities proceeds one pressure at a time, its uncertainty treated as a sum of separable parts. Compound-event research shows this understates risk when drivers are dependent, and urban ecology that habitat arrangement, not quantity, governs which species persist. These have not been joined, so the contribution of green-infrastructure configuration, as against extent, to ecological loss under polyuncertainty, the condition in which several stressors act at once and interact, is unknown. First we measure co-occurrence in the hazards 1,070 cities disclosed to CDP in 2021. Among the 871 naming at least one, the mean names 3.60 of ten families, and extreme heat and water scarcity are named together by 33.8 percent against 27.2 percent under independence, a lift of 1.24 against a fixed-margins null of 1.11 at z equal to 4.7. The dependence is structured, since dry-regime hazards cluster while wet-regime hazards separate, so the pair the model compounds is empirically motivated and blanket super-additivity is not. Second, a patch-network metacommunity model compares fragmented, corridor, and redundant networks across seven scenarios. Under an imposed super-additive interaction regime, the model produces a joint heat-drought loss approximately 1.3 times the sum of the corresponding single-stressor losses. The configurational consequence, by contrast, is emergent. Holding total green area constant, configuration is second-order under any single stressor and first-order under polyuncertainty, the fragmented network falling to a health index of 0.29 against 0.53, while corridors and pocket parks raise it to 0.57 and 0.61 against 0.34 for enlarging its isolated parks.