Urbanization Facilitates Bullfrog Invasion Success and Exacerbates Native Amphibian Declines: A Natural Experiment...

Urbanization and biological invasions synergistically threaten global biodiversity. The COVID-19 lockdowns created a unique quasi-experimental reduction in human mobility. We leveraged this period to conduct paired field surveys during both suppressed and resurgent human activity across urbanization (urban/rural) and invasion (invaded/uninvaded) gradients in China's Yangtze Basin, assessing the invasion dynamics of the American bullfrog (Lithobates catesbeianus). Key findings reveal three core patterns: (1) Urbanization facilitates invasion-bullfrog densities averaged 1.69 times higher in urban than rural habitats, a pattern unexplained by measured habitat variables; (2) Invasion impacts are species-specific: Bufo gargarizans abundance increased in invaded sites (with the stronger response in urban areas), while four native frog species (Pelophylax nigromaculatus, P. plancyi, Fejervarya multistriata, Microhyla fissipes) declined, with P. nigromaculatus, P. plancyi and Microhyla fissipes experiencing intensified suppression in urban landscapes; (3) The reopening and resumption of human activity drove a 12.1% greater increase in bullfrog densities in rural versus urban areas, suggesting that rural establishment is dispersal-dependent. Mechanistically, urbanization accelerates invasions through anthropogenic dispersal networks and permanent water sources, while habitat fragmentation forces native species to aggregate, intensifying competition and disease transmission. These findings underscore the need for differentiated urban-rural conservation strategies to mitigate invasion impacts.