Large parks and city-wide tree cover boost butterfly diversity across 22 major U.S. cities

Abstract

Accelerating global urbanization necessitates a better understanding of how to manage cities that promote biodiversity. However, we currently lack multi-year, multi-city studies, which limits a generalizable understanding of how both within and between city differences impact the spatial and temporal dynamics of urban biodiversity. Here, we tested hypotheses about the drivers of butterfly diversity within and across urban parks by applying Bayesian oc-cupancy models to five years of iNaturalist community science data from 2,550 parks in 22 major U.S. cities. We found that cities with bigger parks supported more species per park, including more disturbance- and edge-avoidant species. This was driven by a positive effect of park size on butterfly species colonization rates. We also found that attributes of habitat quality (plant diversity within parks and tree cover surrounding parks) contributed to butter-fly species occupancy. Park connectivity increased species persistence, but the overall effects on butterfly species occupancy varied across cities. Finally, we found that the total area of tree cover throughout a city, rather than the size or connectivity of individual parks, was the primary determinant of city-wide diversity: Increasing total tree canopy cover from below-average ([~] 6%) to above-average ([~] 22%) increased city-wide species richness by[~] 10%. These findings highlight the need for cities to maintain large parks while also increasing city-wide tree cover to support biodiversity across local to regional scales. By integrat-ing high-resolution community science data across the continental U.S., this study provides mechanistic insight into how cross-scale processes shape urban biodiversity dynamics and identifies generalizable recommendations for improving urban conservation management. Significance statementAdvancing urban sustainability hinges on better understanding how to promote biodiver-sity in urban landscapes. Yet, it is largely unclear how urban biodiversity is affected by both within- and between-city differences. Applying Bayesian occupancy models to 5 years of iNaturalist community science butterfly data from 22 U.S. cities, we found that cities with larger parks supported more species per park, including species of higher conservation concern. Simultaneously, increasing city-wide tree canopy cover area, including area both within and outside of parks, increased total city-wide butterfly diversity. Integrating biodi-versity conservation recommendations from this study into urban management planning will reinforce the benefits of participating in community science programs, thereby strengthening positive feedbacks between the health of people and nature.