Urban warming leads to higher abundance of two key herbivores and reduces tree ecosystem services

Urbanization and global warming are important forces of ecological change. The amount and diversity of herbivory increases during warmer periods in the fossil record and at lower latitudes, which suggests that warming may increase herbivory. Because the world is urbanizing at a rapid rate, urban warming could also affect herbivore densities and, therefore, plants we depend on for ecosystem services. We hypothesized that unexplained herbivore outbreaks in cities could be a result of city warming, known as the urban heat island effect. Our results show warming directly causes higher abundance of scale insects and mites, two of the common and destructive urban tree pests. We also show insect communities on urban trees change as thermal minima increase, such that warmer trees have more abundant and diverse herbivore fauna. Predators, however, do not respond to warming. Moreover, an herbivore removal experiment showed herbivores on warm trees disproportionally affect tree growth and photosynthetic rates. Our findings indicate warming threatens urban forests, which support people’s health and wellbeing in the world’s most populated habitats. Our findings also predict that global warming over the next century will lead to higher herbivore abundance and concurrent decreases in tree services.