No plants or animals evolved in cities. Some insects, like many tree pests, thrive in cities. Others, like many bees, become sick or scarce or disappear all together. Despite the challenges, cities actually support a surprising amount of biodiversity and important ecosystem functions. For example, pollination and decomposition carry on in cities, albeit with a novel cast of characters. Trees and plants support urban food webs but are often too few or in too poor condition to provide all the ecosystem services we rely on. We study how urbanization affects trees, plant pests, and beneficial insects to sustain biodiversity and ecosystem services.