Cities, smaller settlements and industrial areas are structurally complex ecosystems and characterised by their highly dynamic spatial structure. Diverse patch types include buildings, paved surfaces, transport infrastructure, parks and gardens; excavations, bare ground and refuse areas. Patches undergo periodic destruction and renewal. Human population density is high, relative to other ecosystems, and dependent on large subsidies of imported resources (particularly water, nutrients and food). Interactions among patch types and human social behaviours produce emergent properties and complex feedbacks among ecosystem components.