Both increases and decreases in diversity have been documented in urban areas, with suggested explanations ranging from species-productivity relationships to habitat fragmentation, introduced species, disturbance and pollution. For landscape planning and urban wildlife management, it is imperative to understand which processes act in determining diversity in urban habitats. For over ten years we have monitored arthropod communities with pitfall traps in the Central Arizona Phoenix area in residential areas and compared them to desert and desert remnant sites. We document both increased and decreased diversities, depending on habitat type. Residential mesic yards had high abundances and similar high diversity fluctuations as desert habitats, albeit with different community compositions. Residential xeric yards and desert remnants experienced dramatic declines in diversity over the ten year period, possibly explained by landscaping practices and island biogeography, respectively. Our results stress the complexity and seemingly unpredictable effects of urbanization on arthropod diversity superimposed on seasonal and regional climatic patterns.