Urban ecosystems cover less than 3% of the Earth's land surface, yet more than half of the human population lives in urban areas. The process of urbanization stresses biodiversity and other ecosystem functions within and far beyond the city. To understand the mechanisms underlying observed changes in biodiversity patterns, several observational and experimental studies were performed in the metropolitan area of Phoenix, Arizona, and the surrounding Sonoran Desert. The first study was comprised of seven years of arthropod monitoring using pitfall traps in common urban land-use types. This study revealed differences in community structure, diversity and abundance over time and between urban and wildland habitats. Urban habitats with high productivity had higher abundances of arthropods, but lower diversity compared to wildland habitats. Arthropod abundance in less-productive urban habitats was positively correlated with precipitation, but abundance in high-productivity urban habitats was completely decoupled from annual fluctuations in precipitation. This study showed the buffering capacity and the habitat heterogeneity of urban areas. To test the mechanisms controlling community diversity and structure in urban areas, a major field experiment was initiated. Productivity of the native shrub Encelia farinosa and bird predation of associated arthropods were manipulated to test whether bottom-up or top-down forces were more important in urban habitats compared to wildland habitats. Abundance, richness and similarity were monitored, revealing clear differences between urban and wildland habitats. An unusually cold and dry first season had a negative effect on plant growth and arthropod abundance. Plants in urban habitats were relatively unaffected by the low temperature. An increase in arthropod abundance with water availability indicated bottom-up forces in wildland habitats, whereas results from bird exclusions suggested that bird predation may not be as prominent in cities as previously thought. In contrast to the pitfall study, arthropod abundance was lower in urban habitats. A second field experiment testing the sheltering effect of urban structures demonstrated that reduced wind speed is an important factor facilitating plant growth in urban areas. A mathematical model incorporating wind, water and temperature demonstrated that urban habitats may be more robust than wildland habitats, supporting the empirical results.

This study examined crayfish diet within varying hydrologic environment in lotic systems using stable isotope analysis of crayfish and basal resources to add depth to previous findings. Crayfish are numerous and are omnivorous, opportunistic feeders, feeding on invertebrates, vegetation and detritus. Arizona streams stand apart from the Eastern and Northwestern aquatic ecosystems of the United States because Arizona has no native crayfish species. Two species have been introduced and become widely established in Arizona (Orconectes virilis and Procambarus clarkii), with concern for further introduction of crayfish species and more information on how these two species impact the native species in the streams is needed. Previous studies have focused on crayfish abundance with hydrologic variation and crayfish diets within a lentic system, but few have focused on how the diet of consumers varies with hydrologic variability. Crayfish are hardy and have a dramatically increasing population within Arizona and therefore inhabit systems with a wide range of hydrologic variability which may contribute to spatial variability. The results show that crayfish diets do show a significant level of seasonal variation in some study locations, in both C source and trophic level. Hydrologic variation was also shown to impact crayfish diet at several study sites, with increasing magnitude of event (both floods and droughts) correlating with a change toward more aquatic C sources and lower trophic position in several of the study sites. In some locations, the correlation was not as strong with variation and diet change and showed less change in C source and rather showed an increase in trophic position.