Plant-arthropod-bird relationships in a desert city

Background/Question/Methods

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 includes seven years of arthropod monitoring using pitfall traps in common urban land use types. To further test mechanisms controlling community diversity and structure in urban areas, we manipulated plant productivity and bird – arthropod relationships associated with the native shrub Encelia farinosa over two years. This aimed to test whether bottom-up or top-down forces were more important in urban habitats compared to wildland habitats. Finally, we tested the sheltering effect of urban structures on urban plant growth. 

Results/Conclusions

The first study revealed distinct differences in community structure, diversity and abundance over time and between urban and wildland habitats. Urban habitats with high productivity potential 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 demonstrates a remarkable buffering capacity and habitat heterogeneity of urban areas. For the manipulation experiment, abundance, richness and similarity were monitored, confirming clear differences between urban and wildland habitats. An unusually cold and dry season had a negative effect on plant growth and arthropod abundance in outlying areas, whereas plants in urban habitats were relatively unaffected by the low temperature. An increase in arthropod abundance with water availability suggests bottom-up forces in wildland habitats, whereas the bird manipulations suggest that bird predation may not be as prominent in cities as previously thought. In contrast to ground dwelling arthropods, arthropod abundance on the shrubs was lower in urban habitats. Our last experiment demonstrated that reduced wind speed is an important factor facilitating plant growth in urban areas. Our studies demonstrate novel features of the urban environment, and point out further research directions.