Urbanization is frequently cited as a major driver of species losses worldwide; however, most studies in urban areas use a space-for-time substitution approach to document effects of urbanization through time. Ultimately, understanding the effects of urbanization on biodiversity requires long-term datasets. We examined long-term changes in bird assemblages at 12 riparian sites in the greater Phoenix metropolitan area and nearby Sonoran Desert region, featuring a range of human modifications and levels of water flow. Riparian areas in arid cities represent a key habitat type that is sensitive to human modification and supports high levels of species diversity. We used long-term data to: (1) explore variation in bird communities as a function of water permanence and degree of human-modification; (2) identify which environmental variables best describe differences found across riparian site types; and (3) assess how riparian bird communities, abundance, and species richness have changed through time. Engineered riparian sites supported more broadly distributed generalists; whereas, natural riparian sites supported more specialists. Sites with perennial flows had more vegetation and water compared to ephemeral sites and engineered sites had more impervious surface compared to natural sites. In nearly all comparisons, bird species richness, diversity, and abundance declined across riparian types during the period of study, even for common species. Bird communities in natural settings have changed more than communities at engineered sites. Overall, the riparian bird community is shifting toward urban dwelling, resident species that are characteristic of riparian sites with less water and more impervious surface.
Many projects have been undertaken to restore urban rivers in arid regions. At the same time, passive discharge of urban water sources has stimulated redevelopment of wetlands and riparian forests along stretches of dewatered rivers. In Phoenix, Arizona, for example, some segments of the dewatered Salt River have been actively restored by planting and irrigation, whereas others have revegetated in response to runoff from storm drains and effluent drains. Our research documents how biotic communities differ between these actively restored and 'accidentally' restored areas, and between wetter and drier urban reaches. We addressed these objectives with a multi-taxa, multi-season sampling approach along reaches of the Salt River. We quantified plants using cover estimates in quadrats, birds using fixed radius, point-count surveys, and herpetofauna (amphibians and reptiles) using visual-encounter surveys. One notable finding was that wetland plants had greater richness and cover at accidentally restored sites compared with actively restored, dry urban, and non-urban reference sites. Birds and herpetofauna, however, were most species-rich at actively restored and non-urban reference sites, and riparian birds were more abundant at sites with perennial flows compared with ephemeral reaches. From a landscape perspective, the range of management approaches along the river (including laissez-faire) is sustaining a diverse riparian and wetland mosaic. Urban water subsidies are sustaining freshwater forests and marshlands, the latter a regionally declining ecosystem. In urbanized rivers of arid regions, mapping and conserving perennial stream flows arising from stormwater and effluent discharge can be an important complement to active restoration. Figure 2. Species accumulation curves for (A) vascular plants, (B) bird species, and (C) herpetofauna species along the Salt River in central Arizona. Plants were surveyed during the pre-monsoon dry season. Birds and herpetofauna were surveyed during warm seasons (March to September). 798 H. L. BATEMAN et al.Figure 5. Non-metric multidimensional scaling (NMDS) graph showing locations of seven vegetation sampling sites in each of two seasons (July and September). (A) NMDS axis 1 separates plant species (small dots) by flow permanence, and (B) NMDS axes 2 and 3 separate species by degree of urbanization. 802 H. L. BATEMAN et al.Figure 6. Non-metric multidimensional scaling (NMDS and SE ellipses) graphs for bird species (plotted as four-letter codes; Appendix 3) sampled among seven river reaches. (A) NMDS axis 1 separates bird species by season, with most waterfowl and marshland birds being abundant during winter (ellipses: fall is black, winter is grey, spring is grey dotted, and summer is black dotted). (B) NMDS axis 2 separates bird community by amount of flow, with urban dry sites most dissimilar to other reaches. 803 URBAN NOVEL WATER SOURCES
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