Novel water sources restore plant and animal communities along an urban river

Abstract

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

Artificial Aquatic Ecosystems

“…The impoundment of the Everglades behind Tamiami Trail, a road that interrupted sheet flow, is one prominent such example [28]. Similarly, humans accidentally transformed the bed of the Salt River, dried through damming upstream, into wetlands, at stormwater outflows [15]. Transformed waters generally retain their regulatory status [23,24] regardless of intent.…”

“…Water quality of artificial aquatic systems such as ditches responds to catchment land use in much the same way as that of waterbodies of natural origin [8,98], and agricultural land cover impacts reservoirs food webs [99]. In the Salt River in Arizona, level of urbanization explained much of the variance in communities of plants, birds, non-avian reptiles, and amphibians, for reference, restored, and accidentally restored river reaches alike [15]. A study in the Florida panhandle found that natural streams, altered streams, and ditches within the same forested region had similar macroinvertebrate and fish assemblages [100].…”

“…In many cases, the design goal itself can inherently produce a major ecological cost, as in wetland drainage by agricultural ditches [115,116], or can result in unintended disservices arising from synergies and trade-offs in ecosystem services [117,118]. Nonetheless, many designed artificial aquatic systems also provide a range of additional ecosystem services beyond the purpose of their design [15,89].…”

“…One indication of the limitations of many restoration projects is the finding that accidental aquatic systems can sometimes provide equal or greater services compared to deliberate, designed systems. For example, "accidentally restored" wetlands at stormwater outflows in the dry bed of the Salt River in Arizona had greater wetland plant richness and cover than comparable actively restored sites, though the reverse was true for birds, non-avian reptiles, and amphibians [15].…”

“…Thus, what artificial aquatic systems go unregulated may say as much about what we socially undervalue as what we ecologically undervalue. Relatedly, the same accidental wetlands that host birds and remove nitrogen in the Salt River in Phoenix, Arizona, provide somewhat unsafe, legally unauthorized sources of water and cool places to rest for homeless people [15,16,27], which calls into question the design of non-aquatic infrastructure whose functions may have been deputized to or externalized on artificial aquatic ecosystems. When science and policy overlooks artificial aquatic systems, it risks overlooking the people impacted by them as well.…”