Riparian systems in the southwest provide important ecosystem services for human society, including recreational values and maintenance of high biological diversity. Changes in groundwater or restoration can dramatically alter vegetation composition and structure in Southwestern riparian systems with subsequent impacts on wildlife populations. Here we quantify expected vegetation and avian changes related with groundwater or restoration scenarios as pro-active, scientific inputs for non-market ecological valuation studies. On the San Pedro, we used space-for-time substitution to quantify expected surface water presence, proportion of riparian forests and shrublands, and bird abundance as a function of groundwater change scenarios. We applied a parallel modelling strategy for benefit transfer on the middle Rio Grande to quantify vegetation and bird changes as a function of restoration scenarios.
Projected changes in vegetation floristics and physiognomy had different effects on bird guilds on the two rivers. Canopy-nesting birds were projected to decline with conversion of tall, riparian forests to shrublands on both rivers, and midstory and understory nesting birds responded strongly to different levels of understory clearing on the Rio Grande. In both rivers, increases in surface water presence or restored wetland increased the densities of water obligate birds. Replacement of cottonwood forests with exotic shrublands had opposite effects on migrating bird abundance on the two rivers, with decreasing abundance of spring migrants with increasing salt cedar on the San Pedro, but increasing abundance of migrants with increasing Russian olive on the middle Rio Grande. Despite the different mechanisms occurring on these two river systems, the vegetation and avian attributes that we considered were able to capture the substantial changes that could occur given different policy options and have important implications for restoration in the region. The vegetation, avian, and water outputs related with realistic policy scenarios provide the foundation for ecological valuation and more informed decision-making on both rivers.