Effects of Drought and Surface Water Depletion on Birds in Desert Riparian Woodlands

Riparian woodlands in the desert southwest are an extremely important resource because they constitute <1% of the desert landscape, yet typically support incredibly high species diversity. Climate change models predict less winter precipitation in the southwestern U.S. and higher frequency and intensity of drought. Moreover, ground water withdrawal (and subsequent loss of surface water) to support the growing human population in the desert southwest has the potential to further degrade or eliminate riparian woodlands throughout the region. The goal of this study is to quantify the value of riparian woodlands to the health and persistence of riparian bird communities. Specifically, we quantified the extent to which surface water influences the abundance, diversity, and reproductive success of breeding birds that inhabit riparian woodlands. We found that riparian areas contained 68% more species and 75% more individual birds compared to adjacent desert vegetation, with this pattern holding true for both the breeding and non-breeding bird communities. We found that the presence and extent of surface water was positively associated with both total relative abundance and species richness of riparian birds. At the species level, we found that the majority of riparian birds analyzed were positively associated with surface water. We found negative associations with surface water for several other riparian birds. Arthropod biomass increased at sites with increased water. Riparian trees with decreased water stress had more arboreal arthropod biomass compared to riparian trees with increased water stress and we found that aerial arthropod biomass of flies (Diptera) appeared to be positively associated with increased extent of surface water. These results suggest a possible causal connection between increased water and increased bird abundance and diversity in riparian woodlands within our study area. We also observed decreased nesting attempts by Bell’s Vireos and Yellow Warblers at one study site (Rincon Creek) that appear to have resulted from extensive dormancy/die-back of riparian vegetation at the site. We believe that riparian bird communities in Arizona are threatened in 2 ways by future water loss.

First, should long-term drought conditions persist and/or ground water levels fall to the point where surface water flows are reduced or eliminated, populations of breeding and migrant bird species are likely to decline. Second, should long-term drought conditions persist and/or ground water levels fall to the point that riparian vegetation is negatively affected, populations of other species are also likely to decline. Results from our study provide quantitative data that will allow resource managers to better predict how the abundance and diversity of riparian birds will be affected by future reductions in ground and surface water levels in the desert southwest.