Effects of ground water withdrawal and surface water depletion on riparian birds in the southwestern U.S.

Riparian woodlands in the desert southwest are an extremely important resource because they constitute <1% of the desert landscape, yet typically support >50% of the breeding birds. Ground water withdrawal to support recent urban developments has the potential to degrade riparian areas throughout the region. For example, riparian woodlands along the Upper San Pedro River in southern Arizona are currently threatened by ground water withdrawal from nearby Fort Huachuca Military Reservation, the City of Sierra Vista, and Cochise County.

Maintaining the health of riparian woodlands (and their associated bird communities) is a top priority for the agencies mandated to protect and enhance natural resources in the desert southwest. This is an especially important issue for Fort Huachuca and other military installations in the southwestern U.S. (e.g., Fort Hood, White Sands Missile Base). Groundwater withdrawal has the potential to curtail installations’ missions and reduce military readiness should ineffective action be taken to protect the health of vulnerable riparian woodlands.

Methods: We conducted this research in 29 replicate study sites located in riparian woodlands throughout southeastern Arizona (e.g., Aravaipa Creek, Cienega Creek, the San Pedro River, and the Santa Cruz River). These sites had extensive riparian vegetation but varied in the amount of surface water present and the extent and health of riparian vegetation. We surveyed birds, sampled vegetation, and measured surface water at each site from 2006-2008. We also sampled avian food resources (i.e., aerial arthropods) and monitored nests of riparian bird species at a subset of these study sites.

Our project objectives were to:

1) Gain a better understanding of the connections between ground water, surface water, and the health of riparian bird communities in the Southwest.

2) Examine the underlying ecological processes (e.g., food availability) that may influence these connections.

3) Develop models that will allow resource managers to predict how abundance, diversity, and breeding success of riparian birds will be affected by future reductions in ground and surface water levels in the Southwest.

Results: We found that presence and extent of surface water in Arizona’s riparian woodlands 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 were positively associated with surface water, including breeding birds like the Black Phoebe, Common Yellowthroat, Yellow Warbler, Song Sparrow, and Lesser Goldfinch, and long-distance, migrant birds like the Yellow-rumped Warbler and Wilson’s Warbler. In contrast, we found negative associations with surface water for several other riparian birds including Bell’s Vireo and Yellow-breasted Chat. These results provide some of the first quantitative data demonstrating the importance of surface water to many riparian birds in the southwestern U.S.

Results from our avian food sampling indicated that Diptera biomass was greater at “wet” versus “dry” study sites, suggesting a possible causal connection between increased surface water and increased bird abundance and diversity in riparian woodlands.

We also observed decreased nesting attempts by Bell’s Vireos and Yellow Warblers at one study site that appears to have resulted from extensive dormancy/die-back of riparian vegetation at the site. The exact cause of this tree dormancy/die-back remains undetermined, but almost 9 years of drought in the region may be a contributing factor.

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 (e.g., Black Phoebe, Common Yellowthroat, Yellow Warbler, Song Sparrow, and Lesser Goldfinch) and migrant (e.g., Yellow-rumped Warbler and Wilson’s Warbler) 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 breeding species such as Bell’s Vireos and Yellow Warblers are 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.

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Funding for this research has been provided by the Department of Defense Legacy Resource Management Program, the Arizona Game and Fish Department Heritage Fund, the Arizona Bird Conservation Initiative, the National Park Service, and the U.S. Geological Survey. Logistical support has been provided by The Nature Conservancy, the U.S. Bureau of Land Management, The U.S. Forest Service, Pima County Natural Resources Parks and Recreation Department, and the U.S. Fish and Wildlife Service. We thank the numerous technicians and volunteers who have made this project possible through their dedicated field work.

• LaRoche, D., Conway, C. J., Kirkpatrick, C. K., and D. Swann. 2009. "Evaluating the influence of surface water on bird diversity and abundance in southwestern riparian woodlands". American Ornithologist's Union annual meeting, Philadelphia, Pennsylvania.

• Kirkpatrick, C. , C. J. Conway, D. D. LaRoche, and D. Swann. 2009. "Quantifying impacts of surface water and ground water depletion on avian communities in desert riparian woodlands of the southwestern U.S.   DOD Sustaining Military Readiness Conference, Phoenix , Arizona". Poster.

• LaRoche, D., C. Kirkpatrick, and C. J. Conway.  2008.  "Quantifying impacts of surface water and groundwater depletion on riparian bird communities in the desert southwest". Public presentation, Cascabel, Arizona.

• LaRoche, D., C. J. Conway, and C. Kirkpatrick. 2008. "Evaluating the efficacy of sticky traps to measure arthropod abundance in riparian woodlands". American Ornithologists’ Union annual meeting, Portland , Oregon. Poster

• Kirkpatrick, C. , and C. J. Conway. 2007. "Quantifying impacts of ground water withdrawal on avian communities in desert riparian woodlands of the southwestern U.S.". DOD Sustaining Military Readiness Conference, Orlando , Florida. Poster.

• Kirkpatrick C., and C. J. Conway. 2006. "Effects of surface water depletion and ground water withdrawal on Arizona’s riparian bird communities". Arizona Game and Fish Department’s Arizona Bird Conservation Initiative annual meeting, Phoenix, Arizona.

• LaRoche, D., C.J. Conway, and C. Kirkpatrick.  2007.  "Evaluating the efficacy of sticky traps to measure arthropod biomass in riparian woodlands."  The Wildlife Society 14th Annual Conference, Tucson, Arizona.