Amphibians and odonates in Sonoran Desert wildlife waters: issues of habitat quality

Background/Question/Methods

Wildlife waters are an important part of how wildlife managers support resources for populations of game as well as threatened and endangered species. In the southwestern United States, waters have been established for Desert Bighorn, Sonoran Pronghorn, and other species. To assess the impact these managed waters have on non-target organisms, we surveyed tinajas (natural or modified rock pools) and artificial catchments in the Sonoran Desert for organisms with complex life cycles, where the larval stage is aquatic and the adult stage is terrestrial. We used visual searches and dip net sweeps to sample man-made catchments and natural tinajas for tadpoles and dragonfly larvae. We also captured adult dragonflies at water sites. We quantified habitat quality by measuring chemical attributes of the water including pH, conductivity, temperature, nitrate, and ammonia; and physical characteristics of the site including shading, volume, and substrate. We calculated friction surfaces and least cost paths for amphibians and dragonflies separately to evaluate metrics of isolation on species richness.

Results/Conclusions

There were significantly more species of amphibians and dragonflies at natural waters than at constructed wildlife waters. Amphibians were negatively related to ammonia in the tinajas, and dragonflies were positively related to surface area. Additionally, dragonfly diversity was associated with connectivity of tinajas on the landscape; the pattern disappeared when artificial catchments were included in the analysis. Water quality was variable, but the most striking finding was that ammonia concentrations were extremely high. Ammonia concentrations in the constructed water sites are much higher than the National Recommended Water Quality Criteria set by the U.S. EPA. The reason that the levels of ammonia are so much higher in the constructed catchments versus the natural basins is likely that the tinajas are generally located in ephemeral streams that experience flash floods which scour out debris from the basin. The constructed catchments are built to reduce evaporation and catch and contain as much water as possible and do not flush out. Because of this design, they are at best inhospitable and at worst ecological traps for organisms that have aquatic life history stages.