Diversity, biogeography, and conservation of aquatic animals in southern Sonoran Desert oases

Authors: Michael T. Bogan, Nohemí Noriega-Felix, Sylvette L. Vidal-Aguilar, Lloyd T. Findley, David A. Lytle, Oscar Gutierrez-Ruacho, Andrés Alvarado-Castro, Alejandro Varela-Romero

Year: 2012 (xliv)

Abstract

The arid El Aguaje, Santa Ursula, and Bacatete mountain ranges in southern Sonora, México contain numerous canyons supporting spring-fed aquatic habitats, isolated from one another by formidable volcanic cliffs and vast expanses of desert. While the relict tropical vegetation in such canyons has been previously studied, their aquatic life forms have received very little attention. In this study, we mapped sites in these mountain ranges that support freshwater habitat, identified distinct microhabitat types at each site, and inventoried their aquatic biota. We identified eight sites with perennial freshwater in the Sierra El Aguaje in 2008 and an additional 11 sites in the sierras El Aguaje, Santa Ursula and Bacatete in 2009. These 19 sites supported more than 220 taxa of aquatic animals, with richness at individual sites ranging from 10 to 123 taxa. We delineated four microhabitat types (oasis, tinaja, riffle, and seep) which each supported distinct aquatic invertebrate communities. Communities were not distinct by mountain range, but rare species were occasionally found only in a single mountain range. Aquatic vertebrates were not diverse across the study area but were often locally abundant, especially big-eyed leopard frog (Rana magnaocularis). Two native freshwater fishes, desert chub (Gila eremica) and Sonoran topminnow (Poeciliopsis occidentalis), were found at two sites each, and exotic (introduced) tilapias were found at all four sites in the sierras Santa Ursula and Bacatete. Tilapia presence was strongly associated with a shift in aquatic invertebrate community structure and reduced invertebrate density and species richness. The condition of these diverse aquatic habitats ranged from nearly pristine to heavily impacted. Groundwater withdrawal, increasing human visitation, and introduction of exotic fishes are all significant threats to the diversity of aquatic organisms in these habitats.