Effects of Salinity and Density on Tadpoles of Two Anurans from the Rio Salado, Puebla, Mexico

Numerous studies have explored environmental factors that drive the distributions of anuran larvae. However, the causal links between physical or chemical factors and tadpole distributions often remain untested. The Rio Salado is an intermittent, naturally saline river in Puebla, Mexico, that is increasingly being impacted by removal of water for commercial salt production. Using tadpoles of Exerodonta xera (Hylidae; Puebla Treefrog) and Incilius occidentalis (Bufonidae; Pine Toad), we experimentally examined the hypothesis that the distribution of tadpoles along the Rio Salado results from the effects of salinity on tadpole survivorship, growth, and metamorphosis. We also examined the effect of tadpole density and the interaction of tadpole density and salinity, because pool size likely affects tadpole density. Increased salinity did not affect survivorship in I. occidentalis but reduced survivorship in E. xera by approximately 20% at both 0.4 parts per thousand (ppt) (0.4 g L-1) and 0.8 ppt (0.8 g L-1). Higher salinities delayed metamorphosis by up to 9 wk in E. xera and by 3 or 4 d in I. occidentalis. Tadpole density had a nonlinear effect on survivorship in E. xera, and higher densities delayed metamorphosis in I. occidentalis. There were no significant interactions between salinity and tadpole density in either species. Our results suggest that even though they can be found in pools averaging 0.8 ppt, tadpoles may not emerge from such pools because of delayed metamorphosis at salinities around 0.8 ppt. Decreasing pool volume and increasing tadpole density may further delay emergence of I. occidentalis. The removal of water from the Rio Salado to produce salt may threaten amphibians that use the river for breeding.