Hybridization among ambystomid salamander species is common. Ambystoma tigrinum mavortum, the barred tiger salamander may have been introduced into California from released pets, or as fishing bait imported from the upper midwest. Whatever their origin, they have bred with the endemic California tiger salamander, Ambystoma californiense, a situation that has been well known for a number of years. In a new study Ryan et al (2012) find that in the intensively farmed Salinas Valley, California, the threatened California tiger salamanders (Ambystoma californiense) have been replaced by hybrids between California tiger salamander and introduced barred tiger salamanders (Ambystoma tigrinum mavortium).The authors conducted an enclosure experiment to examine the effects habitat modification and relative frequency of hybrid and native California tiger salamanders have on recruitment of salamanders and their prey, the Pacific chorus frogs (Pseudacris regilla). They tested whether recruitment differed among genetic classes of tiger salamanders (hybrid or native) and pond hydroperiod (seasonal or perennial). Roughly six weeks into the experiment, 70% of salamander larvae died in four out of six ponds. Native salamanders survived (n = 12) in these ponds only if they had metamorphosed prior to the die-offs. During die-offs, all larvae of native salamanders died, whereas 56% of hybrid larvae died. The authors necropsied native and hybrid salamanders, tested water quality, and queried the California Department of Pesticide Regulation database to investigate possible causes of the die-offs. Salamander die-offs, changes in the abundance of other community members (invertebrates, algae, and cyanobacteria), shifts in salamander sex ratio, and patterns of pesticide application in adjacent fields suggest that pesticide use may have contributed to die-offs. That all survivors were hybrids suggests that environmental stress may promote rapid displacement of native genotypes.
RYAN, M. E., JOHNSON, J. R., FITZPATRICK, B. M., LOWENSTINE, L. J., PICCO, A. M. and SHAFFER, H. B. (2012), Lethal Effects of Water Quality on Threatened California Salamanders but Not on Co-Occurring Hybrid Salamanders. Conservation Biology. doi: 10.1111/j.1523-1739.2012.01955.x