The Curious Case Of The California Tiger Salamander

Credit: Jon Page

Wednesday, March 10th, 2010--

A salamander native to Texas is wreaking havoc on the wetlands of Northern California. Introduced into California 60 years ago by bait dealers, Barred Tiger salamander (Ambystoma mavortium) larvae are voracious eaters that grow faster and larger than the salamander living there, the native California Tiger salamander (Ambystoma californiense).

Over 20 salamander generations, natural crossbreeding between the two species has formed hybrid salamanders, which together with the invasive salamander are devouring Pacific tree frogs, California newts and fairy shrimp, prey that share the same aquatic breeding sites.

“All tiger salamanders are predators that pretty much eat whatever fits in their mouths,” says Ben Fitzpatrick, a professor of Ecology and Evolutionary Biology at the University of Tennessee and lead author of a study looking at the spread of invasive genes into a threatened native species.

In their adult form, invasive Barred Tiger salamanders are often mistaken for the native species. However, the invasive larvae – also known as waterdogs – have a longer larval period and tend to grow larger, swallowing tadpoles and other prey that are often too large for the native salamander larvae.

Fitzpatrick and his colleague Brad Shaffer, a professor of Evolution and Ecology at the University of California Davis, are interested in tracking the interactions of the two salamander species on a genetic level. They say that the Tiger salamander is a case study of how two species, which otherwise would never have met, have hybridized and altered native wetland food webs.

Using genetic markers specific to each species, the team was able to determine the extent of the hybrid salamander’s range. The invasive Barred Tiger salamander has introduced three genes into the population that have now spread up to 100km from the hybrid zone, the area where the first invasive salamanders were released. The team has dubbed these genes “superinvasive” due to their extensive proliferation, but doesn’t know what biological processes they influence.

Fitzpatrick and his colleagues point out that scientists don’t necessarily consider this spread of genes ecologically detrimental. “Natural gene exchange shouldn’t be considered a threat from a conservation perspective,” warns Fitzpatrick. “But this situation is the result of an unnatural introduction where an invasive species is having an objectively negative impact on the native ecosystem,” he adds.

“This is a really difficult issue for conservationists because hybridization blurs the distinction between native and introduced populations,” says Fitzpatrick. But for scientists it’s an opportunity to track what natural selection favors and how it works on a gene-by-gene basis.

- Aleszu Bajak