Hybrid "Superpredator" Invading California Ponds

Christine Dell'Amore
National Geographic News
June 29, 2009
Mating between the rare California tiger salamander and the introduced barred tiger salamander has created a monster—at least for animals that dwell in the ponds of California's Salinas River Valley. (See a map of the region.)

The new hybrid "superpredator" grows larger than either of its parent species, and its bigger mouth enables it to suck up a wide variety of amphibian prey, said lead study author Maureen Ryan, of the Center for Population Biology at the University of California, Davis.

Mostly on the menu are smaller pond species, such as the Pacific chorus frog and the California newt—both of which were "dramatically reduced" in population by the hybrid in the experiments.

This may be the case in natural ponds as well, Ryan said.

"[The hybrids] seem to be more voracious and a little more aggressive," Ryan said. "Just watching their behavior, they'll go after each other and the other prey."

Cannibal Ambush

Barred tiger salamanders were introduced to California in the 1940s and '50s from Texas.

Hybrids of the invaders and native salamanders now occupy about 20 percent of the indigenous amphibian's range in the Salinas Valley.

The native California species is listed as endangered in Sonoma and Barbara Counties and threatened in the rest of its range under the U.S. Endangered Species Act.

(Related: "Interspecies Sex: Evolution's Hidden Secret?")

To find out how the hybrid is impacting local ponds, Ryan collected tadpoles (juvenile frogs), larvae (juvenile salamanders) and eggs from various species from various sites within the valley and observed them in outdoor experimental ponds.

She and her team found that the hybrid larvae not only ate other amphibians, they also preyed on the native species' larvae.

Ryan's previous research has shown that the hybrid larvae even deploy an ambush strategy: When something swims by, the creatures attack and "jump and suck at the same time," she explained.

All aquatic salamanders are suction feeders, but the hybrids are more effective because of their large size, she added.

The hybrids have another strange adaptation, she added: Tadpoles will sometimes develop extra rows of teeth and become cannibals, something not seen in the native species.

Also at Risk

Other amphibian species are in danger if the hybrid's range continues to spread throughout the valley.

For instance, the Santa Cruz long-toed salamander, also listed as endangered in the U.S., lives in a very small range in Monterey County.

If hybrids moved into this area, they "could put a serious dent into the whole global population of the [long-toed] salamander," Ryan said.

Karen Lips, an amphibian biologist at the University of Maryland, College Park, said in an email that "the results of the paper show that the hybrids are having a significant impact on the other amphibians in these ponds."

And there are other examples in which salamanders have become top predators, added Lips, who was not involved in the research. In woodland ponds, for example, the amphibians dictate the populations of insects and other invertebrates.

Ethical Quandaries

Getting rid of the hybrid poses "ethical quandaries," study leader Ryan said.

"From a conservation perspective, there [are] a lot of deep questions about what to do with this," she said.

After all, the hybrid is part endangered species, so "do we protect [them] because they're part native?"

Overall, Ryan said, her "real concern" is for the survival of California's native salamander, which has shown to be no match for the half-Texan interloper.

The hybrid's more aggressive predation "benefits the hybrid and harms the native, speeding up the process of converting populations into more hybrids."

Research appears this week in the journal Proceedings of the National Academy of Sciences.

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