Managing ponds may help save native salamanders

The California tiger salamander relies on seasonal ponds to breed. But for decades, the species has been under pressure from non-native salamanders introduced in the 1950s and 1960s for use as fishing bait.

These invasive barred tiger salamanders interbred with the native population, creating hybrids that grow faster, eat more, and often take over entire pond ecosystems.

Now, scientists have built detailed computer models to test how water duration, or hydroperiod, and targeted hybrid removal could help native salamanders recover. The findings, published recently in Ecological Applications, suggest that managing pond conditions may tip the balance in favor of native species. The web-based app to run the models is available online.

“Hybrids often consume nearly all other species in a pond,” said Robert Cooper, assistant professor in the Department of Fish and Wildlife Conservation and lead author of the study. “By the end of a season, you’re often left with only hybrids.”

Over three years, research team members studied salamander growth and survival in 18 experimental ponds. They combined genetic, ecological, and demographic data to simulate long-term outcomes under different scenarios, including hydroperiod changes and removal of hybrids using rapid genetic testing.

The results show that pond hydroperiod, an indicator of pond ecological trends, is the most important factor affecting salamander survival and population size, even more so than hybrid presence. Longer hydroperiods, typically over 110 days, lead to larger and more stable populations. But when ponds dry in fewer than 90 days, populations tend to collapse regardless of genetics.

“We often think of endangered species management as a series of challenges that require single solutions,” said Brad Saffer, a collaborator on the project and a distinguished professor in the Department of Ecology and Evolutionary Biology at UCLA and director of the UCLA La Kretz Center for California Conservation Science. “The California tiger salamander is an endangered species for which hybridization is an existential threat for which management of pond hydroperiod may be a practical solution."

“This study demonstrates that the optimal hydroperiod for management depends on the context,” Shaffer said.  “Longer hydroperiods are beneficial for pure California tiger salamander populations, but shorter ones can dull the negative impact of hybridization. It's not a one-size-fits-all solution. The paper also quantifies the conservation gains that will result when new field-based molecular technologies become available, and those gains are both substantial and worth the technological investment.”

Hybrid salamanders still perform better than natives across most conditions. That means changing water levels alone will not be enough to restore native populations.

The researchers found the most effective strategy is a combination approach. In areas with only native salamanders, keeping ponds wet for more than 120 days helps boost population growth. In hybrid zones, shortening hydroperiods limits hybrid success and makes them easier to remove. Pairing water management with rapid genetic testing and targeted removal can slow hybrid spread and protect native salamanders.

“Having hard numbers helps avoid strategies that might unintentionally drive extinction,” said Cooper, also an affiliate of the Global Change Center, Fralin Life Sciences Institute, and the Invasive Species Collaborative. “This gives managers a science-based way to prioritize actions.”

The model also showed that hybrid removal is most successful when done early and intensively. Efforts are far less effective if spread out over time or started too late. Researchers said new field-based genetic tools could make hybrid identification faster and more cost-effective.

“Early interventions and constant vigilance are critical if we want to manage the threat from this invasive species,” said Erin Toffelmier, assistant professor in the Department of Fish and Wildlife Conservation and affiliate of the Global Change Center. “Once hybrids become established, time becomes the enemy and delays compound the challenge and the cost to restoring the native species. Our goal should really be to act early to give the native salamanders some breathing room to recover on their own.” 

Building the models and gathering field data presented major challenges for researchers. Most suitable salamander habitat is on private land, and the team spent over a year securing permits and constructing ponds. Researchers then checked traps and recorded data daily from spring through summer for three years.

The results reveal just how stark the differences between hybrid and native salamanders can be, not just in behavior but in physical size.

“The size difference is dramatic,” Cooper said. “In side-by-side comparisons, hybrids are noticeably larger and more robust.”

Beyond salamanders, the study offers a road map for other conservation efforts. Hybridization is a growing concern worldwide as invasive species move into new areas. By integrating genetics, ecology, and population modeling, the Virginia Tech team has created a flexible framework that could be used to manage hybrid threats in other endangered species.

“The good thing is that these populations have a high capacity to rebound,” Cooper said. “As long as we act as good stewards of these systems, they can recover quickly.”

Original study: DOI 0.1002/eap.70116

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