Arizona removed invasive bullfrogs and rebuilt native frog habitat; Chiricahua leopard frog breeding sites grew from 7 in 2013 to 19 by 2019

Most people only think about frogs when one begins to croak outside their bedroom window. A small spotted frog underwent a long process of second chances in the Southeastern Arizona region. According to the US Fish & Wildlife Service's case study on restoring leopard frog habitat, partners in the Cienega Creek watershed eradicated invasive American bullfrogs and re-established suitable habitats for the threatened Chiricahua leopard frog, whose population had proliferated by the end of the tracking period, albeit with fine print.

The effort was a multi-faceted one, not carried out by a single agency. The Frog & Fish Restoration Outreach Group ran the program from 2010–2013, and the Cienega Watershed Partnership continued it under the same name through 2019. The effort involved partners such as the University of Arizona, Arizona Game and Fish Department, Bureau of Land Management, and the US Forest Service.

The bullfrog problem


The villain in this story is not indigenous to Arizona. The invasive American bullfrogs were introduced to the Cienega Creek watershed in the mid-1980s and were associated with local declines of the leopard frog, primarily due to predation and disease transmission, with chytrid being a particular concern. Bullfrogs are resistant to the effects of chytrid but able to act both as vectors and predators.

The problem extended far beyond a single watershed. Researcher Peter Daszak and colleagues, in a 1999 review article titled "Emerging infectious diseases and amphibian population declines," published in the journal Emerging Infectious Diseases, found that chytridiomycosis and other emerging wildlife diseases had been implicated in a large proportion of amphibian declines across the globe, and California research from earlier in the decade reached similar conclusions: Sarah J. Kupferberg's 1997 study, "Bullfrog (Rana catesbeiana) invasion of a California river: the role of larval competition," published in the journal Ecology, found that bullfrog tadpoles reduced the survival rates of native yellow-legged frog tadpoles by 48% in experimental enclosures.

The bullfrogs were the hard part

Bullfrogs were eradicated from the Cienega Creek complex in 2013, though the process involved anything but simple steps. Efforts focused on intensive adult and juvenile removals during early-season breeding events to prevent recruitment of new frogs into viable breeding populations. Field crews drained ponds and cattle tanks and searched them for frogs, pulled tadpoles out with fine mesh traps, and shot adults with .22-caliber rifles at night.

Vigilance continued even after the bullfrogs were gone. Researchers kept watch on a network of stock tanks located in a buffer zone alongside Highway 82 to watch for any bullfrogs attempting to recolonize the region from the valleys to the north and south.

The case study also notes the contributions of local ranchers, with whom researchers established partnerships to provide water infrastructure serving both the needs of cattle and local wildlife. Captive rearing and reintroduction were not simply a post-eradication afterthought: head-starting at the Phoenix Zoo and releases into Cienega Creek sites, alongside wild-to-wild translocations, ran alongside the eradication effort through 201 3 to keep leopard frog populations from disappearing entirely while bullfrogs were still being cleared out. Over the course of the project, partners raised and released 4,769 captive-reared frogs into ten new sites.

Growth, with an asterisk

Here is where the good news begins. The area of reintroduction expanded from seven breeding sites spanning roughly 42 square kilometers in 2013 to 19 sites encompassing nearly 84 square kilometers by 2019, about 1 6 to 32 square miles. But here is where the fine print begins. Chytrid fungus was first detected in the area in the fall of 2013, when a large, sudden die-off of frogs hit the wildlife ponds. By 2019, high adult frog mortality from disease had been reported at the constructed ponds, while frogs at spring-fed sites were surviving the winter more successfully. The case study notes the potential for bullfrogs to return, or for an increased impact from chytrid, either of which could undermine the gains made by that point.

Why the frog still needs protection

The Chiricahua leopard frog was listed as threatened under the Endangered Species Act in a final rule published on June 13, 2002, under which the Fish and Wildlife Service designated the species threatened throughout its range. As of the project's 2019 findings, continued monitoring and bullfrog removal were judged necessary to sustain the frog's recovery in the Las Cienegas area, a hopeful note, and a reminder that while frogs may need a second chance, so too do we all.