Rampant Bullfrog Invasion Upsets Western Ecosystems

American bullfrogs are voracious predators, devouring even the rarest endangered species in the Western United States.

In the Midwest, summer evenings are filled with the distinctive sounds of crickets, cicadas, and bullfrogs. Their resonant croaks are reminiscent of foghorns or the deep tones of a didgeridoo.

In this region, such sounds are not new. Bullfrogs are indigenous to the Eastern US, extending from the Mississippi River to the Atlantic, thriving naturally as part of the native ecosystem. Growing up in Iowa, I had a fondness for these creatures and spent countless summer days trying to catch them.

Interestingly, in states like Colorado and California, summer nights echo similarly with bullfrog calls. However, these regions are not their natural habitat. Here, bullfrogs pose a threat, endangering native species such as the Oregon spotted frog, exclusive to the Pacific Northwest.

Bullfrogs were introduced to the Western US over a century ago, mainly as a food source. Over time, these frogs, which can grow to the size of a small cat, have expanded their presence, invading countless waterbodies, and consuming whatever fits their vast mouths. Conservationists consider them among the gravest invasive threats in the West and beyond.

Uniquely, the bullfrog is both a native and invasive species within the same country, depending on which side of the geographic divide they inhabit. This duality makes their treatment complex—they can be pests in one state yet a natural ecosystem component in another.

While bullfrogs undoubtedly cause harm, they represent a larger problem rather than the root cause itself. Their presence exacerbates what researchers term the 'Starbucksification' of nature, where environmental homogeneity prevails, resulting in diminishing ecosystem resilience.

Bullfrogs’ Western Takeover

A peculiar culinary interest helped pave the way for the bullfrog invasion.

During the Gold Rush era of the 1800s, a surge in popularity for frog legs, a delicacy linked to high-end French cuisine, arose in the West. Initially, local amphibians like the California red-legged frog were harvested to meet demand. However, as native populations diminished due to overharvesting, pragmatic entrepeneurs sought to farm bullfrogs from the East.

There was a brief period where bullfrog farming seemed viable.

In a 1922 issue of the Riverside Daily Press, a journalist proclaimed the successful breeding of bullfrogs in California, predicting a flourishing industry tied to their farming.

Yet, these aspirations fell short. Bullfrog farming proved to be formidable and unprofitable: they take extensive time to raise, necessitate enormous amounts of live feed, and are susceptible to diseases. Worse still, they yield little meat in return for these efforts.

Despite the industry not catching on, the bullfrogs themselves certainly did.

Escaping confines and introduced both deliberately and accidentally, bullfrogs proliferated across the Western landscape. They colonized ponds, lakes, and other aquatic habitats, alarmingly abundant in regions such as Arizona, California, and the Pacific Northwest. One pond can house thousands of these amphibians, although precise population figures remain elusive.

Biologically equipped for expansion, female bullfrogs can produce up to 25,000 eggs—vastly more than native species.

Human modifications of Western landscapes further facilitated their spread. Permanent warm water resources became widespread only with increased agriculture and irrigation demands, according to invasive species researcher Tiffany Garcia. This altered environment, with its reservoirs and canals, proved to be to the bullfrogs’ liking.

“Human activity has created ideal conditions for these frogs,” Garcia observed. They’ve become entrenched thanks to landscape changes instituted by settlers and resource users.

Reflecting on the issue, Garcia suggested reframing our perspective: rather than viewing it as an invasion by a single species, it's an invasion community where bullfrogs co-exist and even cooperate with other non-natives.

Predatory Bullfrogs on the Loose

Bullfrogs are known for their indiscriminate consumption habits—anything they can swallow, they will.

This poses a serious risk to already vulnerable species such as the Chiricahua leopard frog and the northwestern pond turtle, nudging them closer to extinction.

According to Sidney Woodruff, a researcher, bullfrogs contribute to the decline of native reptiles and amphibians, alongside factors like habitat loss and climate variation.

In a study focused on Yosemite, Woodruff noted that areas populated with bullfrogs saw fewer native pond turtles. In places where bullfrogs were present, only the more substantial turtles survived.

The research underscores the significant threat posed by the bullfrogs to native pond turtle populations.

Invasive bullfrogs often create heightened struggles for native creatures, especially when living alongside other invasive species, as explained by JJ Apodaca. Non-native crayfish, extensively feeding on plants, make native organisms even more susceptible to bullfrog predation.

Bullfrogs also contribute to disease transmission. One notable pathogen, chytrid fungus, linked to declines in over 200 amphibian species globally, has been associated with bullfrogs in the region.

Eradicating Bullfrogs Isn’t Straightforward

Attempting to exterminate bullfrogs entirely isn’t a practical solution for Western ecosystems. They have already established themselves widely.

Garcia noted the impracticality of such efforts, pointing out that bullfrogs, once removed, frequently return. They are firmly embedded in transformed landscapes.

Even a successful eradication wouldn’t restore ecosystems to their pre-invasion states, as bullfrogs remain both a cause and a reflection of broader environmental changes.

Echoing this sentiment, Brendon Larson remarked on the irony of modern agricultural practices favoring non-native species, only for us to express surprise at their success.

Inaction isn’t advisable either, as unchecked bullfrogs pose ongoing threats to vital native species.

The recommended strategy is targeted removal where high-value species are endangered or conservation efforts are underway, reducing competition and predation pressures.

Woodruff’s research in Yosemite highlights this approach's success. By drastically reducing bullfrog numbers in certain waterbodies, her team observed increased survival and breeding of native pond turtles, offering hope for population recovery.

Following the bullfrog cull, a return of native fauna, including frogs, salamanders, and snakes, was observed, along with a rejuvenated acoustic environment with the calls of native chorus frogs resounding once again.

Although complex, managing bullfrogs is necessary. For Woodruff, it’s especially challenging, given her Southern roots where bullfrogs are native, yet necessary as conservation for native species remains the priority.

“I cherish these frogs,” Woodruff admitted. “They’re simply following their instincts to survive. But my commitment lies with preserving native species.”