The appearance of American bullfrog DNA has been detected not far from the sole known habitat of Pithecopus rusticus, a diminutive and critically endangered tree frog native to Brazil. While bullfrogs originate from the eastern United States, they've become invasive in other regions. Uncovering genetic traces of these intruders in the elevated grasslands of Santa Catarina suggests potential trouble for P. rusticus, as detailed by researchers in the March edition of the Journal for Nature Conservation.
These vibrant frogs, adorned in neon green, orange, and black hues, are petite in size. Adult males measure around 35 millimeters from snout to vent, akin to the span between a golf ball and a quarter. Despite the species' initial discovery in the Água Doce community of Santa Catarina state in 2009 and its subsequent description in scientific literature in 2014, efforts to locate additional populations through surveys in similar high-elevation grasslands nearby have proven futile.
"Given the swift decline of habitat quality and the limited population size, there's a pressing imperative to locate additional populations for conservation endeavors," emphasizes ecologist Julia Ernetti from the State University of Campinas in Brazil.
Facing fruitless field surveys, Ernetti and her team pursued an alternative approach: detecting traces of the frogs' DNA within the environment. In December 2020, coinciding with the breeding season for most amphibians, they collected 24 water samples in and around the Wildlife Refuge of Campos de Palmas. Positioned approximately 2 kilometers from the initial discovery site of P. rusticus and boasting similar environmental conditions, the refuge held promise as a potential habitat. However, it remained isolated from the known population by a thoroughfare.
Despite thorough analysis, the team failed to unearth any DNA belonging to P. rusticus. Nonetheless, the examination did unveil indicators of the presence of the American bullfrog (Lithobates catesbeianus) within the refuge. "The presence of DNA traces acts as a warning sign of their potential occurrence and the plausible detriment to indigenous species," Ernetti underscores.
While startling, the discovery of frog traces in the refuge is hardly surprising, remarks Ernetti. She explains that farmers introduced bullfrogs to Brazil in 1935 for commercial purposes, leading to documented farmed and wild populations in Santa Catarina and its neighboring state, Paraná.
The situation is dire, asserts Nathan Snow, a wildlife biologist at the U.S. Animal and Plant Health Inspection Service in Fort Collins, Colorado. "Bullfrogs epitomize invasiveness: they're adaptable omnivores, outcompeting native amphibians for resources and reproducing prolifically."
In addition to competing for resources and territory, bullfrogs pose the risk of spreading diseases like the lethal chytrid fungus, which has decimated amphibian populations throughout the Americas. Furthermore, their calls may disrupt communication among native species. "Any disruption to this communication could directly impact native species' breeding success, potentially leading to population declines and heightened extinction risk," Ernetti explains.
The potential for bullfrogs to prey on P. rusticus further threatens its already fragile existence, though no evidence of such predation has been observed yet. "The precarious existence of P. rusticus means any losses due to bullfrogs would pose a significant problem," Ernetti emphasizes. "Our detection underscores the urgent need for effective strategies to detect and eradicate exotic and invasive species, aiming to safeguard native species."
However, as Snow notes, removing established bullfrog populations is incredibly challenging. "There are limited, if any, successful methods to manage invasive bullfrogs," he and wildlife biologist Gary Witmer highlighted in a 2010 review. "For sensitive ecosystems like the subtropical highland grasslands of Brazil," Snow warns, "a bullfrog invasion could prove devastating for endemic and endangered amphibians."
