Bait, Bite, and Beware: The Complex Web of Invasive Species Management in Guam

Written by Hazel Quarterman and Katrina Ford

Invasive species management is expensive and a growing global issue.  According to a new report by the Intergovernmental Platform on Biodiversity and Ecosystem Services, more than 37,000 plants and animals are in places they do not belong, costing an estimated $423 billion annually.

In Guam, there are many invasive species, including the brown tree snake (Boiga irregularis), the cane toad (Rhinella marina), and rodents such as the Pacific Island Rat (Rattus exulans), and the Black rat (Rattus rattus). Since their introduction, these invasive animals have proved to be problematic, both financially and from a conservation perspective. In Guam, it costs nearly $4 million a year in repairs and eradication efforts to manage invasive species. The brown tree snake, BTS, in particular, has become a significant area of focus for natural resource managers.

To mitigate damage to the ecosystem and economy, baiting programs have been implemented to control the population. For instance, dead mice packed with acetaminophen (Tylenol), a substance toxic to BTS, are used to target and eliminate resident BTS. Using toxicant-laced baits for BTS and rodents has been successful but controversial in Guam. A toxicant is a toxin that has been introduced into an environment.

A brown tree snake (BTS) scavenges in Guam a night. (Photo curtesy of Jacob E. Hill)

It works, but are there consequences

While using toxicants to reduce BTS and rodents in Guam seem to work, questions remain about this approach. Some researchers, like Olin “Gene” Rhodes Jr., director of the University of Georgia Savannah River Ecology Laboratory, is concerned about the magnitude of other species that scavenge BTS and rodents. 

“We should think about the consequences of our actions before we undertake them and be cognizant that there may be very negative, unintended consequences,” Rhodes states. 

Rhodes and colleagues from other institutions released a study focusing on the indirect impacts toxicant baits may have on the scavengers feeding on BTS and rodent carcasses in Guam.  Using BTS and rodent carcasses, researchers experimented to determine what scavengers living in Guam would eat the remains. After more than 1,300 trials and a year of data collection, the scientists determined that both invasive and native species were scavenging BTS and rodent carcasses.

Of the nine scavenger species identified, the most dominant were themselves invasive species. Most feedings recorded were by cane toads and brown tree snakes.

Jacob E. Hill, a graduate student at the University of Georgia’s Savannah River Ecology Laboratory with the Rhodes’s Laboratory adds, “an unanticipated finding of this study was the extensive scavenging by cane toads. These results suggest that due to their scavenging behavior, invasive species may have a more substantial role in food webs than previously acknowledged.”

Researchers also observed limited feeding by native hermit crab (Calcinus guamensis) and coconut crab (Birgus latro). Scientists are unsure how the toxicant may affect native crabs.  All of these findings pose additional conservation questions.

If we do not practice sound environmental stewardship and only focus on what we perceive to be good for us, we often pay the costs of the unattended consequences we create,” adds Rhodes.

The study was completed by Jacob E. Hill, Kelsey Turner, Josh Bradley Smith, Matthew T. Hamilton, Travis L. DeVault, William C. Pitt, James C. Beasley, and Olin E. Rhodes.