Project Highlight: Modeling Brown Treesnake Management Strategies

By Kylie Baker

The Brown Treesnake Problem in Guam

The accidental introduction of brown treesnakes to Guam after World War II, likely via U.S. military transport, has had devastating effects on the island’s ecosystem. These invasive, nocturnal, and arboreal predators have wiped out many native bird populations, leaving only a few forest species on the island. Though efforts have been made to prevent the spread of brown treesnakes to areas like Hawaii, removing them from Guam itself has been a logistical challenge. Their elusive nature and resistance to conventional eradication methods make large-scale removal efforts difficult and expensive. To effectively manage this invasive species, a combination of innovative methods and strategic planning is required.

**Brown Treesnake**. *Photo Credit: Björn Lardner*

Development and Application of the Computer Model

A key part of these efforts is a Pacific Northwest Cooperative Ecosystem Studies Unit (PNW CESU) project led by Dr. Sarah Converse, principal investigator from the U.S. Geological Survey’s Washington Cooperative Fish and Wildlife Research Unit at the University of Washington. University of Washington research scientist Ms. Kelly Mistry and former post-doctoral scientist Dr. Staci Amburgey, who is now with the Washington Department of Fish & Wildlife, have also played crucial roles, with Amburgey contributing three years of foundational research before Mistry took over in the fall of 2022.

In collaboration with Dr. Amy Yackel Adams of the U.S. Geological Survey’s (USGS) Fort Collins Science Center, Mistry has developed a cutting-edge computer model to simulate various eradication strategies. This model, based on past research, simulates snake population dynamics—growth, reproduction, and natural mortality—at the population level, rather than focusing on individual snakes. This approach allows researchers to evaluate the effectiveness of different control methods without the logistical and financial burden of real-world testing. These methods include: manual snake capture, trapping, baiting, and aerial toxicant delivery.

The project’s main goal was to develop a model capable of predicting the likelihood of successfully eradicating snake populations under various management strategies, along with evaluating the effect of these strategies on the abundance of snakes large enough to prey on birds. While testing these strategies is ongoing, Mistry’s work demonstrates the model’s functionality in simulating real-world scenarios and assisting land managers in their decision-making processes.

Testing Strategies and Preliminary Results

Each method tested in the model has pros and cons, including differences in cost, which must be weighed against their effectiveness. Manual snake captures have been used widely, and a major benefit of this method is that it targets all sizes of snakes. However, it’s labor-intensive and costly. Trapping methods are particularly effective for larger snakes, while toxic baits target adults, as mature snakes shift from eating small reptiles to birds and mammals. Bait tubes, designed to allow only snakes to enter, and aerial drops of toxicant-laced mice are also part of the control arsenal, though aerial delivery tends to be the most expensive approach.

Mistry’s model allows the team to test various eradication strategies and trade-offs between cost and effectiveness. Although the model doesn’t simulate specific spatial areas, it adjusts coverage percentages for both ground-based methods and aerial toxicant drops.

**Flowchart showing the steps of the brown treesnake population model: population simulation, eradication method application, and population estimation for future decisions.** *Credit: Kelly Mistry.*

In an initial test run of four strategies, the model produced mixed results. While none of the strategies had a high probability of completely eradicating the snake population, all four showed promise for significantly reducing the number of larger snakes. Mistry is continuing to analyze these early results, but has successfully demonstrated the model’s functionality and use in effectively simulating different strategies.

One strategy involved regular aerial toxicant deliveries, which effectively reduced the population of larger snakes. However, without real-time monitoring, such as visual surveys or trap captures, it is difficult for managers to evaluate the effectiveness of this approach as it’s happening. Monitoring is key to gauging the success of any eradication effort, and the lack of immediate feedback in this strategy contrasts with others that integrate more direct observation methods.

Another strategy combined aerial drops with frequent visual surveys, trapping, and bait tubes over a ten-year period. This produced varied results, performing well in some scenarios and poorly in others, depending on the conditions. Mistry plans to investigate these variations further.

A third strategy focused solely on visual surveys, trapping, and bait tubes, omitting aerial toxicant delivery. Surprisingly, this approach performed relatively well in controlling the larger snakes, even though these methods are typically not as effective for larger individuals. However, it was less successful at targeting smaller snakes and ended up being the most expensive strategy overall.

The fourth strategy combined aerial drops with occasional visual monitoring to estimate population size and track progress. Monitoring can be expensive, but without it, assessing the effectiveness of aerial drops is impossible. This strategy performed similarly to the first, with only slight differences in cost depending on the scenario. Both strategies were among the most optimal for reducing snake populations, particularly larger individuals. However, the importance of monitoring for long-term learning and evaluation is still being analyzed in future research.

The model also helps managers make informed decisions in light of the cost-effectiveness of these strategies. While some strategies performed similarly in reducing snake populations, their costs vary. The aerial delivery system, for example, proved effective but comes at a high financial cost. By testing the trade-offs between cost and effectiveness, the model provides crucial data for managers trying to balance resource allocation with eradication goals.

Future Directions and Acknowledgements

Initially planned to wrap up by the end of 2023, the project received additional funding from the CESU to extend the analysis. Mistry has already completed around 1,200 runs of the model, testing different starting conditions for population size and age structure to refine the results.

Moving forward, Mistry will hand off the model to U.S. Geological Survey scientists, training them to use it as a tool for evaluating eradication efforts. While the U.S. Geological Survey leads the research, the eventual goal is for U.S. Department of Agriculture (USDA) staff, who manage on-the-ground suppression efforts in Guam, to use this tool to evaluate and plan their strategies. The ultimate goal remains to eradicate the invasive brown treesnake population, with the model serving as a guide for focusing these efforts for maximum impact.

A special thank you to Dr. Sarah Converse, Ms. Kelly Mistry, Dr. Staci Amburgey, Dr. Amy Yackel Adams, and Dr. Shane Siers from the USDA for their critical contributions to this project.