Author Archives: gbuckley

The Problem of Brown Tree Snakes: Why Trojan horses only work in Troy

Several years ago, while working as an Animal Care Technician for the USDA’s National Wildlife Research Center (NWRC), I became intimately familiar with the case of the Brown Tree Snake (Boiga irregularis). For those of you who are unfamiliar with the story, it goes something like this:

At the end of World War II, war-time ships were busy with activities around Guam, an island about a thousand miles south of Japan. A few Brown Tree Snakes hitched a ride to Guam, an island their species had never been to. The snakes were probably drawn to the mice already stowing-away on the ships. However, once on Guam the species started to flourish. Their numbers, with more abundant prey resources than in their native ranges, started to show exponential growth.

This snake has wiped out several bird and lizard species, including some that were endemic to Guam. This has caused an increase in the abundance of spiders on the island. Determined to regain our beautiful Helen taken by the Trojans (and also to alleviate the infrastructure problems caused by the snakes) Uncle Sam was rallied to war, and set out across the ocean to destroy its enemy.

The first attempts at controlling the snakes focused on using trained dogs, and eliminating individual snakes by hand. These measures were put into effect in 1994, 40 years after the introduction of the snake to the island. These methods were largely unsuccessful in lowering snake populations. By 1999, no headway had been made in crashing the gates of Troy, and a new strategy was needed.

Researchers from the NWRC quickly collected specimens and started testing the most effective control methods. It was found that over-the-counter grade aced into a dead mouse was enough to kill brown tree snakes. Over the next 14 years, different delivery methods were tested and implemented.

As of 2015[1], the newest tree snake eradication efforts involve injecting acetaminophen into dead mice, attaching them to a parachute, and dispersing them, by helicopter, throughout the jungle of Guam[2]. Here’s how it is supposed to work:

The little parachutes get caught in the canopy, dangling their deceptive prize for the snakes to find. Snakes that eat the mice are exposed to the toxin inside, and quickly die. It is more or less the population control equivalent of the Trojan horse, or in this case the “Trojan mouse”. The dead snakes reduce the size of the reproducing population, and (hopefully) the total population can be brought under control. It seems simple and straight-forward. However, remembering some simple concepts from evolution and behavior, the story gets more complicated.

For instance, Charles Darwin noted great variation among individuals within a species, and used this to develop his theory of natural selection. The process of natural selection is one by which the varieties of a species that are most successful in an environment reproduce more, causing an increase in the abundance of that variety, compared to other varieties. Through the many environmental pressures, species change through time to fit the constraints of their surroundings. Modern biologists have recognized that many behaviors act as variable traits that are influenced by heredity. In other words, natural selection acts on behaviors as well.

Given this, consider the snakes on Guam. Prey selection is a variable behavior in brown tree snakes, with snakes “eating a wide variety of vertebrate prey, including reptiles, birds, and small mammals…”3. In relation to the quality and abundance of these food sources, when one food source becomes scarce or dangerous, snakes that don’t rely on this food source will have higher reproductive success than a snake that does.

By dropping our “Trojan mice”, we are simply adding a human selection pressure to snakes. We are not eradicating the brown tree snake, because we cannot possibly get every snake to take the bait.  By killing the snakes that will sometimes eat a dead mouse, we have simply given a leg up to the snakes that avoid mice as a prey item. These include the snakes that eat only birds and lizards. Effectively, we have increased the relative abundance of snakes that prefer “rare meats”, and are punishing snakes that are likely to provide beneficial services, such as rodent control.

Other activities employed against the brown tree snake include “detector dogs, traps, nighttime fence searches, oral toxicants, barriers, prey reduction, and public outreach activities.[3]” The total costs of these programs is about $3,000,000 annually (although last year the budget was increased by $500,000 “for the manufacturing of an automated bait delivery system for the Toxicant Bait Drop Project[4]”). Seems like a lot of money to control some snakes.

Given that there are roughly 2,745,400 brown tree snakes on Guam (50 snakes/hectare x 54908 hectares on Guam), I propose a simpler and more effective solution. Put a bounty of $1 on snakes.

Considering the high density of snakes of the island, to make a salary of $31,000[5], a hunter would have to collect about 124 snakes per working day (31,000 snakes/250 working days in a year). To trap this many snakes, a hunter would need to cover an area of only 6.2 acres per day, or roughly 6 football fields. Given an 8 hour day, traps, trained dogs, and the human technological advantage, it seems reasonable that the activity of snake hunting could become economically profitable. Profitable hunting has been a proven eradication method of any species; look at the history Passenger Pigeon[6] if you need proof that it works.

Therefore, it seems unreasonable that we spend $3,000,000 a year applying a selection pressure, when we could spend $2,745,400 to eliminate all the snakes and provide around 85 islanders with jobs until the snakes are eliminated. If they really are a threat to biodiversity, and could potentially cost us billions of dollars, it would make sense to pick a more proven and effective method of eradication. Let’s leave the elegant deception to the ancient Greeks and focus on methods modern capitalists can support.

–Gabe Buckley
B.S. Zoology with a minor in Business Administration
Profession Science Masters in Zoo and Aquarium Management, class of 2015

[1] See page 73. http://www.doi.gov/oia/budget/upload/2015-OIA-Greenbook-Master.pdf

[2] http://www.npr.org/blogs/thetwo-way/2013/02/22/172695707/dead-mice-are-going-to-be-dropped-on-guam-from-helicopters-really

[3] See page 75. http://www.doi.gov/oia/budget/upload/2015-OIA-Greenbook-Master.pdf

[4] See page 73. http://www.doi.gov/oia/budget/upload/2015-OIA-Greenbook-Master.pdf

[5] the average salary of Fishing and Forestry Occupations in Guam. See “Annual mean wage” in “Farming, Fishing, and Forestry Occupations”. http://www.bls.gov/oes/current/oes_gu.htm#45-0000

[6] http://www.audubon.org/magazine/may-june-2014/why-passenger-pigeon-went-extinct