Science: Impact of Tree Snakes in Guam
Monday, August 11, 2008; 11:00 AM
Washington Post staff writer Marc Kaufman answered questions about his Science Page story on Monday, Aug. 11 at 11 a.m. ET about the impact of a non-native species into a new environment.
He was joined by Haldre Rogers, a doctoral student in biology at the University of Washington.
In Monday's Science Page story, Kaufman writes about the findings of Rogers's study, which showed that the hundreds of thousands of brown tree snakes that were inadvertently introduced on the Pacific Island of Guam 50 years ago not only destroyed the bird life there, but is now changing the way Guam's forest grows and will most likely cause substantial thinning and clumping of trees in the years ahead. In addition, the snakes appear to be indirectly responsible for an explosion in the spider population.
Marc Kaufman: Good morning. Having a little technical difficulty here, but hopefully we're now up and running.
Today's chat is about invasive species, and how they can effect a total environment. This involves one of the most dramatic and infamous cases in the world -- how the brown tree snake came to Guam on board cargo ships, and within decades had destroyed most of the bird populations on the island. Haldre Rogers and colleagues are now looking at broader implications for tree growth and general island well-being, or lack of it.
Los Angeles, Calif.: Has this snake reached Hawaii, and if so will there be any way to combat it?
Haldre Rogers: There have been several snakes captured in Hawaii (about 8), but all have been captured near the point of entry (i.e. on the runways of the airport) and most were dead upon arrival. We have no reason to believe there is an population of brown tree snakes on Hawaii. If a population of snakes were established on Hawaii, we would have to use the same control methods we are using on Guam, which include trapping, poisoning, visual searching and sniffer dogs.
Anonymous: Do experts believe there may some day be a chance that an explosion of spiders may reach an overpopulation threshold and that their numbers will decrease, insects will increase and more birds may then return to Guam? May imbalances eventually lead to corrections in the imbalances?
Marc Kaufman: Yes, it is already happening with the brown tree snakes. Their population expanded so quickly because there were some many birds they could easily catch and consume. Now they have to work harder to feed on lizards and rodents, and so the population has pretty much stabilized. Same will probably happen with the spiders -- they will reach a point where their food supply won't support the population, or where some predators finds them.
But there has already been terrible damage done as a result of this invasive species, and there are many, many other examples throughout the world and over many years. Often, native species never make it back, or remain a shadow of what they once were.
Munich, Germany: I'm reminded of New Zealand, where imported weasels that were intended to control rabbit populations, instead preyed upon the flightless native birds of New Zealand and brought many species close to extinction. There seems to be no possibility of removing the weasels or rabbits now from New Zealand.
You mention that it's not possible to eradicate the snakes from Guam. Is this because of financial constraints or because no one wants to tangle with 10 ft. snakes?
Haldre Rogers: It is not currently possible to eradicate the snakes from Guam because we lack an effective method as well as a way to finance the control. The US Department of Agriculture and US Geological Service have been researching brown tree snake control methods since the 1990's. They have developed and refined the methods we use today, but they have not found a method that alone, or in conjunction with other methods, could eradicate the population. Our current control methods are: 1) trapping- these modified minnow-traps use live mice as bait and are effective at capturing medium to large snakes in environments with low rodent densities, such as Guam. 2) Visual search- trained searchers with headlamps are effective at capturing small snakes by walking along roadsides, fence lines and in the jungle at night. This is very labor-intensive, though. 3) Acetaminophen- the equivalent of a dose of children's Tylenol is placed in a dead-mouse and left in a bait tube. The acetaminophen is poisonous to the snakes, so the snakes die after ingesting the mice. 4) Dogs- we have trained dogs that search aircraft and cargo before leaving the island and dogs that search in the jungle for snakes. The dogs are very good at finding snakes in controlled areas (such as aircraft wheelwells), and appear to be useful in the jungle as well, particularly when in conjunction with visual searchers. The dog may alert to a snake's scent, then the visual searchers can pinpoint the location.
The problem with each of these methods, is that they either target only a certain size of snakes (mostly medium to large snakes) or they are very labor intensive. None of them would be feasible or effective on an island-wide scale.
Damascus, Md.: Will some of the native trees on Guam go extinct because the birds are gone, or will they just be reduced? And if they do go extinct, will they just be replaced by other trees? Is that a bad thing?
Haldre Rogers: I am not sure what will happen to the native trees on Guam, but I think there is a definite possibility that some species will go extinct. Other species will likely be reduced, but persist. 60-80% of the tree species in the native forests of Guam are dispersed by birds. If all of these are reduced, we will see a huge change in the forests. I believe other trees will take their place, but even so, the forest will be changed dramatically. Species that are important to local people for carving or medicinal purposes may be reduced or lost. In addition, we retain a hope that someday we will be able to eradicate the snakes and reintroduce birds. However, if we lose many of the trees that serve as food sources for the birds, the forests may not be suitable for supporting the reintroduced populations when or if that time comes.
Bethesda, Md.: Very interesting and sobering. Are there other examples of invasive snakes or other species so dramatically changing an environment?
Marc Kaufman: The examples are legion. The zebra mussel, which comes from Eurasia, has caused enormous problems for American lakes and rivers, and has had a huge negative effect on shipping. Another invader of concern is the European green crab. This crab, about 3 inches across , loves to eat young Dungeness and other shore-dwelling crabs. introduction around the world has destroyed shellfish industries from the Northeastern United States to Tasmania, to South Africa. And then, of course, there's the infamous snakehead -- a fish that can travel on land and eats everything in sight. That's been a big story here in the D.C. area.
We also all know about kudzu, an invasive plant that has taken over thousands of acres in the south.
But not all invasives are so threatening. The starling, for instance, is an invasive from Europe, too.
Vero Beach, Fla.: Thanks for a fine story. The arrival of a predator on an oceanic island is always bad.
A new nontechnical book "Where the Wild Things Were: Life, Death, and Ecological Wreckage in a Land of Vanishing Predators" by Will Stolzenburg (my copy hasn't arrived yet) takes a look at the opposite problem: a lack of big predators. The Northeast's plague of deer would perhaps have something to do with the lack of wolves.
Back to Guam, the island is not only cursed with snakes. A scale insect has destroyed the island's abundant cycads. An expert gave a gloomy presentation at a cycad conference six months ago, even though there's now a degree of biological control.
Marc Kaufman: You're correct that islands are especially vulnerable. I'm told, for instance, that many birds on Hawaii have died and may well go extinct because of the introduction of the mosquito (a non-native species.) That has brought diseases the birds are unable to fight. Adding to the grim picture, even the bird sanctuaries--established at higher elevations where mosquitos have not gone in the past -- are endangered because of global warming, I'm told.
Haldre Rogers: When working on the Brown Treesnake Project in Guam, I often heard suggestions for eradicating the snakes. Two of the most common were introducing a predator, such as the mongoose, and putting a bounty on the head of each snake. The mongoose has been introduced to islands worldwide to control rats, and has been highly unsuccessful at that job, primarily because rats are nocturnal and the mongoose is mostly diurnal (active during the day). It has been remarkably successful at eating birds and bird eggs, however. In addition, it is very difficult to eradicate once it has established. The Brown Treesnake does not have any predators from its native range that could be introduced to control the snake on Guam, nor are there other species that we think might be able to eradicate the snake without harming native species as well.
The second commonly suggested control method is to put a bounty on the head of each snake. Apparently, this was done in the 1990's, and a brand new pickup truck was offered as the grand prize for the person who brought in the most snakes. The winning contestant brought in a whopping total of 400-something snakes, not even a drop in the bucket compared to the total population which is roughly estimated to be around a million snakes. Likely, that lucky person had his entire extended family capturing snakes for him, because they are extremely difficult to locate if they were capturing them by hand, and they would have had to put up many traps in order to capture 400 snakes. Their skin is not good for tanning, so leather belts and wallets and boots are not a possibility. And finally, they are very bony and not especially delicious to eat (so I've been told). There are a few recipes floating around for Brown Treesnake adobo (a Filipino food), but I have not heard of people eating it for food; rather I think it's been mostly a source of entertainment.
Washington, D.C.: I read the article, with respect to the officials having no success in controlling the snake populations. Have they considered introducing a limited number of nocturnal predator species such as the mongoose to control the snakes, or have they thought of importing hunters to kill the snakes?
Haldre Rogers: See number 2 re: your mongoose question.
As for importing hunters, we have many trained searchers on Guam. However, a good searcher, on a good night, might be able to catch 1-2 snakes an hour when searching along the edge of roads. They would catch much fewer if walking through jungle. And it would be impossible for them to capture the entire population of snakes using visual searches. The snakes are well-camouflaged and nocturnal, so they are difficult to find.
Washington, D.C.: Seems to me the broader question is whether man should interfere with the natural flow of things and try to correct things. What's your thinking on this?
Marc Kaufman: There have been so many examples of well-intentioned efforts to introduce invasive species to solve perceived problems, and so many have gone awry.
Here's one example, as bemoaned by the Grand Canyon River Guides. Note that the tree was introduced as both an ornamental and to "stabilize" the riverbanks:
Exotic tamarisk, or salt cedar, is ubiquitous throughout the Colorado River drainage below elevations of about 6,000 feet. This small Eurasian tree is now the dominant phreatophyte (streamside) species along river banks in the American Southwest and in central Australia. Tamarisk was brought to the U.S. as early as 1805, and it was widely available as an ornamental on the West coast by the 1870's. Used for ornamental and bank stabilization purposes by 1900 in the lower Colorado River and Rio Grande rivers, tamarisk appeared in the Grand Canyon between 1922 and 1938. Its broad tolerance of drought and inundation, its enormous fecundity and wind-dispersal of seeds quickly allowed tamarisk to spread. It now occupies much of the new high water zone and pre-dam terraces, and it has invaded most of the river's tributaries as well. Today, tamarisk occupies more than a million acres of riparian habitat in the West where it is widely reviled by habitat managers.
And here is the big picture from Ben Collen, who is an extinctions researcher at the Zoological Society of London:
"Between 1960 and 2000, the human population of the world has doubled. Yet during the same period, the animal populations have declined by 30 per cent. It's beyond doubt that this decline has been caused by humans." The study pinpointed five major factors in the rapid decline of nearly 4000 species between 1960 and 2000; they are the human behavior of: climate change, pollution, the destruction of animals' natural habitat, the spread of invasive species, and the overexploitation of species.
Philadelphia, Pa.: Do you know how and when the brown tree snake first arrived on Guam? And why wasn't anything done to control them while it was still possible.
Haldre Rogers: The snake was accidentally introduced to the island in the mid 1940's, just after the end of WWII, when the US military was bringing cargo from the Admiralty Islands back to Guam. One pregnant snake or a pair of snakes, or a bunch of eggs, must have been on a piece of cargo, and escaped into the Guam jungle undetected. No one knew the snake was present on Guam until the mid-1950's. In 1955, there were 12 snake sightings or captures in the area near where they were introduced. At this point, no one knew they were a problem. In fact, the snake was first identified as the Philippine Rat Snake, and people thought it might be helpful in controlling rats. In the 1960's-1980's, wildlife biologists noticed a decline in the bird population, but no one attributed it to the snake. In the mid-1980's, Julie Savidge (not at Colorado State University) did her PhD project investigating the reasons behind the disappearance of birds on Guam. She convincingly determined that the snake was to blame. However, by that point, the snake was widespread and the birds were nearly completely extirpated.
Now, control efforts are primarily aimed at preventing snakes from leaving the island and finding any new populations on other islands when they are still small and we have a chance of eradicating them. If a soldier had found that first snake on the cargo boat before it escaped, or someone had seen it in their backyard near the port and killed it, we wouldn't be in the situation we are in today.
Marc Kaufman: Thanks for your questions. Haldre tells me that in a year or so, she will have more data about the effects of the snakes (and absence of birds) on different tree species on Guam, and some interesting results about the bird-spider-insect-plant food web. So stay tuned.
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