When USGS research zoologist Gordon G. Rodda was a graduate student at Cornell University studying behavioral biology of alligators —or later, completing a post-doc at the Smithsonian Institute studying the social behavior of green iguanas in Venezuela or following that, as a statistics and sociobiology instructor at the University of Tennessee—he did not foresee that his professional future was in snakes. Lots of snakes, and in places they don’t belong.
In 1987, when Dr. Rodda was hired by Tom Fritts of the U.S. Fish and Wildlife Service to work on the brown treesnake project on Guam, researcher Julie Savidge (now at Colorado State University) had just completed work identifying this snake as the perpetrator of massive losses of birds on the island. Because managers and some scientists were still unconvinced that this snake was the problem and budget decisions were at stake, Dr. Fritts’ responsibility was to verify the conclusions of her work.
Unhappily for Guam and its naïve prey species (those that have not evolved to defend against a novel predator like the brown treesnake on Guam), Dr. Rodda compiled ample evidence to support Dr. Savidge’s conclusions. Indeed, this invasive snake was causing the extinction not only the island’s birds, —many of which were endemic and found nowhere else—but some lizards and mammals as well.
Managers needed to understand the biology and behavior of the snake in order to contain it, if possible. Where does it live? What does it eat? How many are there in a given place at a given moment? These are questions for which Dr. Rodda and his team have since been seeking answers.
Dr. Rodda has reviewed reptile and amphibian introductions in a variety of analyses, with a focus on discovering how natural history traits might be used more effectively for predicting which species are likely to be harmful. He reported the close association between the traits of species in the pet trade and invaders (Rodda and Tyrrell 2008). He clarified the reasons for contradictory results in classical analyses by separately analyzing the phases of invasion (transport/establishment /spread/ecological disruption). Previous failure of natural history traits to predict invasiveness has led many observers to rely on other traits, such as climate matching or history of invasiveness in other locales. The problem with the “natural-history-free” approach is that it would have failed to predict or prevent some of the worst invasions, such as the Brown Treesnake on Guam. Dr. Rodda’s novel computational approaches are facilitating more precise predictions of potential invaders.
“These findings are part of a larger ‘revolution’ in thinking about the efficacy of screening potential invaders,” Rodda writes. “While ironclad algorithms are not yet available, the rapid pace of incremental improvements have buoyed researchers in the field with the hope that we can move beyond the current restrictions only on the importation of species that have already proven damaging.”
To learn about a creature, you have to be able to find it. Though the brown treesnake is arboreal, that doesn’t mean it won’t be found on the ground—or in your house. But much more difficult is finding it where it predominantly lives, high up in the trees of Guam’s forests. Dr. Rodda and his team conducted research to define and test the elements of detectability for this snake (such as timing of a survey, light sources, and developing a good “search image” for this snake at different life stages and sizes).
“In the forest, the brown treesnake is hard to see, and you have to know where to look. Not everyone can develop a good search image. We found that screening people for their ability to find a specific image in a complex, busy picture, then field-training these snake searchers at night in Guam’s forests, was an investment that really paid off.”
Prior to Dr. Rodda’s arrival on Guam, Dr. Fritts found 55 snakes in a one-hectare plot in one night. In the snake world, such a high density of a single snake species was unprecedented. To account for movement of snakes in and out of an area and correct for under-counting, Dr. Rodda embarked on a population biology study. He investigated prey species abundance, snake habitat preferences, and snake behavior, and developed a monitoring plan for counting snakes. Not only did he discover that these snakes were abundant in Guam’s common (but non-native) forest type, his counts revealed an average of >100 snakes per plot on any given night. Worse, the predator biomass in some cases was greater than prey biomass, an unsustainable imbalance. What did this mean?
This was to be the beginning of many lessons learned through Dr. Rodda’s research that has bolstered the knowledge base regarding invasive reptiles and even the phenomenon of invasive species as a whole. One of these lessons relates to the importance of prevention.
Since Guam was already overrun with these snakes (Rodda and colleagues estimated up to 13,000 snakes per square mile; Rodda et al. 1999), the next effort was to prevent them from entering other snake-free islands like the Northern Mariana Islands, Micronesia, the Hawaiian Islands, and even the continental United States. Shipping and air traffic from Guam to these other islands is a daily affair. Through “stowaway” events, Dr. Rodda learned that these snakes can get into confined places and live a very long time without food or water in cargo. So, he established the Brown Treesnake Rapid Response Team, a group that now has more than 90 trained snake searchers on Guam and islands in the Central Pacific who respond to sightings on snake-free islands. The team regularly trains local volunteers from these vulnerable islands (virtually any receiving shipments or air traffic from Guam) who are on call to respond to snake sightings and try to capture the reported intruder.
“Prevention is paramount. It is much cheaper than intervention once a snake population establishes,” says Rodda. “With naïve species in abundance, invading predators can remain well-fed with little effort while they continue to multiply. So it is crucial to keep alien species from sensitive environments.”
Without rigorous prevention, Rodda notes, it is extremely difficult to control, let alone remove, an introduced species that can thrive in its new environment. In Guam’s case, this includes snake detection efforts to prevent them from leaving the island as cargo stowaways as well as extreme vigilance on receiving islands.
A third avenue is equally important: new invasive species coming in to Guam.
“A lizard from the island of Manus, now found in Guam and nearby islands, is a case in point: what will this lizard do, and is it ‘setting the table’ for the irruption of the brown treesnake on nearby islands? That is, will this species provide more food for when the existing prey populations dwindle enough to otherwise bring down the snake numbers?”
With aggressive citizen education, snake response training, and identification and prevention measures on Guam as well as ports of entry on vulnerable islands (Hawaii, Northern Marianas, Micronesia), the work of Dr. Rodda, his colleagues, and the Rapid Response Team has played a vital role in keeping these islands snake free. Preventive actions and vigilance must continue, however, if the islands are to remain that way.
Another concept that caught Dr. Rodda’s attention early on was how irreversible such invasions are, and that Guam is a poster child for that concept.
“Many invasive species take over the role previously occupied by a native species. While it might displace that native species, it doesn’t necessarily disrupt ecosystem processes. But when the brown treesnake came to Guam, it occupied a new role as a novel, arboreal night-time predator on birds, lizards, and mammals, a role to which these animals were not adapted. That reality caused mass extinctions.”
In such cases, the original ecosystem is simply not recoverable.
“If the brown treesnake gets to any of these other snake-free islands, it will find a veritable banquet of prey animals—and the same thing could happen there. It’s so important to get ahead of the curve and implement prevention efforts at the get-go, before an alien species becomes a problem.”
If, for example, an ecosystem experiences some habitat loss—say, through change like deforestation or an event like wildfire—it might be moderately recoverable over time and with appropriate management. But if the situation is more like that of Guam’s, where by the time the island’s faunal demise could be attributed to the brown treesnake, it was way too late—the snakes had ecologically taken over. “Once that happens,” Rodda warns, “the odds of recovery are vanishingly small.”
Over 8,000 miles away, there is real concern that this magnitude of ecological change could be taking place in another biologically diverse ecosystem, the Florida Everglades.
In 2005, Dr. Rodda and other brown treesnake experts were invited to attend a planning meeting in Florida hosted by the National Park Service and other agencies related to Burmese pythons in the Florida Everglades. His work on Guam was known to NPS researcher Skip Snow, who recognized the parallels in the two situations. The first question was, “How many do we have?”
Dr. Rodda began by evaluating ongoing field work to judge the level of detectability of the snakes, then extrapolating that to a population estimate. The answer was “hundreds, and possibly thousands.” And though most snakes were sighted along the main road through the Everglades, there were other, more remote sightings miles away, implying that the unobserved areas in between were hosting these animals as well. In conjunction with the population estimation, Rodda and his Florida partners worked on constructing and testing traps.
Ultimately, with FORT colleague Dr. Robert Reed, Dr. Rodda conducted a risk assessment of nine species of invasive giant constrictor snakes. By then they knew that the populations of Burmese pythons were numerous and widespread, and given the vast unpopulated and difficult-to-access areas of the Everglades, probably not controllable without massive funding and effort. However, other species of constrictor snakes were being found in more limited areas, and researchers began to figure out what could be accomplished. As in the Pacific, a response team was also established and training provided to people most likely to encounter the snakes, including emergency responders, wildlife personnel, and even delivery drivers and letter carriers.
“The Florida situation is fundamentally different from Guam’s in two respects”, says Rodda. “One, Florida is connected to the continent, and there are no geographic barriers to prevent the spread of these snakes. Two, Florida has non-target species, like the endangered indigo snake, so ‘snakes’ as a type of animal cannot be killed indiscriminately. In these ways it is much more difficult in terms of management challenges.”
Rodda and his colleagues are still trying to determine what impact the snakes are having. In the guts of captured snakes, researchers have found everything from small birds and rodents to full-sized deer. Other prey species include the federally listed wood stork and Key Largo woodrat. The snakes have also taken domestic fowl and pets.
Other losses are beginning to be observed. For example, marsh rabbits, once commonly observed in Everglades National Park and its environs along roads and other developed areas, appear now to be absent from many areas. Ongoing studies are examining the impact pythons may be having on the prey base in the Everglades.
There have been successes, like the capture of an elusive boa constrictor on No Name Key by a response team member; keeping pythons off of Key Largo with its federally endangered Key Largo woodrat and Key Largo cotton mouse; and possibly containing a population of breeding northern African pythons to their current location in southwestern Miami (with the goal of eventual eradication). These situations involve limited geographical areas, making effective interdiction more likely.
“Unfortunately,” says Rodda, “our successes aren’t as striking as the number of times we fail to keep an unwanted species out. Real success, in terms of invasive species, is measured by eradicating the last individual of that species in a target area.”
“The more we know about the biology and behavior of these species, the more effective we can be at developing means of prevention, control, and in some cases, eradication,” he continues. “It is critically important to making our learning count, to use it to provide control and containment methods and tools to managers, and train people to be aware and to respond immediately to new sightings. We need to be in front of these situations before they become insurmountable problems.”
Dr. Rodda’s vision in knowing the questions to ask, applying research to collect information vital to understanding a species in order to figure out how to control it, and in engaging local to national agencies and the public in awareness and reporting, has made a considerable difference in preventing the spread of invasive snakes in the Central Pacific and in Florida. Dr. Rodda hopes that the Guam and Florida situations encourage land and wildlife management agencies to document ecosystem changes early so that incursions of alien species can be prevented, contained, or even stopped.
Dr. Rodda plans to officially retire from the USGS in the spring of 2012 but will continue some work as an “emeritus” volunteer. When asked what this work will entail, Dr. Rodda’s response was no surprise:
“I’m going to work on prevention.”
Fritts, T.H., and G.H. Rodda. 1989. The brown tree snake, an introduced pest species in the central Pacific Islands. Endangered Species Technical Bulletin 14(7): 5-7.
Rodda, G. H., T. H. Fritts, and D. Chiszar. 1997. The disappearance of Guam's wildlife: New insights for herpetology, evolutionary ecology, and conservation. Bioscience 47:565-574.
Fritts, T. H., and G. H. Rodda. 1998. The role of introduced species in the degradation of island ecosystems: A case history of Guam. Annual Review of Ecology and Systematics 29:113-140.
Rodda G. H., Y. Sawai, D. Chiszar, and H. Tanaka. 1999. Problem snake management: The Habu and the Brown Treesnake. Ithaca, NY: Cornell University Press.
Rodda, G. H., and K. Dean-Bradley. 2002. Excess density compensation of island herpetofaunal assemblages. Journal of Biogeography 29:1-10.
Rodda, Gordon H., and Savidge, Julie. 2007. Biology and Impacts of Pacific Island Invasive Species. 2. Boiga irregularis, the Brown Tree Snake (Reptilia: Colubridae). Pacific Science 61(3): 307-324.
Rodda, G. H., and C. L. Tyrrell. 2008. Introduced species that invade and species that thrive in town: Are these two groups cut from the same cloth? Pages 327-341 in J. C. Mitchell, R. E. Jung Brown, and B. Bartholomew, editors. Urban herpetology. Herpetological Conservation, vol. 3, Society for the Study of Amphibians Reptiles, Salt Lake City, UT.
Reed, R.N. and G.H. Rodda. 2009. Giant constrictors: Biological and management profiles and an establishment risk assessment for nine large species of Pythons, Anacondas, and the Boa Constrictor: U.S. Geological Survey Open-File Report 2009–1202. 302 p.
Rodda, G.H., C.S. Jarnevich, and R.N. Reed. 2011. Challenges in identifying sites climatically matched to the native ranges of animal invaders. PLoSOne 6(2): 1-18.