The first invasive snake

Click here to read this post in Spanish!

Haga clic aquí para leer este blog en español!

Wolf Snakes (Lycodon aulicus) have become established
on Mauritius, where they threaten native skinks and geckos

Reptiles have been moving around the globe for a long time, often assisted by humans. Skinks and geckos had dispersed to the remotest Pacific islands by about 1600 BCE, at least partly thanks to the aid of the first human colonists of those regions. Brown Tree Snakes (Boiga irregularis) were brought from Australasia to Guam during World War II. In more recent decades, Burmese Pythons (Python molurus) have reached the Everglades, California Kingsnakes (Lampropeltis californiae) the Canary Islands, and Indian Wolf Snakes (Lycodon aulicus) the island of Mauritius in the Indian Ocean. In many cases, these introduced populations of snakes have become invasive, disrupting the native ecosystem in numerous ways, mostly by eating their way through populations of native prey. The indirect effects of these dramatic population declines are unpredictable and profound. For example, on Guam the loss of native forest birds as a result of snake predation led to an explosion of spider populations, with a 40-fold increase in the number of webs compared with nearby islands without invasive snakes that still harbored a native bird community. Although species have been colonizing new ecosystems for a long time, the rapid rate at which they are now being facilitated by global trade is a serious ecological concern. But how new is this problem, exactly?

Where was the first recorded population of introduced snakes? Incredibly, three species of snakes were introduced to the Balearic Islands in the western Mediterranean as far back as 2200 years ago. Having won the Second Punic War, the Roman Republic was expanding west into the Iberian peninsula, which they had taken from Carthage. As a result, transport and trade between the western and central Mediterranean were more regular than ever before, which may help explain the introduction of several species of amphibians and reptiles native to either the European or African mainland to the Balearics. The native people of the Balearics had served as mercenaries under both Rome and Carthage and were renowned for their skill with the sling, but Rome conquered their archipelago anyway shortly after the war and purposefully settled over 3,000 Spanish and Roman colonists there. It's likely that many of these people, understandably, missed their mainland homes, including the native plants and animals to which they were accustomed. They probably brought pet chameleons and tortoises with them, and surprisingly, keeping snakes as pets was also common, so they may have purposefully or accidentally introduced snakes from mainland Europe and Africa for this reason.

Ladder Snake, Rhinechis (Elaphe) scalaris
Their name reflects their dorsal pattern rather than their climbing prowess.

One species, the Ladder Snake (Rhinechis [Elaphe] scalaris), is endemic to the Iberian peninsula. It is a large, adaptable snake that eats mostly small mammals, similar to a North American ratsnake. Although it is easy to see how these snakes could have stowed away on ships, perhaps boarding to eat rats or mice that fed on grain or other goods, it has also been suggested that the Ladder Snake was introduced partly because it played a totemic purpose in mythology and religion. People encouraged non-venomous mammal-eating snakes to take up residence in and near their homes to keep populations of rats and mice under control, and having snakes around the home was thought to maintain the sexual potency of the home's male inhabitants. There is also some evidence that mammal-eating snakes were gathered up and released in areas where epidemics were rampant to help control rat or mouse vectors. This may have led to the association between the Roman god of healing, Aesculapius, whose staff is still a symbol of medicine today, and the Aesculapian Snake (Zamenis [Elaphe] longissimus), a relative of the Ladder Snake.

False Smooth Snake (Macroprotodon mauritanicus)

A smaller species, the False Smooth Snake (Macroprotodon mauritanicus [formerly cucullatus]), is native to northern Africa and southern Spain, where it preys upon small lizards. It might have been introduced to the Balearics accidentally, but no one is really sure how it got there. Apparently, False Smooth Snakes are at least partially responsible (introduced weasels, cats, and genets probably also contributed) for the extinction of an endemic species of lizard, Lilford's Wall Lizard (Podarcis lilfordi), a ground-dwelling, frugivorous species that once dispersed the seeds of a perennial shrub, Daphne rodriguezii. Since the wall lizards began to disappear from the large islands of the Balearics about 2000 years ago, the plants have suffered from a lack of seed disperal, a service formerly provided by the lizard, which would eat the fruit and crap out the seeds. On tiny offshore islets this relationship is still going strong, but on Menorca and Mallorca, where there are many snakes and no lizards, seedlings of D. rodriguezii sprout only underneath their parents, a losing strategy for a young plant.

Viperine Watersnake (Natrix maura)

Finally, the Viperine Watersnake (Natrix maura), a semi-aquatic natricine native to both southwestern Europe and northwest Africa, was introduced to both Menorca and Mallorca in ancient times. During naval battles, both the Phoenicians and the Carthaginians apparently used to throw open jars full of snakes into enemy warships to cause panic among the combatants (apparently even back then nobody could tell the difference between venomous and harmless snakes), which possibly led to or reinforced its populations on the islands. In the Balearics, these watersnakes eat endemic Mallorcan Midwife Toads (Alytes muletensis) (which they consume with impunity despite the frogs' toxins thanks to the snakes' immunity to a wide range of toxins), so a program of active eradication within the range of the frog has been enacted. The Viperine Watersnake could also have been responsible for the extinction of other endemic species of midwife toads never described but historically present.

Snakes may actually be some of the most problematic potential invasive species because they are difficult to detect and almost impossible to eradicate. Research has shown that if you're going to stop an invasive species, you had better stop it early or not at all, a tall order in the face of snakes' impressive crypsis and secretive behavior. Snakes' low energetic requirements allow them to persist through lengthy periods of resource scarcity, and their flexible metabolism allows them to quickly take advantage of resources when they are available, both adaptations to eating infrequent large meals. This scenario is ideal for an individual animal in transit or freshly introduced to a novel environment, who may need to have the ability to remain motionless without feeding or reproducing for long periods of time. Given snakes' long history with people, it's no wonder that Northern and Banded Watersnakes have become established in California, Aesculapian Snakes in Britain, Cornsnakes in the Cayman Islands, Catsnakes in Malta, Monocled Cobras and Habus in the Ryukyu Islands, and many other examples.

ACKNOWLEDGMENTS

Thanks to Rob, Javier Gállego, Aviad Bar, and Jose Zuñiga for the use of their photos.

REFERENCES

Austin, C. C. 1999. Lizards took express train to Polynesia. Nature 397:113-114 <link>

Bruna, E. M., R. N. Fisher, and T. J. Case. 1996. Morphological and genetic evolution appear decoupled in Pacific skinks (Squamata: Scincidae: Emoia). Proceedings of the Royal Society of London. Series B: Biological Sciences 263:681-688 <link>

Lazenby, F. D. 1947. Greek and Roman household pets. The Classical Journal 44:245-252 <link>

Fisher, R. N. 1997. Dispersal and evolution of the Pacific Basin gekkonid lizards Gehyra oceanica and Gehyra mutilata. Evolution 51:906-921 <link>

Pleguezuelos, J. 2002. Las especies introducidas de Anfibios y Reptiles. Pages 501-532 in J. Pleguezuelos, R. Márquez, and M. Lizana, editors. Atlas y Libro Rojo de los Anfibios y Reptiles de España. Dirección General de Conservación de la Naturaleza-Asociación Herpetológica Española, Madrid <link>

Rocha, I. R. S. 2012. Patterns of biological invasion in the herpetofauna of the Balearic Islands: Determining the origin and predicting the expansion as conservation tools. MS thesis. Universidade do Porto <link> <download>

Rogers, H., J. Hille Ris Lambers, R. Miller, and J. J. Tewksbury. 2012. ‘Natural experiment’ demonstrates top-down control of spiders by birds on a landscape level. PLoS ONE 7:e43446 <link>

Traveset, A. and N. Riera. 2005. Disruption of a plant‐lizard seed dispersal system and its ecological effects on a threatened endemic plant in the Balearic Islands. Conservation Biology 19:421-431 <link>

Life is Short, but Snakes are Long by Andrew M. Durso is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.

This blog is supposed to be about snakes, but if you can't make exceptions for family, then you're a jerk

Click here for this article in Spanish!

Haga clic aquí para ver este artículo en español!

Sphaerodactylus elegans of Cuba

Geckos are some of the most diverse and widespread squamates on Earth. They range in size from the dwarf Sphaerodacylus of the Antilles (16-18 mm) to the (probably) extinct Kawekaweau or Delcourt's Giant Gecko, Hoplodactylus delcourti, of New Zealand (as long as a tuatara and as thick as a man's wrist). Except for a few species, none have eyelids, and most have adhesive pads on their toes that allow them to climb slick surfaces. Some are parthenogenic. A few owe their success partly to humans, and it is these that we consider here today.

Several species of gecko have been introduced to the United States by humans, most accidentally, as stowaways or through the pet and greenhouse trade. One of the most ubiquitous is the Mediterranean House Gecko, Hemidactylus turcicus. Described by Linnaeus in the first edition of his Systema Naturae, it is a yellow-tan, nocturnal, insectivorous gecko about six inches long. Whereas most of our non-native lizards are limited to a few small areas in Florida, H. turcicus can be found in 17 states, from California to Maryland. It isn't continuously distributed across the southern US, but rather locally common in urban sites, due to many separate introductions, the earliest of which occurred in Key West before 1915. One such population is in a middle school in Cary, North Carolina.

Hemidactylus turcicus

Legend has it that a science teacher during the early 1980s was keeping some H. turcicus in a terrarium. All was well until an absentminded student left the lid off one day. The lizards escaped, and a small population has been living in the walls of the school campus ever since. At least, that's what everyone thought. An ongoing summer of research conducted by a wildlife student at North Carolina State University, my brother Kevin Durso, in 2012, has revealed that the population is much larger than anyone thought.

Bags of geckos

On the first night, Kevin counted 82 geckos on the walls of the campus buildings. He came back with reinforcements - tall college students and volunteers armed with lizard nooses and water guns, for blasting geckos off ceilings and walls. On some nights, students at the middle school come out with their teachers and parents to see what the research is all about. Kevin & Co. are marking each gecko they capture with glow-in-the-dark elastomer, so they can be individually identified upon later capture. Once captures of new, unmarked geckos begin to decline, he can begin to estimate the total size of the population using a mathematical model. Whether this will happen sooner or later is still hard to know. Kevin is also keeping track of the exact location of each gecko sighting, so that a home range size estimate can be made. He works at night, when the geckos are most active. Perhaps this is why no one knew the true size of the gecko population until now - how often is anyone at school at night? Not if I can help it, Mom!

Super Soaking a gecko off a wall

Previous research on house geckos has revealed that they inhabit similar areas, both climatically and in terms of microhabitat, in their native and non-native range. A population on the Stephen F. Austin State University campus in Nacogdoches, Texas, ate mostly grasshoppers, moths, and isopods. Their great success in southern North America has been attributed to low predation pressure, little interspecific competition, and a life history which maximizes survival at all ages. House geckos in southern Louisiana are host to native North American parasitic worms, so there is some potential for parasites to regulate populations of these non-native lizards.

Injecting a gecko with glow-in-the-dark elastomer

What factors have allowed the Cary H. turcicus population to grow so large? What effects do these non-native geckos have on the local ecosystem, from the arthropods they eat to the birds and snakes they are eaten by? Have they been spreading around the Triangle area since the 1980s, brought home on schoolbuses in students' backpacks and coats? Only time, and further research, will tell.

ACKNOWLEDGMENTS

The College of Natural Resources and the Office of Undergraduate Research at NC State University provided support and funding for this project. Thanks to Konrad Mebert, Miguel Landastoy, Alex Morrison, Kevin Durso, and Sandy Durso for photographs.

REFERENCES

Davis WK (1974) The Mediterranean gecko, Hemidactylus turcicus in Texas. Journal of Herpetology 8:77-80

Rödder D, Lötters S (2009) Niche shift versus niche conservatism? Climatic characteristics of the native and invasive ranges of the Mediterranean house gecko (Hemidactylus turcicus). Global Ecology and Biogeography 18:674-687

Rose FL, Barbour CD (1968) Ecology and reproductive cycles of the introduced gecko, Hemidactylus turcicus, in the southern United States. American Midland Naturalist 79:159-168

Saenz D (1996) Dietary overview of Hemidactylus turcicus with possible implications of food partitioning. Journal of Herpetology 30:461-466

Selcer KW (1986) Life history of a successful colonizer: the Mediterranean gecko, Hemidactylus turcicus, in southern Texas. Copeia 1986:956-962

Life is Short, but Snakes are Long by Andrew M. Durso is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.