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Tuesday, January 28, 2014

The first invasive snake

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.


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


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>

Creative Commons License

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


Unknown said...

Interesting post. I was born in Mallorca and used to visit yearly to visit family. I remember seeing many lizards but don't recall actually ever seeing a snake, not that I was looking for them. I will have to make sure I do a little herping next time i visit my relatives in Mallorca. Natrix Maura looks like a very pretty snake indeed.

Andrew Durso said...

Thanks Joey! Whether native or introduced, snakes are hard to find, so I'm not surprised you haven't seen one there. Good luck finding one!

john said...

Andrew, I'm a little surprised that you did'nt mention the Blind Snakes sp?, called Pot Snakes in Hawaii. The details of their arrival are fuzzy in my mind; But this is a species of snake that originated in Asia somewhere and island hopped throughout the South Pacific with the first human inhabitants. It is believed that Polynesian explorers carried potted plants on their boats and the Blind Snakes stowed away in the potted plants. These snakes are probably the first invasive snake species.

Andrew Durso said...

Good point John. I don't know of any studies that attempt to date the first blindsnake invasions. One of the reasons I didn't mention Brahminy blindsnakes is that they are parthenogenetic, and also they feed frequently, so I didn't think they really fit the paradigm of invasive snakes I was describing here. But don't worry, I am planning another whole post on them soon!

Unknown said...

Monocled Cobras in the Ryukyu Islands?????
I would like to see the source of this.

Andrew Durso said...


According to Lever's (1999) 'Naturalized Amphibians and Reptiles of the World', pp. 134-135: "...some 3500 cobras had been imported from Malaysia to Okinawajima in the Ryukyu archipelago by the Rido Company for entertainment purposes. Between 1991 and 1993 seven cobras were observed or captured in the wild near Nago City...the species' survival in Nago City was confirmed in 1999..."

They cite:

Ota, H. 1999. Introduced amphibians and reptiles of the Ryukyu Archipelago, Japan. Pages 439-452 in G. H. Rodda, Y. Sawai, D. Chiszar, and H. Tanaka, editors. Problem Snake Management: the Habu and the Brown Treesnake. Comstock Publishing Associates, Ithaca, New York.


Shiroma, H., S. Katsuren, and M. Nozaki. 1994. Cobra species in Nago, Okinawa. Ann. Rep. Okinawa Pref. Inst. Health Env. 28:89-93.

which I have not read but I have interlibrary loaned a copy of the latter. When I get it I'll let you know what it says.

John Scanlon, FCD said...

The introduction of Lycodon to Mauritius apparently occurred quite early (pre-1850), so it may have overlapped on the main island with Casarea and/or Bolyeria, which have not been recorded alive there. L. aulicus rather resembles C. dussumieri in habing a relatively long and narrow snout and relatively long, 'canininorm' anterior teeth which are useful for puncturing hard-scaled skinks. The extinction of bolyeriids outside the tiny Round Island is attributed to pigs, goats and general habitat buggeration, but maybe Lycodon played a role too.

Andrew Durso said...

John, thanks for the fascinating information. I had no idea that introduction took place so long ago. We like to think that competition probably doesn't play a very strong role in structuring snake communities, but this is one case where it definitely may have (not to discount the potential role of predation by Lycodon on young Bolyeria/Casarea).

Unknown said...

What a really interesting read, I knew of the python issues in the US, but never thought of snakes as invasive, and showing they have been for so many years, incredible.

I always assumed these things were a modern invention brought about through careless pet ownership etc. (or rather callous pet ownership, like the terrapin/turtle invasions that occurred due to a certain cartoon! I know we have those in Milton Keynes in various areas since people realised they lived a long time and got quite large).

I love the thought that a species could become invasive after being used as a weapon just seems so far fetched.

Anyway, thank you for posting such a wonderful article, it's really opened my eyes to something new.

Andrew Durso said...

Thanks so much for your comment Ashley. I too was surprised that careless pet ownership was nothing new and amazed by the weapons story. There are an incredible number of introduced amphibians and reptiles around the globe, to say nothing of the introduced birds, mammals, plants, and invertebrates, some of which have become so ingratiated in the native landscape that scarcely anyone recognizes that they weren't originally from there.