Invasives creeping out of place

To find the first and only "original" remains of a snake in the Canary islands, in the Atlantic sea, we need to go back to the late Miocene, a geological epoch especially renowned for the repeated desiccations of the Mediterranean sea. Paleontologists have found just one vertebra of what was something like a boa living in the archipelago 5-10 million years ago. As far as we know, since then no snakes lived in the Canary islands, at least until the early 2000, when the archipelago has experienced the very unfortunate introduction of the California kingsnake (Lampropeltis californiae). This species, probably escaped or released from captive facilities, is now an important environmental problem in the archipelago due to the enormous social alarm among the population not accustomed to the presence of snakes, and the damage caused to many endemic reptile species, such as the Gran Canaria giant lizard (Gallotia stehlini). To remove this major ecological threat, over 1000 snakes were captured since 2007, and now thanks to the 1 million euro Lampropeltis project supported by the EU through the LIFE+ programme, the authorities expect to set up the conditions for the eradication of the species.

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California kingsnake (striped albino pattern). Photo © Ramón Gallo Barneto

The lucrative trade of species for pet amateurs carries the inherent risk of escape or abandonment of animals kept in captivity, and as a consequence the potential establishment of wild self-sustaining populations of a number of invasive alien species. In fact the California kingsnake is only one of the many species known to have succeeded in getting naturalized outside their native range as a side effect of the pet industry. Another major example among the snakes is the Burmese python (Python molurus bivittatus) in the Everglades, Florida. The spread of this species is a major concern in the region, because it is a predator that can grow over 5 metres long, and as such is able to eat nearly any native animal, possibly even panthers and alligators.

Snakes are often introduced also as cargo stowaway. In this way many species manage to colonise even remote oceanic islands, where they can represent a serious threat to some of the most and unique living creatures of the world. A renowned case of accidental introduction concerns the brown tree snake (Boiga irregularis). Soon after World War II this native of the Solomon islands, northern Australia and Papua New Guinea, was transported as stowaway by military cargos to the island of Guam, Mariana Islands. In this western Pacific island this ecologically disrupting predator is having a dramatic ecological and economic impact. In particular is negatively affecting several species beside having killed off at least 8 of the island’s 11 species of native birds, as well as some indigenous lizards and bats. Now there is a growing concern that the species could take advantage of the frequent aircraft flights from Guam to the Hawaii and make its way to this archipelago in central Pacific, where its ecological and economic impact would be even greater (according to some studies it might cost over 1.7 billion dollar per year if successfully introduced).

Brown Tree Snake. Photo © Daniel O'Brien

Accidental introductions linked to military activities are likely to have occurred also in ancient times: perhaps snakes were used to frighten enemies even during Roman assaults. As suggested for the origin of the viperine snake (Natrix maura) in the Balearic, Spain, some introduced populations of snakes in the Mediterranean islands could be actually linked to such battle related events. Also this snake is having a major ecological impact in terms of loss of indigenous species, and changes in community structures and function. In fact in the Balearic Islands, the introduced viperine snake is known to represent a serious threat for the endangered endemic Mallorcan midwife toad or ferreret (Alytes muletensis) in Mallorca, and was probably involved in the extinction of the species in Menorca.

Other than inflicting ecological harm, non-indigenous snakes can be dangerous to humans, as some species are also poisonous. A typical poisonous snake introduced outside its natural range is the habu (Trimeresurus flavoviridis), a Japanese native viperid introduced in Minnajima, Okinawa Island. Also the brown tree snakes is well known for its venomous bites: in Guam the estimated cost for hospitalisation and intensive care for people affected by snakebite (especially infants) is about 25,000 dollars per year. Along with the ecological damages and the health problems, the brown tree snake can also provoke significant economic impacts, like frequent power outages and damage to the electric lines due to the attitude to crawl along the wires: yearly cost for direct damages and lost productivity is conservatively estimated at 1 million dollars.

Thus, nothing to do with the notorious old rumor, still very popular in countries like Italy, according to which snakes, and particularly vipers, are recurringly broadcasted by helicopters - by either environmentalists or parks authorities - to restore their wild populations. Here the problem for nature conservation professionals is how to prevent the further release or spread of snakes and other harmful alien species outside their natural range, where they clearly represent a key driver of biodiversity loss.