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Snakes on Ibiza and Formentera (Part I)

20 January 2023


Historically, both Ibiza and Formentera have been snake-free. The current plague of snakes that can be found in Ibiza and Formentera is the result of irresponsible human action.

Once again, a capricious attitude and a lack of awareness of the fragility of our ecosystems, -which we prove to have as a species time and time again-, has led to the current plague of snakes on Ibiza and Formentera. A plague of snakes that seriously threatens the survival of the only endemic land vertebrate of Ibiza: the Pitusas lizard (Podarcis pityusensis).

This, in practice, means that in a ridiculous period of years, a unique biological jewel in the world such as the Pitusas lizard may end up disappearing from the ecosystems of Ibiza and Formentera (Podarcis pityusensis).

sargantanes o serps Specimen of the Pitusas lizard (Podarcis pityusensis).

The problem is serious, the figures given for the possible extinction of the Pitusas lizard are as close as 2030 (E.Montes).

The process that gives rise to a new species is a long process, lasting thousands of years, in which a series of environmental and social characteristics have to come together to bring about the isolation and birth of a new species.

The fragility of island ecosystems is partly due to the unique genetic characteristics of their species. These species have developed different mechanisms of adaptation to the environment over thousands of years, which are specific to stable environmental conditions. That is to say, in general, when a factor appears that radically changes an aspect of the ecosystem, such as temperature, rainfall, exotic competitors or the appearance of a predator for which they have no defence mechanisms, the species most affected by these changes may be abruptly driven to extinction.

dragones ibiza
Geckos or "dragós" are another of the many species that can be affected by the predatory action of horseshoe and ladder snakes..

It is important here to stress the extent to which this is relevant to global biodiversity. The species that inhabit the islands are, in many cases, totally isolated from the species from which they originally came. Species that, over thousands of years, have followed a slow but constant process of evolution apart from the rest of the species, which leads them to become a unique species in the world, i.e. an endemism that will not be found anywhere else in the world. It is this endemism of an island ecosystem that conditions its great fragility in the face of abrupt changes in its ecosystem, such as the destruction of its environment or the introduction of invasive species, since the total population of individuals that make up this endemism at a global level is the total population existing in a handful of square kilometres that are very easily altered by human action.

So we should not take lightly the warnings of those experts in the field who have been warning us for years of the serious threat posed by invasive snakes in Ibiza and Formentera.

Proof of this is the already confirmed disappearance of the lizard population on the islet of S'Ora, where the last specimen was detected in 2017, and which is attributable to the arrival of invasive snakes, something that was confirmed before the lizards had disappeared from the islet. The lizard population on the islet of S'Ora has remained isolated from the rest of the Pitusas populations since the last Glacial Maximum (E. Montes, 2021), that is, some 20,000 years. 20,000 years of genetic richness, down the drain of evolution in just a handful of years due to the irresponsible action of man.


What species of snakes are we talking about?

The snakes we are dealing with are basically 2. The vast majority of them are the horseshoe snake (Hemorrhois hippocrepis), and, to a lesser extent, but already naturalized, the ladder snake (Rhinechis scalaris). Both species were, and are currently, introduced through the importation of century-old olive trees for ornamental garden purposes. The first record was made in 2003. A third snake, the bastard snake, was recorded in 2003 (Malpolon monspessulanus) has also been detected on more than one occasion, but its naturalization on the island (natural reproduction) has not been confirmed to date, which means that all the specimens detected are considered to be specimens from the mainland.

sargantanes o serps sos sargantanes
Adult horseshoe snake (Hemorrhois hippocrepis), captured in the area of Ibiza Airport.

Due to the greater impact and presence of the horseshoe snake compared to the ladder snake, we believe it is necessary to focus on the former. Horseshoe snakes in Ibiza and Formentera have already developed some peculiarities that differentiate them from the populations in the south of the Iberian Peninsula (from which they originate). They are larger; that is, they have developed a so-called gigantism that can reach up to 2 metres in length. But despite growing larger, the females reach sexual maturity at a smaller size in relation to the total size they reach as adults in Ibiza and Formentera. Female horseshoe snakes in Ibiza reach sexual maturity at around 48% of their maximum length, while in the mainland populations, they reach sexual maturity at around 63%. This means that a female of around 63 cm is already able to reproduce. In the case of males, this size is reduced to 50 cm.

On the other hand, there is evidence that its breeding season may be being extended to September-October, although it is centred on the months May to July (although it has not been possible to ascertain that this translates into two clutches and not late clutches).


Why should we be concerned about invasive snakes in Ibiza and Formentera?

As has already been indicated, invasive snakes are posing a serious threat to the survival of current ecosystems due to their predatory action. The main endangered species is the iconic lizard of the Pitiusas Islands (Podarcis pityusensis). As an endemic species, it is also of incalculable value from a genetic and evolutionary point of view.

However, snakes in general, and the horseshoe snake in particular, are multipurpose predators. Although it is currently estimated that 56% of their diet consists of lizards, the other 44% is made up of micro-mammals, birds and their eggs. So there is no need to wait for lizards to become extinct before we know what will happen to other species. If action is not taken and effective solutions are not found as quickly as possible, we are all responsible for this: public administrations (regardless of their competences), organizations, companies and individuals.

In addition, as already mentioned, it is considered that horseshoe snakes have a greater reproductive potential in Ibiza and Formentera compared to the mainland populations from which they originate. Another factor to take into account when assessing their degree of danger to the ecosystems is the almost total absence of predators. Let it be known, only domestic cats and kestrels (Falco tinnunculus) exert predatory pressure, but this is minimal: they only hunt a few small juveniles.

It is therefore urgent that the snake population on Ibiza and Formentera be controlled, and this is a collective and individual responsibility that we must demand of ourselves and the public administrations.

eZy Watermark 26 11 2022 08 47 59a
Traps are normally removed in October-November, but in areas with high densities, if it is possible to keep them all year round, it is advisable to leave them in place, as it is possible that occasional captures may still occur beyond these months.

Find out more:

Snake control in Ibiza and Formentera.
Instructions for the manufacture of alien snake traps.
- Tesis E.Montes.
- Feasibility of ophidian control in Ibiza and Formentra (Asociación herpetológica española, 2012).
- The horseshoe snake.
- Gallery.


About Nova Falcons:

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-Controls on birds, mammals y reptiles.

-Wildlife surveys, censuses and monitoring.

-Survey and control of invasive and protected species.

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Wildlife control in Ibiza