Notes on snake control in Ibiza and Formentera (1/3)
NOVA FALCONS, COMPANY SPECIALIZED IN THE CONTROL AND CAPTURE OF SNAKES
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Horseshoe whip snake (Hemorrhois hippocrepis) and ladder snake (Zamenis scalaris).
The plague of snakes in Ibiza and Formentera, especially the horseshoe whip snake (Hemorrhois hippocrepis), is one of the most serious environmental problems of the Pitiusan archipelago (with the permission of the aberrant urban development and mass tourism that we "enjoy"). This pest is a serious threat to numerous species, including the Pitiusa lizard (Podarcis pityusensis), which is a local endemism that is threatened with extinction due to the introduction of this colubrid by humans. For those who are not aware of the relationship of the Pitiusas with the invasive species of snakes, we encourage you to read the previous entries on this subject, available both in the section of divulgation of Nova Falcons, and in the section of services of Nova Control, where we deal with the problem in greater depth:
Specimen of the Pitiusas lizard (Podarcis pityusensis), a local endemism that is in danger of disappearing from the Pitiusas ecosystem.
The purpose of this entry (divided into 3 parts to make it more enjoyable) is none other than to share the self-training process we carried out as we studied some specimens of horseshoe snakes that we captured in 2022, as well as the results obtained. Since the main objective was not to carry out a scientific study, but rather to learn more about this invasive species and collaborate in its control, we did not carry out an experimental design prior to the start of the captures (so it has been necessary to correct this, as far as possible, a posteriori). This, together with a low sample size, i.e. few specimens analysed to apply statistical analyses that allow us to generalise the results to any other environment and place, does not allow us to establish complex relationships between the different parameters studied. However, these data do help us to achieve our main objective, which was to gain a deeper knowledge of this species, and thus improve our specialisation in order to be able to carry out effective monitoring. Now, by sharing part of the results in this entry divided into 3 parts, if possible, we also provide small informative notes to help those who want to learn more about the horseshoe snake in the local area, and, of course, to put into practice the control of the species, something for which the involvement of everyone is necessary.
Snake control as an invasive species
Nova Falcons provides a wildlife control service in a public area where there is no control or eradication plan for invasive species (including the horseshoe snake), and which also borders the Ses Salines Natural Park. Therefore, we take the opportunity to do our bit to control this species on the island of Eivissa, acting in such a sensitive area as the environment of the Parc Natural de Ses Salines de Eivissa i Formentera with a strictly voluntary work. In other words, the work in this area did not have any budget from the public administration or any other entity (more on this aspect in the third part of the publication). The total number of traps was 40, installed for approximately 6 months. We will refer to this area as Zone A. During the same period, we also placed a total of 18 traps in the area of Roca Llisa-Jesús, which we will refer to as Zone B hereafter. We consider that these are two interesting points to compare, since the first is of high ecological value, and with a supposedly lower density of snakes, and the second, with a different environmental value, but presumably with a higher density of snakes due to its proximity to the original source of the pest. We added another zone, Zone C, with a somewhat more diffuse methodology, a more dispersed location between zones A and B and with specimens provided by third parties.
The traps used were of the double-compartment cage trap type, one for the live mouse that acts as an attractant for the snake, and another that can only be accessed by the snake. Obviously, the mouse does not suffer any type of damage even if the snake enters the trap, but the fact of using live bait implies a series of costs in resources and maintenance, which greatly increases the workload compared to systems with solid bait or unbaited trapping systems; especially when we are talking about managing dozens of traps, instead of 2-4 in the domestic environment, without having a team of technicians dedicated exclusively to this service.
The total sample was 59 individuals, but in 8 of them, due to their poor condition, biometric data could not be obtained (unusable roadkill, advanced state of decomposition...). Therefore, the biometric data presented here were obtained from a total of 51 snakes, most of them captured with traps, although there are some from manual captures and roadkill (see figure 1 below). Of these 59 snakes, 12 specimens were given to us by Jordi Ferrer, from Posidonia Sanidad Ambiental, so that we could study them and continue learning about this fascinating species, which, it must be said, is not to blame for the environmental damage it causes, the responsibility for which lies with the humans who introduced it to the Pitiusas. Apart from this, within the area of Zone C, different individuals gave us another 8 specimens, including a ladder snake (the only specimen of the exhibition).
Data to take into account in order to understand the graphs and the different sub-zones of Zone A
In order to make something representative (as indicated above, there was no previous intention to make a properly prepared study), we created 3 zones (A, B and C) and divided Zone A into two sub-zones (1 and 2). These two sub-zones of Zone A are based on the difference in results, the physical barriers that divide them and whether or not they are in direct contact with the natural park (only sub-zone 1 is, sub-zone 2 has sub-zone 1 between it and the park).
The comparison to evaluate the performance and results of the traps will only be made between the 40 traps in Zone A and the 18 traps in Zone B, for two main reasons: these traps are managed by us in 100% of the cases, and they are active during the same dates.
As indicated above, Zone C contains data from those traps, our own or third parties', which have not been active during the same months, and which in some cases are also located in other areas of the island halfway between the two points.
Serp de ferradura, horseshoe whip snake (Hemorrhois hippocrepis).
The sum of trap catches in Zones A and B gives a total of 30 specimens for 58 traps, which is not a very high figure (i.e. excluding the trapped specimens from Zone C, and the specimens from the 3 zones coming from other than traps, more on this below).
This low number, in our opinion, has a positive side, as captures are especially low in the two sub-zones of Zone A in relation to Zone B, and more so in the case of sub-zone 1, which is the one that shares the border with the park. Thus, it seems that our data support the hypothesis that the density is still lower in the south than in the north-east of the island, where the other sampling Zone B (Roca Llisa-Jesús) is located and where the traps had a higher yield.
In subzones 1 and 2 of Zone A, 10 snakes were captured with 40 traps (0.25 per trap), while in Zone B, 20 snakes were captured with 18 traps (1.10 per trap). But despite this apparent lower density in Zone A, in the natural park itself and the surrounding area, direct sightings of snakes seem to be increasing compared to other years, which indicates that the snake population is probably growing, although the density is still relatively low and the abundance of prey is high enough to influence the effectiveness of the traps. And we say that the population may be relatively low, because if we add to the 1o specimens the 6 hit-and-runs and the 3 hand captures in both sub-zones of Zone A, we are left with 19 specimens for Zone A. In other words: the traps have a certain degree of capture capacity, but despite the number of traps (4o), we have almost the same captures from other sources as from the traps.
We do not have any roadkill or hand captures of specimens in Zone B, basically because Zone B is not a public space, physically delimited and with restricted access where we can recover specimens from hand captures or roadkill, as it is the case in subzones 1 and 2 of Zone A. Therefore, in order to compare the number of captures between both zones, we limit ourselves to traps. In Zone C, if we have a manual capture.
Total percentage of the sample, including the 3 zones (Trap, manual and run-over).
Evolution in the number of captures
The months in which we captured the most snakes were May, June and July, corresponding to the time of greatest activity of the species, probably due to the beginning of the reproductive period. In August, we captured practically none, which is surprising as this species is thermophilic, but could be explained by a reduction in activity due to the extremely high temperatures. In September, there is a slight peak in captures and then they drop again with the arrival of colder temperatures, although captures continue to be recorded throughout the year (even in January 2023), which means that in Ibiza they do not hibernate completely (figure 2).
Monthly evolution of the total sample, including the 3 zones.
As we collected more and more specimens, it became increasingly clear that there was an opportunity to learn more about the horseshoe snake. Studying the specimens live and not just through the literature has allowed us to learn more about how and when they breed locally, to reaffirm our knowledge of their feeding habits, to observe different patterns of colour, moult, size and health status, all with the long-term goal of assessing whether or not the current eradication method is working. This is an important point, as the number of snakes is likely to decrease in the areas where they are trapped intensively every year, but perhaps above a certain number, the populations, although relatively controlled, if the control system is not complemented with other types of trapping and attractants, may remain stable at the cost of an "artificial" balance between the number of active traps and the amount of food available in the environment. Another important factor to take into account are the areas that may be acting as reservoirs for the species, as long as they have no control in place. Only time and consistent and methodical work on the behaviour of colubrids on the islands of Eivissa and Formentera (for example, regarding their degree of dispersal in mountainous areas not frequented by people), and the long-term effectiveness of the current trapping method, can clarify this last point.
For our part, we do not yet have data from several years for the same area where we have been working with the same methodology, which would allow us to draw conclusions on population trends. However, as indicated above, we have been able to record the biometric data of 51 specimens captured in 2022, from the 3 zones we have used for sampling. Some of the data have already been presented in this entry, but most of them have been collected in the second part of the 3 parts into which we have divided this entry. The third part will focus on accidental captures; the necessary participation of citizens (volunteering) and the role of public administrations and private companies in the control of the pest; ending with a small anecdotal section on black rats (Rattus rattus) and their effect on snake control, and some brief general conclusions.
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Text: own elaboration.
Photographs: unless otherwise specified, Borja Pérez; and unless otherwise indicated, all photos were taken in Eivissa or Formentera.
Acknowledgements: to our technician and biologist Lara Abascal, for her involvement, beyond the strictly work-related, during the whole process of capturing the snakes and the subsequent processing and writing up of the information collected. Thanks also, to all those who have voluntarily provided us with specimens to be included in the sample of the entry.
See the end of part 3 of this entry.
Find out more:
- Nova Falcons: Snakes in Eivissa and Formentera (Part I).
- Nova Falcons: Snakes in Eivissa and Formentera (Part II).
- Nova Control: snake control in Ibiza and Formentera.
- Instructions for the manufacture of traps for non-native snakes.
- Thesis E.Montes.
- Feasibility of ophidian control in Ibiza and Formentera (Spanish Herpetological Association, 2012).
- The horseshoe snake.
- The ladder snake.
About Nova Falcons:
Nova Falcons is a company specializing in wildlife control and environmental services.
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