Notes on snake control in Ibiza and Formentera (3/3)
NOVA FALCONS, COMPANY SPECIALIZED IN THE CONTROL AND CAPTURE OF SNAKES
To access the first part of this entry:
To access the second part of this entry:
Captures of non-target species with the current trapping system for invasive snakes.
Although we made a low number of captures in Zone A in relation to the number of traps (10 captures for 40 traps, in approximately 6 months), it seems that trapping with this system is, so far, the most effective method of capture (for more information on the sampling zones and subzones, see part 1 of the entry). However, there is an extra complication in relation to snake control: accidental captures.
Frequently, and especially when snake captures are lower, we found other species inside the traps: 28 lizards (P. pityusensis), 18 rodents (Mus Sp. and Apodemos silvaticus), 4 shrews (Crocidura ichnusae) and 1 gecko (Tarentola mauritanica).
In total there were 51 accidental captures of non-target species, and they all have something in common: they are captures made in Zone A, next to the Parc Natural de Ses Salines de Eivissa i Formentera and seem to increase significantly in those periods and subzones of Zone A, in which hardly any snakes were captured (Figure 6). The extreme case is August, when only 1 snake and 23 specimens of non-target species were captured. In other words, there seems to be an inverse relationship between bycatch and snake catches, at least in areas with low snake density, either absolutely or in relation to food availability. This could be useful when interpreting bycatch data, as it could indirectly indicate the abundance of snakes in the area and reflect the status of the ecosystem: the fewer snakes, the greater the biodiversity and vice versa. It could also indicate the degree to which the pest is established in the area. In other words, if we assume that the spread is coming from the northwest of the island, we could conclude that the environmental damage in the south of the island is still at a less developed stage.
Because not all traps in Zone C were managed by us, it is excluded from the graph.
During the whole monitoring in Zone A, as already mentioned, it was crucial to check the traps at least twice a week. This greatly increased the monitoring effort that we carried out for almost 6 months with 40 traps in that particular zone, as the usual frequency of checking the traps is usually once a week.
There were some casualties in the first by-catches, as it was not expected that by-catches would occur, something we had not recorded beyond the merely anecdotal in other areas of the island where the pest was more widespread.
The importance of volunteers in controlling the plague of invasive snakes.
The resident population of Eivissa and Formentera, seems to be increasingly aware of the importance of controlling the plague of snakes, as it is beginning to become evident the absence of lizards in areas with higher density of snakes. This has resulted in recent years in an increase in public participation in the campaign to eradicate this invasive species during the spring and summer months. This participation, although admirable, assumes part of a burden that should be borne to a greater degree by the public administration, whose insufficient reaction in tackling one of the island's main environmental problems (late and partial limitations on the importation of olive trees and other ornamental trees, scant distribution of traps, little dissemination of public assistance to manage captures, in too many cases there is a lack of rules, controls and regulations in relation to invasive species and their routes of entry...), contradicts the criteria of the European Commission for the eradication of the invasive species, which is in line with the criteria of the European Union. ), contradicts the criteria of the IUCN (International Union for Conservation of Nature), the highest international authority on environmental conservation: invasive species are one of the main causes of biodiversity loss in the world... and the lizard of the Pitiusas is on its way to being one more example of insular extinction.
Specimen of the Pitiusas lizard (Podarcis pityusensis), a species under real threat of extinction due to predation by invasive snakes. Photo: Roberto Bustamante.
Photo: Roberto Bustamante.
Of course, as a company, at Nova Falcons we believe that the private sector should also do what we can to control this pest. Either by providing voluntary work, free technical assistance, information on the different alternatives to contracting the service for those who do not want to contract it but do want to implement it; as well as implementing self-controls in workplaces, land and establishments where it is appropriate to do so.
For this reason, at Nova Falcons - Nova Control, apart from carrying out snake control services in the private sector, we try to do our bit by providing informative content, free technical assistance and voluntary work, as far as our resources allow.
The current situation in Zone A
During 2022 we had 40 traps active for about 6 months in the sampling area that we called Zone A, based on voluntary work that Nova Falcons undertook by providing the technicians and resources necessary for their active maintenance. This was possible thanks to the temporary cession (during the months of service) of 36 traps that the Consortium for the recovery of the fauna of the Balearic Islands (COFIB) made to us; as is done with other organisations and people who want to participate voluntarily in the control of this invasive species.
Given the location of Zone A, i.e. a public area adjacent to the Ses Salines Natural Park (with which it shares more than 3km of border), we believe that it was very positive that this was done. A total of 10 catches with 40 traps in the Zone Zone A are not many, but if we have to be taken into account that the total catch that was made in such a sensitive space and important local level, as is the Parc Natural de Ses Salines de Ibiza i Formentera in that same year was a single exemplary, the thing you can see something different mode (part, are the other 6 that were collected overtaken and the 3 by manual capture, is to say, retired from the area 19 snakes). Therefore, we consider that it was positive that this work was done, and the logical and natural thing to do would have been to continue it this year, preferably increasing the number of traps, in order to better cover those areas that were moderately covered.
Traps stacked on 12-10-22, the day on which voluntary monitoring in Zone A was terminated, having started on 2-5-22.
Unfortunately, this year in Zone A we have only a symbolic number of 4 traps installed. This is a far cry from the 50-60 that we would have liked to manage (and that is what we have been trying to do since the end of April and during the month of May 2023, with no results). To properly cover all the public space occupied by Zone A, at least 50-60 traps are necessary (i.e. 10-20 more than the ones used last year), but we cannot take them on by ourselves, without the involvement of any of the affected parties. We are talking about 50-60 traps, plus the work and resources to keep them active for 6 months at a rate of 2 checks per week, in an area, which is also in the public domain. Due to the lack of support and obstacles we have encountered when trying to repeat last year's voluntary work, with the sole purpose of trying to help in the protection of the Natural Park in particular, and the control of the pest on the island in general, this year the control is not being carried out.
The black rat (Rattus rattus): the not-so-invisible, invisible friend of Zone A
To end this third part of the entry we called "Notes on snake control in Ibiza and Formentera" (and before moving on to some brief conclusions), we would like to share with you an anecdote about an "annoying friend" that sometimes comes along with snake control: rats. In Zone A, there is (or at least there was during the spring and summer of 2022), at least a fairly "healthy" population of rats, specifically black rats (Rattus rattus).
The problem that rats create for snake control is basically in the form of damage: broken drinking troughs (in the same trap, up to 3 different drinking troughs completely destroyed in different revisions, with the risk of death due to lack of water that this means for the live bait), accidental entry into the snake's compartment... from which they are able to get out, but damaging the entrance trap door in the process, which means that it is always necessary to check that it is in perfect condition, and to straighten or repair it again when a rat has manipulated it trying to get out. But obviously, if a snake enters before this check is carried out, it is possible that it will manage to get out, which means that rats negatively affect the effectiveness and quality of snake control.
Also, living up to their definition as rodents, they cause structural damage to the traps in the form of gnawing, attracted by the smell of the live bait (mice) and their food. They generally damage the area of the snake's entry mechanism, both from the internal and external side when trying to enter and exit the trap.
In one extreme case, they literally smashed the bottom of the mouse compartment, including the wire mesh.
So "lustrous" was the rat population (we don't know the current state of the problem), that they nested under one of the traps...
This allowed us to see the development of the litter through the visits: from the closed-eyed hatchling stage, to watching them scurry away as they began to prowl around the nest.
Although brief, the data collected seem to be in agreement with the main conclusions drawn from the studies carried out to date on the horseshoe snake on the island (see references at the end of the text). Basically, the following conclusions can be drawn:
- The density of snakes in Zone A, located in the area around Ses Salines (south of the island), seems to be lower than in Zone B, located in the area of Roca Llisa - Jesús (closer to the focus from where the plague started); at least, if we base it on the performance of the traps. However, as we have already explained, this netting may be affected by the amount of food available, and consequently, reflect a "false" low density. Therefore, it would be more accurate to say that the Ses Salines area has probably not yet reached the peak of snake numbers (they are increasing), which has resulted in a drastic reduction of certain snake prey (including lizards).
- Snake activity seems to be reduced during August, largely due to heat and drought. Also, as is normal for a cold-blooded animal, it is reduced during the winter months due to the cold. Peaks of activity are found during the breeding period and before the onset of winter, the latter also coinciding with the hatching of the clutches.
- In the case of the horseshoe snake, it is possible that they may be breeding for more months of the year than on the mainland (it is certainly important to continue gathering information on their reproductive cycle on Ibiza and Formentera).
- Horseshoe snakes feed mainly on lizards and rodents, and seem to be subject to low predation pressure.
- Catches (or lack thereof) of non-target species could be indicative of the density, spread and absence of snakes in the area, as well as the state of biodiversity in the area as far as snake prey is concerned, when these can also potentially be caught accidentally in the traps.
- If technically feasible, it is advisable to check the traps more than once a week, ideally 2-3 times, to avoid both deaths and predation from accidental captures inside the traps. This is even more so in those areas where the density of snakes is not yet too high, at least in relation to the amount of food available. That is to say, if we place the traps in areas where we still see lizards relatively frequently, there is a good chance that some of them will enter the traps at some point, and it is advisable not to wait until a week has passed to check the traps in order to avoid their death.
- To date, the ladder snake (Zamenis scalaris) is still much less widespread than the horseshoe whip snake (Hemorrhois hippocrepis) on the island of Ibiza.
It is impossible to know 100% how the snake plague will interact with the different species that make up the local ecosystems if the population continues to increase, because snakes not only alter the populations of those species they prey on: they also alter the populations of those species with which their prey interact. That is to say: all those species, plant or animal, with which the snakes' prey interact (which the snakes are reducing in population), will in turn see their own natural balance within the ecosystem altered (which in turn can lead to third imbalances or new balances in the ecosystem's relationships). In an ecosystem, all its elements are related and interact with each other directly and indirectly, and breaking or altering these relationships may (or may not) bring about far-reaching changes to the ecosystem.
If we agree with this reality, the question that must be asked today is whether or not it is worth taking the risk of assuming consequences that may be irreversible, for example, in the case of the extinction of certain species that are then irretrievable. If the answer is no, then it is obvious that we must demand greater action, paying special attention to such sensitive areas as the Parc Natural de Ses Salines de Eivissa i Formentera, and all its immediate surroundings; something that is not happening at present.
Geckos are other prey that fall within the range of invasive snakes.
Small birds in general, but more specifically their eggs and chicks, and even more so when, as in the case of the common ground nuthatch (Calandrella brachydactyla), they are ground-nesting species, are potential prey for snakes. Invasive snakes are polyvalent predators that today have their attention fixed on the most common prey, the lizards of the Pitiusas and rodents, but as the lizards in particular decline in population, the snakes will turn their attention to other prey that they have at their disposal. This nest has been photographed this spring in Zone A, next to the natural park (or inside... depending on how you want to look at it), a location that has the most important breeding area of the common terrapin on the island of Ibiza.
Álvarez C., Mateo J.A., Oliver &Mayol J. (2010). Los ofidios ibéricos de introducción reciente en las Islas Baleares. Boletín de la Asociación Herpetológica Española 21: 126-131.
Decreto ley 1/2023, de 30 de enero, de medidas extraordinarias y urgentes para la protección de la lagartija pitiusa (Podarcis pityusensis) y la lagartija balear (Podarcis lilfordi) y para la prevención y lucha contra las especies de la familia Colubridae sensu lato.
Fisher, S., Fisher, R. N., Alcaraz, S. E., Gallo‐Barneto, R., Patino‐Martinez, C., López‐Jurado, L. F., ... &Grismer, J. L. (2021). Reproductive plasticity as an advantage of snakes during island invasion. Conservation Science and Practice, 3(12), e554.
Hinckley, A., Montes, E., Ayllón, E., &Pleguezuelos, J. M. (2017). The fall of a symbol? A high predation rate by the introduced horseshoe whip snake Hemorrhois hippocrepis paints a bleak future for the endemic Ibiza wall lizard Podarcis pityusensis. European Journal of Wildlife Research, 63, 1-8.
Montes, E., Feriche, M., Alaminos, E., &Pleguezuelos, J. M. (2020).The Horseshoe whip snake (Hemorrhois hippocrepis) on Ibiza: predator release in an invasive population. Amphibia-Reptilia, 42(2), 249-254.
Montes, E., Feriche, M., Ruiz-Sueiro, L., Alaminos, E., &Pleguezuelos, J. M. (2020). Reproduction ecology of the recently invasive snake Hemorrhois hippocrepis on the island of Ibiza. CurrentZoology, 66(4), 363-371.
Montes, E., Kraus, F., Chergui, B., &Pleguezuelos, J. M. (2022). Collapse of the endemic lizard Podarcis pityusensis on the island of Ibiza mediated by an invasive snake. CurrentZoology, 68(3), 295-303.
Piquet JC, López-Darias M. 2021 Invasive snake causes massive reduction of all endemic herpetofauna on Gran Canaria. Proc. R. Soc. B 288: 20211939.
Richmond, J. Q., Wood, D. A., Stanford, J. W., & Fisher, R. N. (2015). Testing for multiple invasion routes and source populations for the invasive brown treesnake (Boigairregularis) on Guam: implications for pest management. BiologicalInvasions, 17, 337-349.
To access the first part of this entry:
To access the second part of this entry:
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.
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.
- Hits: 186