Recent Invaders in Small Mediterranean Islands: Wild Boars Impact Snakes in Port-Cros National Park

Abstract

Mediterranean islands host unique ecosystems that are particularly vulnerable to invasive species. However, knowledge regarding the precise impact of invasive species on local biodiversity remains limited for many of these systems. Here we report on the negative impacts of invasive wild boars (Sus scrofa) on native snakes on islands in the Mediterranean basin. Capture-mark-recapture was initiated in 2012 on two snake species (Montpellier snake, Malpolon monspessulanus and Ladder snake, Zamenis scalaris) across two islands of Port-Cros National Park. Several wild boars, an invasive species, reached the islands in 2007. They remained confined to small areas of the islands for several years. In Port-Cros, the numbers of wild boars suddenly increased in 2015, and rapidly colonized the whole island damaging vast land surfaces. In Porquerolles, wild boars did not proliferate. This offered an opportunity to examine the impact of wild boar outbreak with a Before-After Control-Impact design (BACI). Snake counts and mark-recapture modeling showed that demographic traits were stable before 2016 for both snake species on both islands. As well as abundance, recruitment, and population growth rate of Montpellier snakes significantly declined where wild boars proliferated but remained constant on the island where they did not. Wild boars probably impacted snake numbers through habitat destruction and direct killing. The rapid decline of snakes (apex predators) and intensive uprooting that strongly damage ground dwelling species (plants, animals) suggest that wild boars represent a serious threat to island biodiversity. As elsewhere around the world, these invasive ungulates proliferate in the Mediterranean basin, they are proficient swimmers and exhibit a remarkably high invasive potential. We recommend vigilance and fast eradication to prevent population outburst; even a few a localized non-proliferating individuals contain the latent potential for devastating outbreaks.

Construction of a refuge wall with crevices to protect European leaf-toed geckos Euleptes europaea and young Turkish geckos Hemidactylus turcicus on the Ile du Levant, France

“…comm.). However, it is now known that these refuge structures are easily damaged by wild boar following their arrival on the islands of Hyères (Ballouard et al, 2021), and potentially result in the predation of the geckos when they turn over the piles of stones and tiles. There are already wild boar present on Ile du Levant and we observed wild boar dung close to the walls throughout the year but they remained undamaged so we conclude that the breeze block walls are not vulnerable to wild boar.…”

Biodiversity Management in a Mediterranean National Park: The Long, Winding Path from a Species-Centred to an Ecosystem-Centred Approach

“…In Port-Cros, the wild boar has been blamed for digging up the soil, in search of food, and as a result degrading some populations of rare and protected plants, notably geophytes (e.g., the narrow endemic Romulea florentii), and of the rare and locally endangered Sardinian frog (Discoglossus sardus) located in some temporary ponds. It has also been blamed for the reduction of the abundance of the snake Malpolon monspessulanus [163]. The so-called 'negative' impact of wild boar must be compared with its 'positive' effects, which are part of an ecosystem approach: (i) the aeration of the soil; (ii) the reactivation of the soil seed bank, including bulbs; and (iii) the return of coprophagous insects which were eliminated by chemical vermifuges used for domestic livestock, as boar droppings have become a real refuge for many of these endangered insects everywhere (Philippe Ponel, unpublished data in [92]).…”

“…Most of the members of the SC of the N-PCNP are of the opinion that the role of a the National Park is not to combat a natural process or its impact on associated species (even if they are rare and protected plants), which is consistent with an ecosystem-based approach; in addition, they take into account the fact that if rare plants (e.g., Romulea florentii) are present today, it is because they have resisted the presence of wild boar-occasionally or permanently-for millennia. In the same way, if the snake Malpolon monspessulanus is still present, while unable to swim across several kilometres of sea and therefore to recolonize islands if it is becoming extinct, it is because it resisted the presence of wild boar over millennia under natural environmental conditions (an obvious fact which is not considered by Ballouard et al [163]). However, taking into consideration the fact that the wild boar is not a threatened species, and that the well-being of the inhabitants is an important feature of the N-PCNP governance, the SC was not opposed to the control of the wild boar, via trapping and hunting (management: T26 and T32).…”

“…Taxonomic lobbies (e.g., bats, birds, flowering plants, marine mammals, snakes, turtles) obviously play a negative, counter-ecological role. For example, (i) the assimilation of biodiversity to the number of species, which is very reductive; (ii) the belief that the higher the number of species, the better; (iii) the belief that disturbed habitats are species-poor and of low interest; (iv) the belief that fire is per se an ecological disaster, whereas, if it is not too frequent, it fulfills useful ecological functions-at Cap Lardier (mainland Provence, N-PCNP), the 2017 fire caused massive germination of the Magnoliophyta Anthyllis barba-jovis, which is of heritage value, and the appearance of a fauna of xylophagous beetles; (v) the belief in the balance of nature; (vi) ignorance of natural fluctuations in populations; (vii) the popular but non-scientific belief that wild boar are invasive (see, e.g., [163]); and (viii) an implicitly fixist vision of the environment. These biases affect the general public and the media, but also sometimes park officers, wardens and even scientists.…”