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

Abstract

Invasive species currently account for a major threat to global biodiversity, and island ecosystems are among the most vulnerable, because of the frequency and success of species introductions on islands. Within Mediterranean islands, reptiles not only are frequently introduced species but are also among the most threatened because of these introductions. The Balearic archipelago is a good example of this, since only two of its current 16 species of reptiles are native. Thirteen years ago, the snake Hemorrhois hippocrepis was introduced by cargo in Ibiza island, and it is in expansion. Individuals obtained from an early eradication campaign showed a fast expression of phenotypic plasticity and acquired larger sizes than those of the source population, probably due to a high prey availability and predator scarcity. The species is thriving at the expense of a small variety of native and non-native prey, but the predation pressure on the endemic Podarcis pityusensis, the only native reptile in the island, is very high, as this lizard represents 56% of the prey in frequency, which might threaten its survival on the long term. Our results on the feeding ecology of the snake are of sufficient concern to justify the maintenance of actions to eradicate this invader.

The Horseshoe whip snake (Hemorrhois hippocrepis) on Ibiza: predator release in an invasive population

“…The invasive population of H. hippocrepis on Ibiza is exhibiting a fast process of phenotypic plasticity; in only 13 years, some island individuals have become larger (105% of body length and 213% of body weight) than individuals from the source population (Montes et al, 2015). However, the study of its feeding and reproduction ecology failed in supporting the extraordinary body condition of individuals and the invasiveness of the population (see also Hinckley et al, 2017 andMontes et al, 2019). Thus, we suggest that their success may be favored by one of the environmental conditions of Ibiza, that is the low predation pressure on H. hippocrepis on this island (Hinkley et al, 2017), given that most of its predators in the source area are absent on the invaded island.…”

“…We discard alternative causes of tail breakage for the Ibiza population, like tail secluding by large and dangerous prey as rats being subdued (Rattus sp. ; Gregory and Isaac, 2005), because of the low frequency of this prey in its diet (only two of 293 individuals were found with that prey in their guts; Hinckley et al, 2017).…”

“…Furthermore, the trophic and reproduction ecology of H. hippocrepis failed to explain the success of that invasive population on Ibiza; the low rate of feeding frequency (20.8% were fed individuals) and the small size for the main prey (a small lizard, 4 g of body weight on average), poorly supports the high body condition of alien individuals (Hinckley et al, 2017); and their much lower female reproduction frequency compared to the native population (biennial vs annual) was not conclusive in determining the invasiveness of the snake (Montes et al, 2019).…”

“…In this scenario, the endemic lizard P. pityusensis is our first concern, representing more than half of the snake diet (Hinckley et al, 2017), while steeply disappearing from the invaded area (unpubl. data of the authors).…”

New insights into the historical translocation of the Algerian hedgehog and pine marten throughout the Balearic Islands (Western Mediterranean): refining the radiocarbon‐based chronology

“…Sanders & Reumer, 1984). The body size change in Hemorhois hippocrepis on Eivissa and Lampropeltis californiae on Gran Canaria are recent examples of an abrupt body size change occurring immediately after the colonisation event (Fisher et al, 2019;Hinckley et al, 2017). According to the available data, the body size of the garden dormouse (Eliomys quercinus) and the wood mouse (Apodemus sylvaticus) seems to have been unaltered on Mallorca and Menorca presumably from their first archaeological records (c. 2000 BC) until the introduction of the weasel on the islands.…”