An introduced snake on Ibiza Island, the Horseshoe Whip Snake

Hemorrhois hippocrepis Photo credit: Accipiter (R. Altenkamp)

Island ecosystems may be more vulnerable to invasive species than any other ecosystems. Island species have often evolved in isolation with reduced competition and predation from mainland species. When new species invade an island they are often able to out-compete the local endemics resulting in serious population declines or extinctions. The Mediterranean’s Balearic Islands have been isolated from the continent for 5.33 million of years. Human-mediated introductions started by the end of the third millennium BCE, when humans colonized the islands and alien species introductions began. Two mammals, the European Pine Marten, Martes martes, and the Least Weasel, Mustela nivalis, together with an introduced snake, False Smooth Snake, Macroprotodon cucullatus, have been considered responsible for the extinction of the native lizard Podarcis lilfordi on the main islands.

Podarcis pityusensis Wikimedia Commons

Until quite recently, all except two of the larger 63 Mediterranean islands larger than 75 km2 harbored at least one snake species. The exception were the westernmost Balearic islands, also known as the Pityusic islands, Ibiza, and Formentera, which were never colonized by snakes. This absence of snakes was recognized by Pliny the Elder two thousand years ago. Between 12 and 13 years ago, three species of snakes (Horseshoe Whip Snake, Hemorrhois hippocrepis; Ladder Snake, Rhinechis scalaris; and the Montpellier snake, Malpolon monspessulanus) were introduced in Ibiza when old ornamental olive trees were imported from the southern Iberian Peninsula. However, there have not been any records of the Montpellier snake in Ibiza during the last six years. The Ladder Snake has been captured infrequently which might mean that it is struggling to establish a population. The Horseshoe Whip Snake, however, is expanding in Ibiza. It is a large, slender-bodied, long-tailed colubrid distributed throughout the Western Mediterranean. It was first reported on Ibiza in 2003.

In a new paper Hinkey et al. (2017) report that specimens of the Horseshoe Whip Snake obtained from an early eradication campaign showed a rapid expression of phenotypic plasticity and acquired larger body sizes than those of the source population. This was probably due to a high prey availability and few snake predators. the Horseshoe Whip Snake is thriving at the expense of a small variety of native and non-native prey. However, the predation pressure on the endemic Ibiza wall lizard, Podarcis pityusensis, the only native reptile on the island, is very high.  The Ibiza Wall Lizard represents 56% of the prey taken by the Horseshoe Whip Snake and the heavy predation may threaten its survival.

The authors conclude the Horseshoe Whip Snake threatens the biodiversity of Ibiza and that the threat may extend to smaller populations of lizards on  surrounding islands. The snake has been observed swimming in the sea and a shed skin was found on one islet. It seems that an eradication effort is needed.

Hinckley A, Montes E, Ayllón E, Pleguezuelos JM. 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):13.

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