Demographic monitoring of the invasive ladder snake on Formentera (Balearic Islands, Spain)

Abstract

The ladder snake Zamenis scalaris was first reported in 2006 in Formentera (Balearic Islands, Spain), until then a snake-free Mediterranean island. Since 2016, intense control campaigns have been carried out and, for the first time, snake captures over 2017-2020 have been used to analyze the sex ratio, age, fat percentage, population density, and seasonality of this invasive population. The male-biased sex ratio found in the present work could be the result of capture biases caused by different detection probabilities between sexes. A reduction of snake body length over time was observed, which could indicate a depletion of the larger snakes due to intense snake capture. The highest capture frequencies of both sexes were reported in May and June, coinciding with the mate searching period in the species native range and indicating that snakes increase their movements during this period. Size-specific survivorship curves indicated a drop in survivorship at medium sizes, which suggested a survival cost associated with reproduction, presumably due to increased mortality during the mating season. Population density decreased from 1.812 snakes/ha in 2017 to 0.669 snakes/ha in 2020. Evidence obtained in the present work has allowed to evaluate the main ecological aspects of Z. scalaris and the current state of the population of this invasive snake on the island of Formentera. Results showed that the population is well established in Formentera, but prolonged control campaigns have caused its decline. This study becomes a starting point to improve the management and control programs of invasive snake populations in the Balearic Islands. Long-term control as well as the improvement of the detection and capture of adult females are strongly recommended to attempt the eradication of Z. scalaris in Formentera.