What has happened to the females? Population trends in the Aesculapian snake at its northern range limit

Abstract

Populations at the edge of their main range of distribution are often exposed to suboptimal environments. They therefore exhibit a greater susceptibility towards habitat changes and often clearly differ in their structure and dynamics from populations in the main range. Here we define population parameters of the Aesculapian snake Zamenis longissimus, a species endangered in Europe, at its northern range limit based on data obtained in the valley of the River San (Bieszczady Mts., SE Poland) in 2009-2013. We focus on the spatial and temporal sex ratio patterns by comparing data obtained in 2009-2013 with published records from 1990 to 1998 and five other northern populations (1 from the main range, 1 at the northern edge of its contiguous range, and 3 entirely isolated ones). Using the capture-mark-recapture (CMR) method, we estimated the population size to be about 230 snakes. Our data show that among adults the percentage of females (17.74%) in the San valley population was significantly lower than in those other five populations and the same population studied two decades earlier. Because of the male-biased sex ratio, we estimated the effective population size to be about 74 snakes. This disproportion between the sexes in adults may be interpreted as an early stage of the population extinction process at the northern range limit of this species’ distribution. It is probably due to the limited availability of egg-laying sites, compelling females to undertake longer movements, which may heighten the risk of mortality.