Decline of a common reptile: case study of the viperine snake Natrix maura in a Mediterranean wetland

The Ebro Delta is a wetland area in which natural ecosystems have been partially replaced by rice fields. This mixed and productive landscape has allowed the establishment of a rich community of organisms. The viperine snake Natrix maura has traditionally been a common and abundant predator because the habitat is favorable and prey availability is high. In June 1995, we conducted a demographic study to evaluate relative densities of snakes in the rice fields. Thirteen years later, we repeated the same study in the same area and season. The field work consisted of 29 censuses of one hectare each, and snakes and their potential prey (green frogs and fish) were counted. In 1995, we found 27 snakes (0.93 animals/ha), these occupying 48% of the sites. Frogs and fish were observed in 23 of the 29 censuses (79%). In 2008, no snakes were found and frogs and fish appeared in only 11 of the samples (38%). In 2008, we also prospected 20 sites in rice fields located next to the natural lagoons. At these sites, we detected a greater number of snakes (25% of the stations). Several factors can explain the clear decline of the N. maura population in the Ebro Delta rice fields: 1) the transformation and degradation of the habitat; 2) the increase in population densities of natural predators such as herons; 3) the decrease in prey availability; 4) the massive use of pollutants in the rice fields; and 5) snake death on local roads and directly by human persecution. We propose that a combined effect of these factors has caused the alarming decline of this predator. The observation of water snakes in rice fields near natural lagoons indicates that protected natural areas act as natural refuges for fauna with reduced mobility, such as viperine snakes. The recovery of the N. maura population in the rice fields of the Ebro Delta depends on an integral change in agricultural management, including the reduced use of pollutants, the recovery of snake prey, and the maintenance of favorable habitats to prevent predation