Distribution of arid-dwelling land snails according to dryness

Although land snails are hydrophilic animals, several species inhabit arid or semi-arid environments. Here, I hypothesize that, for arid-dwelling land snails, both relatively moist environments and extreme arid zones, within their distribution ranges, should be disadvantageous. Therefore, arid-dwelling land snails should show maximal probability of presence and maximal abundances at intermediate levels of aridity. I tested this hypothesis with two land-snails from Sierra Elvira mountain range (SE Spain), Sphincterochila candidissima and Iberus gualterianus. Given that environmental variables as well as snail distribution showed spatial autocorrelation, I performed spatially explicit models, specifically simultaneous auto-regressions (SAR). The results supported the hypothesis, with the distribution of S. candidissima and the abundance of I. gualterianus following a concave-down relationship with aridity. Moreover, both species were less abundant as elevation increased, and I. gualterianus showed a positive association with rocky surface. Therefore, this study highlights that, in arid environments, arid-dwelling land snails show maximal abundance and probability of presence at intermediate aridity levels. Although the reasons explaining why extreme aridity values limit the abundance and distribution of land snails are well detailed, it remains intriguing why these snails decrease in abundance when moisture increases.