Effect of population density on water influx and distribution in the desert snail Trochoidea seetzenii

Abstract

Using the snail Trochoidea seetzenii and a field study, we tested whether water influx in a desert organism is density dependent. We introduced 160 snails (mean body mass ± SD = 1.85 ± 0.25 g) into sites with natural high (~50 snails m-2) and low (~10 snails m-2) density populations. The introduced snails were divided into two groups: 1) free, which could disperse into the local population, and 2) enclosed, which were kept under constant and high density (~50 snails m-2). Water influx of the snails was measured when their water source was only dew and also after a rainfall. In addition, we measured the movement of snails on shrubs after dew and among shrubs after rainfall. The following results emerged: i) water influx was highest for free snails that were introduced into low-density populations; ii) water influx of free and enclosed snails in sites with high-density populations were similar; iii) the number of snails that moved on a shrub in relation to the total number and the average vertical distance movement per snail were density dependent; and iv) movement distance among shrubs was higher for snails that were introduced into high-density areas than for snails that were introduced into low-density areas. We concluded that when T. seetzenii used dew on shrubs as a water source, its water influx was density dependent. This relationship may be explained by the small amount and short duration of dew availability and by the searching behaviour of snails for dew. We assume that density-dependent water influx may be found in many species of desert invertebrates and suggest that density-dependent water influx may be a controller of invertebrate behaviour, distribution, and abundance in arid environments.