Increased population density depresses activity but does not influence emigration in the snail Pomatias elegans

Dispersal is a key trait linking ecological and evolutionary dynamics, allowing organisms to optimize fitness expectations in spatially and temporally heterogeneous environments. Some organisms can both actively disperse or enter a reduced activity state in response to challenging conditions, and both responses may be under a trade-off. To understand how such organisms respond to changes in environmental conditions, we studied the dispersal and activity behaviour in the gonochoric land snail Pomatias elegans, a litter decomposer that can reach very high local densities, across a wide range of ecologically relevant densities. We found that crowding up to twice the maximal recorded density had no detectable effect on dispersal tendency in this species, contrary to previous results in many hermaphroditic snails. Pomatias elegans is nonetheless able to detect population density; we show they reduce activity rather than increase dispersal in response to crowding. We discuss these results based on the ability of many land snails to enter dormancy for extended periods of time in case of unfavourable conditions. We propose that dormancy ("dispersal in time") may be more advantageous in species with especially poor movement abilities, even by land mollusc standards, like P. elegans. Interestingly, dispersal and activity were correlated independently of density; this dispersal syndrome may reflect a dispersal-dormancy trade-off at the individual level. Additionally, we found snails with heavier shells relative to their size tended to be less mobile, which may reflect physical and metabolic constraints on movement and/or survival during inactivity. We finally discuss how the absence of density-dependent dispersal may explain why P. elegans is often found at very high local densities, and the possible consequences for ecosystem functioning and litter decomposition.