Intrinsic differences in dispersal between populations of gastropods separated by a few metres: Evidence from reciprocal experimental transplantation

Differences between populations in dispersive behaviour can strongly influence population and community structure and have important implications for evolution. Differences in dispersal can be caused by intrinsic differences among populations or by different extrinsic cues. In a semi exposed shore in Co. Wicklow, Ireland, intertidal gastropods, Littorina littorea moved long distances in areas with low natural population density compared to areas with high natural density but were not influenced by density per se. A reciprocal experimental transplantation distinguished between intrinsic and extrinsic factors as causes of differences in dispersive behaviour. Snails from the area of high density were transplanted to the area of low density and vice versa. Comparisons were also carried out with controls (disturbed and undisturbed individuals) in each area. Dispersal of transplanted snails was compared with that of snails translocated within each area. Mean distances displaced and percentage dispersal were monitored after two days period. To test temporal generality the experiment was done twice. The two experiments yielded different outcomes. The majority of evidence supported the intrinsic model: transplanted snails dispersed differently from controls in their new area and similarly to controls in their area of origin. However, in one of the experiments there was some evidence suggesting influence of extrinsic factors or an interaction between extrinsic and intrinsic factors. Further research is required to identify whether the observed intrinsic differences are genetic or due to internal conditions that have been modified by the animal's present and/or past environment. This study reinforces the value of repeated experimental transplantation to characterize factors causing differences in behaviour.