I still have snail dispersal in my mind. In Part 1 of this series, I had assumed that the net dispersal distances* were normally distributed. Today, in a paper by Baur & Baur (1993), I saw a plot of the frequency distributions of distances moved per day by the land snail Arianta arbustorum in 2 different habitats.

Fig. 1 from Baur & Baur (1993). Left plot was for dispersal in a forest clearing and the right plot was for dispersal in a 1-m wide grassy strip.The distributions were not normal at all. Baur & Baur fitted an exponential curve to their data. To simulate this distribution with my data from Part 1, which were normally distributed, I converted each dispersal distance to its inverse logarithm, thus creating a lognormal distribution.

Here are the lognormally distributed dispersal distances and their random dispersal angles in the form of a polar plot as in Part 1.

When dispersal distances are lognormally distributed, most will be near the origin, while a few will be far away from the origin. In comparison, in normal dispersal as in Part 1, most dispersal distances will be away from the origin and there will be more of them far away from the origin.

Do all or most snail species disperse lognormally?

*The net dispersal distance for each snail is the distance between the origin (the release point) and the snail's location at the end of the dispersal period.

Baur, A. and B. Baur. 1993. Daily movement patterns and dispersal in the land snail

Arianta arbustorum.

Malacologia 35:89–98.