10 February 2010

Simulation of snail dispersal - Part 2

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.


Duane Smith said...

Does the population dispersal pattern depend of the distribution of food? For example, if food were abundant everywhere (or at least locally at the origin), than population distribution might follow an exponential pattern but if it were uniformly scarce, then population distribution might be normal. Just a wild speculation. But I think it is an empirical question that can be tested.

fred schueler said...

The distribution of Cepaea on the streets in the village here (Bishops Mills, Ontario) certainly looks something like log-normal on most nights: a great number near the edge of the pavement and a few that go out onto the middle of the road.

As for Arianta, my experience in Toronto is that they never come up onto the surface, except to miraculously die and leave their shells, so how could they disperse?