25 October 2006

Land snail dispersal

Dispersal of the proverbially slow land snails over suitable habitats is an interesting topic. Whilst the ranges of some species, such as Zoogenetes harpa, naturally extend across several continents, others are restricted to smaller areas.

Yesterday, I read a short paper by Baur and Baur titled "Dispersal of the land snail Helicigona lapicida in an abandoned limestone quarry" (Malak. Abh. 24:135-139, 2006).

The authors released 300 marked adult snails of the said species in a limestone quarry and then monitored the dispersal of those snails over a 2-year period. Although the recovery rates of the marked snails were low, 16.7% after 1 year and 4.0% after 2 years, the results do provide a general estimate of how far and how quickly this species disperses.

Fig. 1 from Baur & Baur, 2006. Arrows indicate median displacements.

Median dispersal was only 1.7 m 5 months after release, but increased to about 6.4 m after 2 years. These results roughly agree with the measured dispersal rates of other species of land snails. However, smaller species in general tend to move less within a given period than do larger species.

If we assumed that the measured dispersal rate of H. lapicida was constant over a long period and there was a continuous limestone habitat, the approximate median dispersal of the snails would be 32 m after 10 years, 320 m after 100 years and 3200 m after 1000 years. You get the idea. Dispersing barely 3 km over a thousand years isn't much of a dispersal. Over geologically long-enough periods other factors, for example, the movements of the continents, also come into play. Of course, we also have to take into account assisted dispersal facilitated by winds, animals, etc.


pascal said...

I think there's a disconnect in scales when we look at dispersal - at small time scales, dispersal is exceedingly slow, but at large time scales (thousands of years) dispersal could happen relatively quickly.

Perhaps it's a buildup of statistically unlikely events. It's likely that the snail will move a few meters and survive. It's unlikely that any one snail will get carried many thousands of meters (let alone enough snails to establish a genetically viable population). But over long time periods, that likelihood would increase - perhaps as a step function, rather than a standard linear one. Thus, at long time intervals, we see large movements of populations, but at short time intervals, we'd be lucky to see any movement at all.

Step (1945) wrote about finding a Cantareus aspersa resting on a flower pot: "[after] penciling my initials on his shell, I hurled the snail as far as I could. Next morning, he was again attached to the pot though to regain it he had to cross a very broad road and low wall."

That seems like a pretty fast snail to me - probably 10 meters or more in under 24 hours...

Ken said...

Would the dispersal rate be affected much by the availability of food, with snails tending to move more if in low food areas ?


Pascal: I am surprised the snail's shell didn't break after Step threw it and after it landed on the ground.

Ken: Yes, of course. Or, if they came to the edge of a suitable habitat, they would be forced to stop moving.

There are also problems with the methodology that are discussed in the paper. For example, the initial crowding (snails were released in groups at designated spots) may force them to migrate quickly in the beginning and then to slow down.


Comments from Tim Pearce sent in an e-mail:

For studying long-distance dispersal, I think you want to look at the maximum, not the median dispersers. Sure, the median moved 3.2 meters in one year, but the maximum moved 10 times that far in one year. Secondly, what do we suppose about the snails that were not recovered. Could the 250 snails that were not recovered after 1 year have all moved more than 31 meters, and that is why they were not recovered? Not likely, but possible. Studies like this are very important, but also have some limitations. Anyway, for studying long distance dispersal, I think you need to look at the rare long distance dispersers, not at the median of the population. The rare ones can form new colonies.


The authors were aware of the limitations of their study. The other thing is that they looked at the dispersal of marked snails thru an area that was already occupied by the same species. But, in a situation where the snails are dispersing across unpopulated habitat, the maximum distance moved would be meaningful in the case of the species that are selfers. Otherwise, a single unmated snail that moves, say, 50 m away form all the other snails is not likely to start a new colony. Of course, it would be important to know if one mated snail can move far away & start a new colony.


Comment e-mailed by Bert Bartleson:

Concerning Cornu aspersa Müller, 1774, I agree with Pascal that they are certainly relatively speedy, as snails go at least.

We built a new house about four years ago. Our neighbor to the South had a very large number of C. aspersa in their garden when we arrived. During construction our entire lot was cleared down to the sandy subsoil (pretty sterile!). During construction I found a snail that had crossed about 25-30 feet of bare sand and climbed a newly poured concrete wall and stuck itself to the wall. It was a juvenile. After we moved in we had landscaping done on the lot. By the end of one year's time our entire lot (110 X 150 feet) was colonized by the C. aspersa from neighbor to the South. The snails reached the lot to the North which is at least 150 feet away during that first year as well.

I made another observation about C. aspersa a number of years earlier. My son, who was a teenager at the time, borrowed my car for a date. When I went out to the garage the next morning there was a snail stuck to the side of my car. It had hitch-hiked home with my son the night before. All my son would say was that he had been parking at "location X". So I don't know how many miles the snail traveled that night. But this method could certainly speed up dispersal.

My guess is that plant materials infested (colonized) with snails are the most common means of rapid snail dispersal. One of my neighbors bought new plants and then called me to identify the "new snails" that had just appeared within a few days of planting the plants in June, 2005. They were Cepaea nemoralis (Linnaeus, 1758). I had never found them in Olympia prior to 2005.

One of my favorite collecting spots for land snails is a nursery about 120 miles north of here near Mt. Vernon, WA. We found 7 species there last year, some native, some introduced. In addition to the abundant C. aspersa and C. nemoralis, we also found Oxychilus alliarius (J. S. Miller, 1822), Vespericola columbiana pilosa Henderson, 1928, Cochlicopa lubrica (Müller, 1774), Ancotrema sportella (Gould, 1846), and a species of Succinidae.

Bert Bartleson, Olympia, WA., USA