Method
This exercise is based on data collected from three grassland habitats in the Corbières of southern France, in early April 2006. These were along minor roadsides, which allowed us to sample almost a single line of fennel plants in each location. In most cases, a plant had just two neighbours - the one before and the one after. Because plants were sampled in sequence the numbers of snails on neighbouring plants are known along each transect.At this time of year, new growth was sprouting from the base and considerable numbers of active snails could be found here. Above this, last year's (dead) stems carried the snails we counted, which had glued themselves in position.
The first transect was in a lowland area adjacent to a stream, the second alongside a vineyard and the third on an exposed (windswept) hillside road. Films of the habitats are provided, as are some photographs. At least five species of snails were found but these are not distinguished in the counts.
Snails were counted on fennel stalks greater than 5mm in thickness (each stem was measured at its 10cm height), or on the six thickest stalks for each plant. Note that this will not represent the total count for the entire plant (uncounted and smaller stems also had snails on them).
The data is presented in an excel spreadsheet. This shows the counts for each stem, for each plant and each transect, as well as the distance to the nearest fennel plant.
This allows you to plot the count data at each of three different levels:
- Habitat - to compare the transects for differences in abundance and distribution between the three sites.
- Plants - to compare plants within a habitat to see if some plants are favoured more than others.
- Stems - to compare stems within plants to see if some stems are favoured over others - whether distributions are clumped within plants.
We could thus use this data to decide whether the snails are more common in one of the habitats, whether they preferred occupied or vacant plants in each habitat, or whether one or more stalks are favoured in each plant.
We suggest you plot the data as histograms to determine whether the snail species are clustering at each level.
You will need to think about how to plot the data to examine the suggested questions above. Clearly, if a snail seeks the company of other snails we should expect clustering on particular stalks and on particular plants. We might also expect snails to be more abundant in some habitats compared to others.
In fact, there are lots of ways in which this data can be analysed to give more insight into the behaviour of these snails. You may review these within your group and perhaps with your tutor to see what else you could learn about the over-wintering behaviour of the species we counted.
