Slugs and Snails by Robert Cameron

Author:Robert Cameron
Language: eng
Format: epub
Publisher: HarperCollins Publishers
Published: 2016-10-13T16:00:00+00:00

The Cotswold woods are a particularly easy habitat to search. On limestone, numbers can be very large, and a relatively small amount of effort is required to get a good inventory of species. As a rough guide, Beata Pokryszko and I worked out that you should have at least 10 times as many specimens seen or collected as there are species in your sample. In many places this is quite hard to achieve, and the guide is certainly not foolproof. When you collect litter to sort later, you do not know until you have left the area just how many specimens you have. When lists are compared, these sampling problems have to be remembered. Slugs present a particular problem. Not only may they escape most sampling methods in dry weather, they also disappear from litter and soil samples left drying before sorting. Snails dying leave their shells behind. Many slugs can be identified only by dissection of well-preserved specimens. Repeated searches over the same area are the only way to build up a good inventory. Many of the studies of the faunas of particular habitats considered next exclude slugs.

Some Characteristic Faunas

In Britain, the most abundant habitat before human intervention was forest. Of course, these forests varied greatly, depending on climate and on the underlying rock and soil properties. We now have only fragments of these forests left, all modified or managed by us. In addition, there are many woods formed by plantation, or by natural succession on previously cleared land. The richest snail faunas are found in ancient forests on calcium-rich soil, and are at a maximum when those forests contain permanently wet areas and exposed rocks. The Cotswold woods mentioned earlier (Table 26) combine these features, and are among the richest known. Although there is some geographical variation, with faunas in the north lacking a few species found further south (and vice versa), the faunas of such woods are very similar to each other. In one study of 30 such woodland sites (Cameron et al., 2006), each of 400 square metres, among four areas in southern England including the Cotswolds, the mean level of similarity in the faunas was 82 per cent. Of 43 snail species recorded overall, 35 were present in the richest single site. Variation in richness and composition among sites was slight and caused by the degree of disturbance, the presence or absence of wet patches within the site, and the geographical limits of only two or three species.

Away from limestone or chalk, the most important character of forests for snails is the status of the soil. Even in places where the rocks contain little calcium, it is leached downhill, and the presence of wet areas in valley bottoms provides the combination of dampness, increased calcium and palatable leaf litter that many snails need. Provided there is no gross disturbance and some long-term continuity in tree cover, such valley-bottom woods may have very rich faunas, containing a number of species typical of permanently wet conditions, such as Vertigo substriata, Succinea putris and Euconulus alderi.

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