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Terrestrial molluscs living in Britain at present are generally small in size, the majority being less then 10mm in maximum length/width, and many less than 5mm. Their small size and rate of movement means that they are restricted to relatively small areas of ground during life, and that the site where they die is likely to be good indicator of the habitat in which they were living. This does not necessarily hold for an assemblage found some time after death, as the dead shells may have moved large distances, especially as a result of wind and/or water action. When interpreting any death assemblage it is important to consider whether that assemblage is autochthonous, in situ, and representative of the habitat during life or allochthonous, not in situ, and displaced from the site occupied while alive.

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Despite these restrictions, knowledge of the varying ecological preferences can provide useful information concerning the local habitat at the time when those molluscs were living. The value of mollusc analysis in this context depends on the assumption that the habitats preferred by subfossil species are the same, or similar, to those of modern species – the principal of uniformitarianism – but this may not always be the case.

It has long been recognised that different taxa of non-marine Mollusca may be found in different habitats (e.g. Boycott 1934; Evans 1972). Some species live in open country exposed to the sun while others require varying degrees of shade, either continual shade, or that afforded by the base of vegetation. There are many taxa which are more catholic in their requirements, with the ability to live in widely differing environments. Most molluscs also require access to a source of calcium, an essential element necessary to build their shells.

The first detailed reports of the ecological preferences of British non-marine molluscs appeared in the early years of the twentieth century when Arthur Boycott reported on the habitats of freshwater Mollusca (Boycott 1918); he followed this with several brief reports but his seminal papers in the 1930s on both land and freshwater molluscs provided a solid base which other investigators later developed (Boycott 1934, 1936). Boycott recognized terrestrial species as preferring wet places (hygrophiles), dry places (xerophiles) or woodland, while freshwater species were divided into those living in rivers, canals or lakes. Boycott’s work was furthered by Bruce Sparks (1961, 1964) who explored the environmental requirements of fossil molluscs and then expanded this to include archaeological deposits (Sparks 1969).

Meanwhile Michael Kerney was also using molluscan analysis in relation to archaeological sites to investigate the environments in which prehistoric man lived (Kerney 1963; Kerney 1966; Kerney 1968).

But it was not until John Evans published his seminal work Land snails in archaeology (1972) that the value of molluscs in palaeoenvironmental reconstruction entered the archaeological mainstream; while Evans’s work concentrated on the chalk downlands of southern England, he also discussed the value of molluscs in dune environments. Many more recent studies have been reviewed by Paul Davies (2008).

It is now generally accepted that terrestrial molluscs can be divided into three broad categories: shade preferring, catholic and open country (e.g. Boycott 1934; Sparks 1961; Evans 1972, 194), although it is often useful to include marsh as an additional category for those species preferring wetter conditions.

Freshwater molluscs are categorised into four groups: slum, ditch, moving water and catholic (e.g.

Boycott 1936; Sparks 1961). A brackish water group in coastal habitats can tolerate a degree of salinity.

Analysis of molluscan assemblages can take the autecological or synecological approach (e.g. Cameron 1978; Davies 2008, 55). Autecology takes into account the habitat ranges of individual species, from which the average environmental requirements for multiples species can be assessed; in the context of

19 palaoecology, this approach is reliant on the principle of uniformitarianism. In contrast, synecology studies the inter-relationships between communities of species and has the ability to reveal groupings that cross the habitat boundaries described above.

There are many different ways in which the molluscs found either in living or subfossil context may be analysed, and these were summarised by Evans et al. (1992) and discussed by Davies (2008, 62):

1. Total number of individuals

This provides an overall picture of the suitability of the habitat for molluscs, for instance the availability of calcium for shell building and vegetation for food and shade. In the context of wind blown sand this can be very helpful in understanding rates of sand deposition, as a large number of shells implies stability with adequate dietary requirements, whereas a small number suggests rapidly accumulating dunes with insufficient time or lack of calcium/vegetation for high numbers of individuals to collect.

2. Total number of species

This gives some indication of the number and variety of microhabitats and diversity and, as with the number of individuals, to some extent reflects the stability of the ground, as time is necessary for a large number of species to colonize an area. In Scottish dune sequences increasing stability and shade was indicated when ‘non-wet ground’ species numbers increased above 12 or 13, with numbers above 15

‘non-wet taxa’ often coinciding with true shade species, perhaps indicating rich, long, very stable grassland or open woodland (Thew 2003, 165).

3. Distribution of numbers of individuals among species

This is measured by various diversity indices giving an indication of the structure of the environment.

When used in archaeological contexts they are useful to compare assemblages from different sediments to assess how the environment changed either over time or space. Diversity indices will be discussed in more detail below.

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4. Age structure

Unstable environments are likely to have a high proportion of juveniles which were unable to live long enough to mature whereas in stable environments there will be a greater proportion of adults surviving to reach reproductive age. In subfossil assemblages there is often difficulty in differentiating juvenile and fragmented shells which reduces usefulness.

5. Intra-specific variation

The morphology of some molluscs varies either over time or in different environments. For example Pupilla muscorum tended to be larger during the last glacial period than in the Holocene, especially when found in wet habitats (Kerney 1963, 236; Kerney et al. 1964, 160). Calow (1981) demonstrated that Radix balthica is often smaller in exposed environments than in more sheltered places, while Tattersfield (1981) suggested that shell size for some helicid species related more to population density than environment.

6. Stenotopy

Stenotopy reflects the environmental tolerances of a species and is useful when ‘indicator species’ can be identified. This implies that the particular species is capable of inhabiting only a narrow habitat range and if present in an assemblage it is reasonable to assume that the local habitat met those requirements.

Unfortunately, there are few British non-marine species that can be considered as indicator species. An example is Pomatias elegans, a species associated with ground showing some form of soil disturbance such as forest clearance, slope processes, tree throws, etc. (Evans 1972, 134; Thomas 1977, 138).

7. Ecological groups

These reflect the groupings described above, being the preferred habitats for different species of land or freshwater shells. While very useful for many assemblages they do not take into account interactions between species but can usually provide a broad classification of the likely environment. A further problem was highlighted by Cameron (1978), who demonstrated that buried assemblages may not reflect living assemblages, especially if there has been bioturbation by earthworms in a changing habitat, such as the progression from short grazed grass to longer non-grazed grass to scrub to woodland.

8. The behaviour of pairs of species

There may be consistent relationships between two species of mollusc. The proportion of one species may increase as another decreases, or both increase or decrease in unison. On grassland Pupilla muscorum is an early pioneer of bare earth and readily colonizes newly created grassland (Evans 1972, 146) while Vallonia excentrica is more common on grazed grassland, although it may live in taller, damper grassland (Evans 1972, 162); a shift in proportions from Pupilla to Vallonia is a pointer to the commencement of grazing.

21 9. Taxocenes

The use of synecological concepts has led to the development of taxocenes: the identification of groups of species, often from different groups of the broad ecological classifications, that have a tendency to occur in association and may therefore indicate a habitat which would not be suggested from study of individual species. Such groupings may be identified subjectively or by various multivariate methods such as detrended correspondence analysis or principal components analysis. Several taxocenes for both wet and dry ground have been proposed (e.g. Ložek 1990; Evans 1991; Evans et al. 1992; Davies 2008, 24, 64). Although taxocenes may be very useful as an analytical tool in some circumstances they cannot be applied to all assemblages as there are insufficient numbers of shells or species to determine groups of taxa. Also, it is clear that taxocenes may not necessarily apply across different sites where microhabitats may vary over often short distances.