A quantitative study on the implications of sieve mesh size in land mollusc sampling - are snails 0.5-1 mm in size worth including?

摘要

Terrestrial molluscan samples were processed using 0.5 mm and 1 mm mesh sieves to investigate the numbers and identities of species which would be missed if only the latter size is used. Findings indicate that although there were more than double the number of snail shells in the smaller size fraction, only a relatively small number of species are likely to be missed or under-estimated. However,of the six such species identified, one (a Red List species) would have been missed completely, and a further two would have been grossly under-estimated. Furthermore, on a site by site basis, by the inclusion of the 0.5-1 mm size fraction, a median of two new species was added at each site. The proportions of missed species varied by habitat, with grasslands showing a 42 per cent increase. In conclusion, using a 0.5 mm mesh vastly increases the numbers of shells to process, and these will contain a large proportion of immature and dead specimens which can be difficult to identify and problematic to include in datasets. While only a small number of species are likely to occur in this small size fraction, these species can be missed altogether or hugely under-estimated using larger mesh sizes. However, the species which are likely to be missed or under-estimated can be predicted using published databases which give details on size and, importantly, shape, of shells.