AN ASSESSMENT OF THE MOLLUSK FAUNAS OF GRAZING MARSH DITCHES USING NUMERICAL INDEXES, AND THEIR APPLICATION FOR MONITORING AND CONSERVATION

Rare freshwater molluscs that inhabit traditional grazing marsh ditche s in southern En,oland and East Anglia are threatened by changes in ag ricultural practices and inappropriate management and are, therefore, conservation-dependent. A ditch management strategy which encompasses the needs of farmers and the continued survival of rare species needs to be devised. A relatively straightforward and unambiguous method is needed to assess grazing marsh ditches for their molluscan value, to m easure change and to enable a favourable management policy to be deter mined. Part of the mollusc data from recent surveys have been analysed using numerical indices to explore the possibility of developing a sc heme whereby molluscs may be used as biological indicators to determin e frequency and methods for ditch management. Three indices have been used: Conservation Score, Molluscan Conservation Index and Abundance-a djusted Molluscan Conservation Index. All these indices provide some m eans of identifying ditches with high molluscan conservation value, bu t none appears to give an unequivocal result. As bioindicators, the sp ecies present may be useful in determining a ranking order for setting the proportion of ditches in each cleaning cycle. The indices may hav e their greatest application in monitoring change.