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Freshwater sphaeriid bivalves are poorly known ecologically, particularly uncommon species such as Pisidium pseudosphaerium (UK RDB (Red Data Book) staus = 'rare'). In the UK, this species occupies grazing marsh where conservation opportunities might be shared with other threatened molluscs. We surveyed sphaeriids including P. pseudosphaerium and snails in 106 drainage ditches in SE England in 1999. P. pseudosphaerium occupied over half the ditches surveyed, at slightly elevated BOD (6 ± 7 mg l À1 sd) but reduced calcium (64 ± 31 mg l À1 ) and nitrate (0.5 ± 1.2 mg l À1 ). As part of a sphaeriid assemblage comprising Sphaerium corneum, Musculium lacustre and Pisidium obtusale (=Assemblage 1), P. pseudosphaerium occurred in ditches with floating vegetation and abundant snails of conservation importance, co-occurring significantly with either Valvata macrostoma (RDB 'vulnerable') or Anisus vorticulus (RDB: 'vulnerable'). A more diverse sphaeriid assemblage (=Assemblage 2) included species common in wider, deeper ditches with open water. We suggest that traditional ditch management can support both rare and representative snails and bivalves on grazing marsh. Quasi-traditional and rotational ditch clearance will favour common sphaeriids and pioneer snails during early succession; P. pseudosphaerium, V. macrostoma, A. vorticulus during mid-succession; and the snail Segmentina nitida (RDB: 'Endangered') in late successional ditches. In common with threatened snails, P. pseudosphaerium will also benefit from reduced eutrophication. We recommend that P. pseudosphaerium retain RDB status to ensure protection and to emphasise the need for improved ecological information.

ABSTRACT1. The gastropods Segmentina nitida, Anisus vorticulus and Valvata macrostoma occur in drainage ditches on grazing marshes that are now among the most threatened wetland systems in western Europe. Although each of these species is listed in the UK Red Data Book (RDB) and Biodiversity Action Plan, influences on their distribution are poorly understood.2. To improve management information, the within-channel distributions of the three snails were examined in 20 ditches in southeast England. Abundance, vegetation cover and other environmental factors were recorded across the ditch profile near the surface (50.2 m) and at depth (50.15 m from the benthos).3. All three species were significantly more abundant near the surface than at depth. Below 0.6 m, ditches had significantly reduced concentrations of dissolved oxygen (50.6 mg L À1 , 55% saturation), possibly sufficient to limit the occurrence of gastropods dependent wholly or partly on aqueous gas exchange.4. There were no other systematic variations in abundance between the ditch margins and centre channel. However, across all ditches and samples in the survey, each species' abundance varied significantly with vegetation structure. S. nitida was most numerous where there was least open water and hence most vegetation, V. macrostoma among emergent stands and A. vorticulus in ditches with floating vegetation but few submerged plants.5. These microdistributional data support large-scale surveys in illustrating the potential importance of vegetation management for these snails; providing that other requirements are satisfied, ditch management could favour each RDB species by optimizing particular vegetation features. Apparent preferences for vegetation structure rather than particular channel locations suggest that refuges left during ditch clearance could be located anywhere in the ditch. Factors that reduce oxygen concentrations below 1 mg L À1, such as eutrophication, might be detrimental.

Conservation biology draws from a diverse tange of academic fields. Already a number of textbooks are available on the subject. This book is the first attempt to place the concepts of conservation biology into a uniquely Australian context. The book aim1l to provide an introduction to the principles of conservation biology and supply information on the main quantitative methods and procedures important in detecting and solving conservation problems in Australia.