Some observations on the dispersion patterns of two species-populations of littoral, stone-dwelling gastropods (Pulmonata).

This paper, on aquatic snails at Malham Tarn (Grid. ref. 34897666), is concerned with the dispersion patterns of two species-populations of littoral, stone-dwelling gastropods (Ancylus fluviatilis Müll. and Planorbis contortus Linn.) and forms part of a sequence involved with the ecology of this group. It will consider the nature of the patterns observed, and will infer causes, a-posteriori, from correlated distributions in independent environmental variables. This type of information is generally lacking for freshwater, bottom-dwelling invertebrates (Elliott, 1971) and previous reports on the Mollusca have been based on extensive rather than intensive surveys (e.g. Boycott, 1936). The habitat of an organism and consequently its dispersion pattern may be defined at various observational levels, e.g. in the case of the snails in question as: bodies of freshwater; littoral regions; submerged stones; submerged stones of a particular type; a particular part of submerged stones. Clearly the habitat definition chosen will depend on the degree of precision associated with the observations made, and on the type of ecological question being posed. The data presented below are extracted from different levels on such an observational sequence. They begin with a consideration of the overall vertical and lateral patterns of dispersion in the total sampling area and then focus attention on the between- and on-stone patterns respectively. Information on whole-Tarn dispersion patterns has been published elsewhere (Calow, 1973a). [ABSTRACT FROM AUTHOR]