The structural and functional dynamics of selected species -populations of freshwater snails:towards a systems approach

Abstract

The structural and functional dynamics of freshwater gastropods at Malham Tarn are examined and detailed information is presented with regards to two littoral species, Ancylus fluviatilis (Mull.)and Planorbis contortus(Linn.), occupying a semi-isolated part of the Tarn. At all levels, from whole Tarn to micro-situations the snail populations are aggregated. Both P.contortus and A. fluviatilis are semelparous with little overlapping between generations. Their phonological details may vary from year to year depending on external conditions. A.fluviatilis is a herbivore which feeds on epilithic algae, particularly diatoms. P.contortus is a detrivore which is only able to make use of the bacterial fraction of its food. The functional ecology of these two species is considered in terms of the classical energy budget statement and indicates that mucus secretions are important elements of their function and functioning. The philosophical implications of the mechano-reductionist, energy budget statement are discussed and criticised. The relevance of Control Theory and the more generalised Systems Theory are examined in terms of the results from freshwater snails. A more realistic, holistic approach is suggested.