| Summary: | Studies on snail communities in nine dam reservoirs in Upper Silesia were carried out in two periods: 1980–90 and 1995–2004. The observed changes referred to domination patterns. There was found that after introduction of two alien gastropods Physella acuta and Potamopyrgus antipodarum they became the dominants in four reservoirs by the simultaneous percentage decrease of earlier native dominants. The species density decreased in second study period in five reservoirs, mainly in effect of alien species invasion, but in some cases in consequence of various human interventions. Taking into account the whole collection the commonest species in the first study period were Radix peregra and Planorbis planorbis, while Radix peregra, Potamopyrgus antipodarum, and Physella acuta in the second. Inexplicable is however the mass occurrence of Valvata piscinalis in Żywiecki dam reservoir, because in whole Soutern Poland it is one of rarest snail species both in rivers and stagnant water bodies.
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