LYMNAEID HABITATS IN SWAMPY MEADOWS ON AC ID SOIL - EFFECTS OF AGRICULTURAL IMPACTS

Characteristics of lymnaeid habitats were studied in three swampy mead ows on acid soil i) in 1974, and ii) in 1994 or 1995 after disturbance related to cessation or modification of agricultural practices. Chang es in the distribution of three Lymnaea species were noted in the firs t meadow after its abandon with an invasion of Myocastor coypus. Deser tion of the main drainage furrow and colonization of ditches and roden t passageways by L. cf. fuscus were detected in 1994, inducing an incr ease in the overall area of snail habitat. The mean numbers of L. cf. fuscus and L. truncatula were greater, whereas those of L. glabra slig htly decreased. Another aquatic snail, Physa acuta, had invaded the un occupied drainage furrow. Clearing of drainage ditches in the other tw o meadows, followed by the invasion in 1995 of one or two aquatic spec ies (Lymnaea ovata, Potamopyrgus antipodarum, Physa acuta), induced an increase in the habitat area for invading snails as for L. glabra and L. truncatula. The numbers of the first three species were high in th e two ditches; in contrast, there was a reduction in the numbers of th e two lymnaeid species.