The colonization of newly-dredged drainage channels on the Pevensey Levels (East Sussex), with special reference to gastropods

Ten newly dredged drainage channels were studied at regular intervals over several years (1969—73) to find the rate and nature of colonization by plants and animals. The process was rapid and similar in all channels, but with important differences between them.
Dense cover by floating plants, mainly Lemna minor and L. polyrrhiza was general in 4 out of 5 eastern channels, where the mean number of gastropod species per sample was 6.0 at the end of the second summer following clearance. A mean total of 11.2 gastropod species was recorded for these channels during the 2–4 year period following clearance. In 5 western channels where Lemna trisulca, filamentous green algae and submerged plants were dominant, the corresponding mean figures for gastropods were 7.2 and 14.6. Insect taxa were also better represented in the west, but crustaceans, arachnids, leeches and flatworms were more evenly distributed. Extensive sampling in other parts of the Levels in both recently cleared and other channels confirmed the uneven distribution of gastropods and some other species.
Out of 20 species recorded, three groups of snails were distinguished according to their frequency, abundance and geographical distribution. Opportunities for transport were important in determining the pattern of colonization, but environmental factors sueh as drainage pattern, extent and season of dredging, width of channel and treatment with herbicide, were probably more influential.
The information in this paper may be relevant to the siting of the proposed Nature Reserve in the Pevensey Levels announced in the National Press in October, 1977.