Evaluation of bioengineering approaches aimed at controlling pulmonate snails: the effects of light attenuation and mechanical removal of macrophytes.
The effects of the following perturbations on populations of freshwater gastropods from a drainage channel in the Southeast area of Lewes Brooks, East Sussex, UK, were investigated experimentally: light exclusion with black polythene; introduction of the floating plant Hydrocharis morsus-ranae; mechanical removal of macrophytes. Experiments were conducted in enclosures (3.5 × 2 m), positioned along the margin of a drainage channel. Although the 2 treatments involving shading with black polythene and mechanical removal resulted in the virtual disappearance of the macrophytes, their effects on the physicochemical environment were very different. The structure of the remaining plant community was unaffected by the introduction of H. morsus-ranae. The total numbers of Bithynia tentaculata, Lymnaea peregra and Physa fontinalis, as well as their population densities in the inner zone of the enclosures were significantly reduced in the black polythene treatment, compared with the controls. Planorbis planorbis was not affected. Similar trends were observed in the numbers of snails on artificial plant decoys. The effects on snail numbers of mechanical removal of the macrophages were much less marked, the only statistically significant ones being the decline in population densities of L. peregra and P. fontinalis in the inner zones of the enclosures. The possible reasons for the effects and their significance to the control of pulmonate snails are discussed.