The Ecology of the British Freshwater Leeches

Abstract

investigation of the ecology of the group. In other countries three papers have been published dealing wholly or mainly with the ecology of the local Hirudinea. These are by Pawlowski (1936), Bennike (1943) and Sandner (1951). Pawlowski (1936) collected from fifty-seven stations in the Polish Lake Wigryseen and in neighbouring smaller bodies of water. Ten of the thirteen species he collected are also found in Britain, so his results are of interest. He tackled the ecological problem in three ways. First, he recorded all possible information about the food habits of his leeches; secondly, he related the leech fauna to the type of substratum; and thirdly, he classified his habitats as eutrophic, dystrophic or oligotrophic, and considered which species of leech occurred in each type. Sandner, a pupil of Pawlowski, carried out a more extensive survey in the county of Lodz, in Poland, classifying his habitats in a rather arbitrary way into tarns, small ponds, fish-ponds, peat-pools, etc. He also made a number of physico-chemical observations on the water, and attempted to relate these to the presence or absence of certain species of leech. Bennike (1943) sampled 215 habitats scattered over the whole of Denmark, and attacked the problem mainly from the physical and chemical standpoints. He measured certain selected characters of the water in a proportion of the places visited, and related these to the species present. None of these workers used quantitative sampling methods, but Bennike was forced to conclude that 'it is not the qualitative but the quantitative composition of the fauna that characterizes the various types (of fresh water)'. It was therefore decided that an attempt would be made to study the ecology of leeches in Britain by making quantitative collections. From a study of the above-mentioned papers it was thought that in all probability the most important single factor determining the distribution of leeches would prove to be the distribution of the appropriate food organisms, but clearly it was impossible