CHAPTER 10 



ECOLOGY 



1. Relations with the Inanimate Environment 



In this chapter we shall consider the ways in which the distribution 

 and abundance of leeches is affected by factors of the environment, 

 both living and non-living. We may begin by considering the 

 inanimate environment, the characteristics of which have the 

 advantage of being more easily measurable. With few exceptions 

 leeches live in aquatic habitats. The exceptions fall into two 

 groups, the land leeches which live in damp situations like tropical 

 jungle and feed from time to time on passing terrestrial animals 

 (Haemadipsidae, p. 32), and amphibious leeches which leave the 

 water for part of the year to live in soil or under stones, feeding on 

 worms or slugs which they swallow whole (e.g. Trocheta and 

 Haemopis). The remainder, the truly aquatic leeches, show a 

 pattern of distribution which may be correlated with the physical 

 and chemical characteristics of the water. There have been four 

 main studies of the ecology of leeches in Europe. Pawlowski (1936) 

 and Sandner (1951) worked in Poland, Bennike (1943) in Denmark 

 and Mann (1955) in Britain. Ten species of leech were common to 

 all three areas so there is a good body of knowledge on the ecology of 

 European leeches. For descriptive purposes it is convenient to divide 

 freshwater lakes and ponds into hard, intermediate and soft waters 

 with dividing lines at calcium concentrations of about 24 mg/1 and 

 8 mg/1. These calcium figures give a fair indication of the total 

 amounts of dissolved solids in the water and at the levels mentioned 

 it has been shown that there are marked changes in the composition 

 of the fauna of fresh waters (Ohle, 1934; Boycott, 1936). In hard 

 waters the most abundant leech is almost always Helobdella 

 stagnalis and the same is true in the intermediate group, provided 

 that the body of water is a reasonably large one. Small ponds have 



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