Food utilization and niche overlap in three species of lake‐dwelling leeches (Hirudinea)

The diets of Erpobdella octoculata, Glosiphonia complanata and Helobdella stagnalis on the stony littoral zone of eutrophic Crose Mere were examined over two years using the precipitin test. The abundances of food organisms were measured. For all leech species, juvenile mortality was greatest in locations where a low ratio of prey abundance to recruited leech abundance occurred, and the intensity of feeding by Erpobdella and Helobdella fell slightly in late summer when large numbers of juveniles of these species were present; these suggest the involvement of food in juvenile mortality, Seasonal changes in feeding intensity were similar for all leech species, with the exceptions outlined above and for Glossiphonia in April when feeding by brooding adults ceased, at which time the severity of any interspecific competition would have been reduced. Within a leech species, there was little change in the food spectrum as the leeches grew. Niche breadth was lowest for Erpobdella and highest for Helobdella. Niche overlap was greatest between Glossiphonia and Helobdella, and there was a trend for overlap values to be lower between Glossiphonia and Erpobdellu than between Helohdellu and Erpohdellu. Erpobdella fed mainly on chironomids and oligochaetes, Glossiphonia on molluscs, oligochaetes and chironomids, and Helobdella on oligochaetes, chironomids, Asellus, molluses and amphipods, all in decreasing order of importance. It is suggested that coexistence in the leech species may be due in part to Erpobdella and Glossiphonia having ‘food refuges’ in chironomids and molluscs, respectively, with Helobdellu being more of a general predator.