Relative importance of factors determining diversity and composition of freshwater leech assemblages (Hirudinea; Clitellata): a metaanalysis

Data on numbers and percentages of leech species sampled in 55 Polish freshwater environments were analysed. Most of these data were published between 1951-1998. The first aim of the present study was to recognize the most important ecological factors affecting the diversity and taxonomic composition of leech assemblages in Polish waters. Evaluating the relative importance of these factors in explaining diversity and composition of the leech fauna was the second aim. The significance of six factors, potentially affecting the composition and diversity of the leech fauna was tested. These were: type of environment, size of environment, geographical location, presence of fish predation, sampling season and level of degradation. Two factors were found to be important determinants of the leech assemblage composition. The first were the specific properties of the ponds. Occurrence and percentages of certain leech species therein were significantly different from those in lakes. The second factor was the presence of fish or the intensity of fish predation. No significant differences in occurrence or percentages of species were found between geographic regions, habitat size and samples collected in various seasons. Lakes were the richest environments in terms of species number. The high mortality caused by sampling of leeches in small, semi-isolated ponds or in lakes localized in protected areas is discussed. Surprisingly, significantly lower species diversity was observed in environments with more intense fish predation.