The consequence of a food refuge collapse on a guild of lake-dwelling triclads and leeches

The diets ofPolycelis nigra, P. tenuis, Dugesia polychroa, Dendrocoelum lacteum, Glossiphonia complanata, Helobdella stagnalis andErpobdella octoculata in an English lake were examined, using a serological technique, and compared between 1981–82 and 1989–90. Leech, triclad and prey abundances were also recorded. Between the two studies, snail numbers crashed whereas the abundances ofAsellus andGammarus increased.Dugesia andGlossiphonia numbers decreased substantially, whilstPolycelis tenuis andHelobdella abundances increased. In the second study, the snail component in the diet was greatly reduced resulting in a broader food niche, particularly forDugesia andGlossiphonia, and greater food overlap between the predators with the exception ofDendrocoelum andErpobdella which do not eat molluscs. It is postulated that the reduced size of the snail refuge, and consequent increase in severity of interspecific competition with other predators, particularlyPolycelis andHelobdella, led to the observed decrease in abundances ofDugesia andGlossiphonia. The decline in the last two genera, perhaps coupled with increased crustacean abundance, could have contributed to the numerical increase of the competitively superiorPolycelis tenuis andHelobdella.