THE CONSEQUENCE OF A FOOD REFUGE COLLAPSE ON A GUILD OF LAKE-DWELLINGTRICLADS AND LEECHES

The diets of Polycelis nigra, P. tenuis, Dugesia polychroa, Dendrocoel um lacteum, Glossiphonia complanata, Helobdella stagnalis and Erpobdel la octoculata in an English lake were examined, using a serological te chnique, and compared between 1981-82 and 1989-90. Leech, triclad and prey abundances were also recorded. Between the two studies, snail num bers crashed whereas the abundances of Asellus and Gammarus increased. Dugesia and Glossiphonia numbers decreased substantially, whilst Poly celis tenuis and Helobdella abundances increased. In the second study, the snail component in the diet was greatly reduced resulting in a br oader food niche, particularly for Dugesia and Glossiphonia, and great er food overlap between the predators with the exception of Dendrocoel um and Erpobdella 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, particularly Polyc elis and Helobdella, led to the observed decrease in abundances of Dug esia and Glossiphonia. The decline in the last two genera, perhaps cou pled with increased crustacean abundance, could have contributed to th e numerical increase of the competitively superior Polycelis tenuis an d Helobdella.