Population Biology of the Freshwater Snail Bulinus globosus on the Kenya Coast. II. Feeding and Density Effects on Population Parameters

Abstract

Captive cohorts of B. globosus [a Bilharziasis vector] were reared in netted enclosures in their natural habitat at different initial densities, and fed on different diets of natural vegetation. Growth, reproduction and survival was measured weekly throughout the cohorts'' lifetime. Snails provided with a supplementary diet of lily leaf showed significantly better growth, higher reproduction rates, longer survival and higher intrinsic rates of natural increase (rm) than those fed on coarse vegetation alone. Snails from enclosures with lower initial densities showed significantly better growth, longer survival and higher rm values, than those from higher initial densities. Field values for growth, reproduction and survival (and therefore rm) correlated with those for the least successful feeding regimes in captive cohorts. Food quality therefore appears to be limiting B. globosus populations in the reservoirs. These results support the conclusion that such reservoirs represent suboptimal habitat for B. globosus (O''Keeffe 1985). It is suggested that low intrinsic rates of natural increase and the poor condition of the snails is responsible for the relatively low prevalence and sporadic occurrence of snails shedding schistosome cercariae.