Density and composition of aquatic oligochaete populations in different farmers' ricefields

Summary The density and composition of ricefield oligochaete populations were estimated in 33 farmers' fields in the Philippines and related to physicochemical properties of the soils and agrochemical use. The spatial distribution of organisms and the investigative sampling strategy were determined from a high-intensity sampling in a single ricefield. Species diversity was low. The dominant species were Limnodrilus hoffmeisteri and Branchiura sowerbyi. The spatial distribution of populations was contagious and densities ranged from 0 to 35 000 m^-2 (maximum 620 kg ha^-1). Mean population density was positively correlated with soil moisture, organic matter, and the quantity of N fertilizer applied. No pesticide impacts were observed.