A comparison among different population models for Limnodrilus hoffmeisteri Claparede (Oligochaeta, Tubificidae)

The lack of any reliable method for assessing the age of individuals collec ted in the field has often been considered a major obstacle for population studies in aquatic oligochaetes. One possible solution could be the adoptio n of other variables, such as weight or stage, for the definition of the po pulation structure; this approach would be useful if allowed good predictio ns about population growth. We measured, under laboratory conditions, the e ffect of age, size and life stage on survival, growth and fecundity of Limn odrilus hoffmeisteri Claparede. The results are used to establish four matr ix population models, based respectively on the classification of the indiv iduals by age, weight and stage and on a mixed classification. Matrix popul ation models make the assumptions that the individuals in a population can be arranged in a number of discrete classes and that time is a discrete var iable. In these models, the population is represented by a vector (each ele ment in the vector is the number of individuals in a class) and the demogra phic coefficients (survival, growth and fecundity) are collected in a squar e matrix. The estimate of lambda, the potential long term population growth rate and its confidence interval were taken from the four models using the jackknife method. The width of the confidence interval is a measure of the effectiveness of the models and thus of the classification of the individu als. The results suggest that weight is inefficient as a criterion for the definition of population structure of L. hoffmeisteri in comparison to age and stage.