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

The lack of any reliable method for assessing the age of individuals collected in the field has often been considered a major obstacle for population studies in aquatic oligochaetes. One possible solution could be the adoption of other variables, such as weight or stage, for the definition of the population structure; this approach would be useful if allowed good predictions about population growth. We measured, under laboratory conditions, the effect of age, size and life stage on survival, growth and fecundity of Limnodrilus hoffmeisteri Claparede. The results are used to establish four matrix population models, based respectively on the classification of the individuals by age, weight and stage and on a mixed classification. Matrix population 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 variable. In these models, the population is represented by a vector (each element in the vector is the number of individuals in a class) and the demographic coefficients (survival, growth and fecundity) are collected in a square 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 individuals. 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.