AGE, STAGE AND SIZE STRUCTURE AS POPULATION STATE VARIABLES FOR TUBIFEX TUBIFEX (OLIGOCHAETA, TUBIFICIDAE)

Age based demography is not applicable to tubificid worms, because of the lack of any reliable method for assessing the age of individuals. 'Stage', namely the degree of reproductive development, and/or size co uld be used instead of age, if that knowledge improved prediction of a n individual's fate and fecundity. We measured, under laboratory condi tions, the influence of age, size, and maturation stage on the fate an d fecundity of individuals, by means of a modification of the standard cohort experiment. As expected all of the three state variables signi ficantly affect the fecundity, though the residual variance is conside rable. As a first step in the evaluation of the effectiveness of a siz e and/or stage based demography for tubificids, the results are used t o establish a size-stage structured matrix population model. The asymp totic behaviour of the model is studied and a basic sensitivity analys is is performed, to evaluate the effects of the observed variability o f the matrix elements, on the eventual finite growth rate.