Stage-classified matrix models and age estimates

When the size of individuals is a better indicator of their chances to survive, grow, and reproduce than their age, the suitable matrix population model is stage-classified. Cochran and Ellner developed a method to assess age-based parameters from such models. We present here, for these age estimates, simplified formulas that are valid whenever there is neither retrogression nor fission: individuals may only die, survive in the same stage, or survive and recruit to the next stage. Our formulas enable one to understand better why, and under which hypotheses, it is possible to compute age estimates from a stage-classified model, and point out some limitations of the method. These limitations in fact come from the basic hypothesis of stage-classified matrix models: stage is considered to be the only variable that influences survival and recruitment rates. As a consequence, age estimates using stage-classified models should be valid if the stages describe precisely enough the life cycles of the studied species, and particularly if senescence is taken into account.