Estimating the fitness of a local discrete-structured population: From uncertainty to an exact number

The fitness of a local discrete-structured population is measured by the dominant eigenvalue λ1 of its matrix model, L, calibrated on empirical data. The data mined in case studies on local populations of Calamagrostis spp., perennial long-rhizome grasses colonizing rapidly open spaces (such as forest clear-cuts or meadows) due to their fast vegetative propagation, do provide for the accurate calculation of the transition part T in canonical sum of L=T+F, but leave the reproduction summand F uncertain. This ‘reproductive uncertainty’ (which is still constrained by the data and botanical knowledge) was amenable to calibration under the hypothesis that the adaptation is maximal for the given data and constraints, while the need to test the hypothesis experimentally motivated a drastic change in the technique of field experiments: from the description of above-ground parts of clones on a sample plot to the excavation of the whole colonies and analysing its system of rhizome reproductive links. In this paper, I report on how the “evolution” of experimental design and calibration technique has achieved an exact number in the estimation of fitness measure. As a by-product of that evolution, a new phenomenon has been discovered in the ontogeny of Calamagrostis epigeios, which has induced unexpected situations where the ever-working λ1(L) fails in the accuracy and needs an adjustment. The data-vs.-model dialectics behind the story are also commented on.