Population Structure and Size-Dependent Reproductive Effort in Umbilicaria spodochroa

Abstract

The size structure and the relationship between individual thallus size and apothecium production was investigated in crowded populations of the saxicolous, umbilicate lichen Umbilicaria spodochroa in coastal cliff communities in southern Norway. There was a strong positive correlation between apothecium production and the mass and diameter of individual thalli. The study also indicated the existence of developmental maturation thresholds, a minimum diameter and mass that has to be reached before apothecium production can commence. Linearity of the relationship between thallus mass, thallus upper surface area and minimum number of apothecia produced after maturation suggests that in these size stages there is a constant proportion of resource allocation to reproduction. The populations exhibited pronounced size hierarchies, commonly with many small individuals and fewer big ones. Thus at any time the few large thalli contribute more to the propagation of the population than many of the more numerous small individuals. The Gini-coefficient of inequality of mass between individuals ranged from 0.48 to 0.70 in the sampled populations and was correlated with population density.