Ecological and physiological explanations for the restriction of a Tasmanian species of Ozothamnus to a single population

Abstract

Vascular plant species that occur in only one population are surprisingly frequent, and not only on oceanic islands. The potential causes of this extreme restriction include the anthropogenic, the historic, the biological and the ecological. Ozothamnus reflexifolius, a one-population composite shrub occurs in an environment unaffected by human activity and is unlikely to be a neoendemic. It does occupy an environment of extreme aridity in a centre of local endemism, indicating a relict status. The species composition of the vegetation containing O. reflexifolius is distinct from the species composition of the surrounding forest. O. reflexifolius is fecund and has seeds with adaptations for potential wind dispersal to other rock plates, which are not in short supply in the vicinity. However, germination trials indicated that O. reflexifolius seeds have a short period of viability. Annual growth rings in the stems of the species are directly correlated with the number of sympodial branchings and shrub height. The age structure of the population implies continuous regeneration. The varying aspects of the large rock plate on which the distribution of O. reflexifolius is centred would prevent loss of the whole population in any fire, a common event in the area, whereas other rock plates in surrounding areas do not have this protection. The species may colonise, and then be destroyed by fire on surrounding rock plates, while being secure in its central population.