Species richness of vascular plants and vertebrates in relation to canopy productivity

Abstract

Three paradigms are proposed to explain the community-physiological processes operating to maintain species diversity in an ecosystem. The species richness (number of vascular plant species per hectare) of Australian plant communities is related to the amount of incident solar radiation intercepted by both overstorey and understorey strata, and the annual shoot growth which results in these strata. The species richness of the vascular plants (of different life-form, e.g. shrub, low shrub, perennial or annual herb) in the understorey will depend on the percentage of net photosynthates which is translocated to shoot and floral apices. The species richness of amphibia, birds, mammals and snakes all appear to parallel the increase in the species richness of vascular plants along the climatic gradient from the semi-arid to the humid zone. Species richness of lizards decreases as the overstorey of the plant community becomes denser along the same climatic gradient. The paradigms derived from the Australian data appear to be applicable to other Mediterranean-type ecosystems. Species diversity of the overstorey is invariably low in Mediterranean regions. If the overstorey regenerates rapidly from densely-packed rootstocks during post-fire succession, the resultant foliage canopy suppresses the growth and diversity of the understorey; the species diversity of vascular plants in these ecosystems is low, but species diversity of vertebrates appears to be unaffected. The processes which determine the species richness of plant communities and associated vertebrates needs immediate research, to provide a basis for scientific management of ecosystems to maintain biological diversity.