The Demography of a Sand Dune Annual: Vulpia Fasciculata: I. The Natural Regulation of Populations

The population dynamics of the winter annual grass Vulpia fasciculata on two dune systems in North Wales is described and a simple model which describes the population dynamics in terms of thedensity-dependent regulation of spikelet number and the density-independent mortality is presented.

Abstract

(1) The population dynamics of the winter annual grass Vulpia fasciculata on two dune systems in North Wales is described. Permanent quadrats were mapped frequently to obtain data on the fates of seedlings and the reproductive performance of each individual. The role of density in the regulation of populations was assessed by the manipulation of natural densities in the field. (2) The behaviour of populations of V. fasciculata on the fixed dunes of the two dune systems studied was remarkably similar. The probability of a seedling surviving to maturity was high, but the potential seed rain on some plots was considerably reduced by rabbit grazing at or just before flowering. There was a peak of mortality during inflorescence development. The causes of mortality during the life cycle were various. (3) There was no evidence for any density-dependent mortality, but above a density of 100 flowering plants per 0 25 m2 it was found that there was a significant negatively density-dependent relationship between the number of spikelets per plant and the density of flowering plants. A simple model which describes the population dynamics of V. fasciculata in terms of the density-dependent regulation of spikelet number and the density-independent mortality is presented. (4) The reasons for the stabilization of the density of V. fasciculata at different levels in different areas on the two dune systems and the reasons for the annual fluctuations in population density within these areas are discussed.