THE DETERMINANTS OF THE DISTRIBUTION AND ABUNDANCE OF THE WINTER ANNUAL GRASS VULPIA-CILIATA SSP AMBIGUA

1 Vulpia ciliata ssp, ambigua is a winter annual grass that occurs primarily in southern and eastern England. A model is constructed which relates this distribution to three climatic factors: the total number of sunshine hours in the spring quarter of the year (March-May), the temperature of the winter quarter (December-February), and the temperature of the autumn quarter (September-November). 2 The species is primarily limited to sandy soils and shingle. A map of Great Britain is created to indicate where the suitable soils on which the species can exist are to be found. The resolution of current databases is such that field verification of soil types on which the species is occasionally found is essential. 3 The density and size of four populations, along a 100-km transect from the heart of the species range in eastern England to the most northerly population, suggest that populations become smaller towards the northern edge of the distribution. 4 The primary reason for the decline in abundance towards the margins of populations and towards the edge of the range is a decline in the fecundity of individuals, which is strongly correlated with the finite rate of population increase. There was a slight increase in mortality towards the margin of populations.