Population dynamics of Vulpia ciliata: regional, patch and local dynamics

1 Data on the population dynamics of the annual grass Vulpia ciliata were c ollected at three levels, from the scale of the regional population down to small (10 x 10 cm) patches. We use these data to explore the degree to whi ch fine scale processes influence large scale patterns of abundance. 2 Populations were characterized by their persistence, despite their small size. The mean half-life of populations was estimated to be around 45 years . Most populations are small (a few m(2)) in area, with only a few as large as a hectare in size. 3 Population regulation occurs as a consequence of density-dependent seedli ng recruitment, This reduces population growth by up to 87%. The nature of this density dependence appeared to be essentially the same across sites an d years. 4 interactions with perennial vegetation also significantly affected popula tion dynamics, through reducing seedling recruitment and survival, and on a verage depressed population growth by a further 30% at one site and by up t o 96% in another population. 5 Plants were aggregated and densities were positively spatially autocorrel ated. This tends to buffer patches against extinction. Mean seed production per plant, was also significantly spatially autocorrelated; however, the s trength of this was minor. 6 Data on small-scale extinction showed that disturbance is an important de terminant of the distribution of numbers of plants within subplots. Compari son of the distribution of subplot densities with the results of a spatial simulation model suggested that disturbance at a relatively large scale (at least 20 x 20 cm) impacts on dynamics at the population scale. 7 An integro-difference equation model for patch expansion shows that popul ations are constrained to an area no larger than around 100 m(2) on a time- scale relevant to the dynamics of this species (about 20 years). 8 We conclude that the most characteristic features of dynamics at the regi onal scale, namely the persistence and very small spatial size of individua l populations, can be readily explained by processes operating at small spa tial scales.