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

1 Data on the population dynamics of the annual grass Vulpia ciliata were collected 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 which 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 seedling recruitment, This reduces population growth by up to 87%. The nature of this density dependence appeared to be essentially the same across sites and years. 4 interactions with perennial vegetation also significantly affected population dynamics, through reducing seedling recruitment and survival, and on average depressed population growth by a further 30% at one site and by up to 96% in another population. 5 Plants were aggregated and densities were positively spatially autocorrelated. This tends to buffer patches against extinction. Mean seed production per plant, was also significantly spatially autocorrelated; however, the strength of this was minor. 6 Data on small-scale extinction showed that disturbance is an important determinant of the distribution of numbers of plants within subplots. Comparison 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 populations 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 regional scale, namely the persistence and very small spatial size of individual populations, can be readily explained by processes operating at small spatial scales.