Effect of spatial scale on factors limiting species distributions in dry grassland fragments

1 Distribution of plant species in fragmented landscapes is the result of both seed and site availability. Little is known about how their relative importance differs at different spatial scales. 2 I sowed seeds of eight dry grassland species into 22 localities that differed in occupancy by these species and followed seedling establishment over 3 years. I compared the number of emerging seedlings at three scales: between previously occupied and previously unoccupied localities, between occupied and unoccupied blocks within occupied localities, and between plots with and without seed addition within occupied blocks. 3 At the two larger scales, I also studied the relationship of the number of seedlings and of the distributions of adult plants to environmental factors. 4 Both seed and site availability are important in structuring the distribution of these species, but site availability becomes less important with increasing spatial scale. The intensity of this effect is, however, species specific. 5 The relationship between environmental factors and pattern of species distribution is also clearly scale dependent, and differs between seedlings and adults. Whilst abiotic factors are the main determinants of seedling distribution, the occurrence of adult individuals is best predicted by the occurrence of other species. This suggests that the present distribution of species in the landscape is determined mainly by historical factors. 6 Conclusions based on the importance of seed and site availability for species distribution in natural communities at one scale cannot be extrapolated to other scales. Only comparisons on multiple scales can provide a full understanding of factors affecting species distribution at landscape level.