To determine the influence of neutral (species spatially determined by dispersal limitation) vs. niche mechanisms (species sorting along habitat gradients) of species distribution at different spatial scales we sampled vegetation and hydrological data in 222 helocrenic springs in siliceous mountain ranges in central Germany and the north west of the Czech Republic. To evaluate the strength of the relationship between hydrological parameters and species occurrence (strength of niche specificity) we modeled the probability of occurrence of bryophyte and vascular plant species using generalized additive models.

We found that a smaller plot size leads to lower response maxima, whilst the species' response optima were not systematically influenced by spatial scale. This is interpreted in terms of a stronger influence of neutral vs. niche mechanisms on the finer scale, which is in line with theory. However, there was still a strong correlation between species occurrence and the pH-value on the finer scale with even more pronounced optima (narrower amplitude) in comparison to the coarser scale, indicating that a strong species-environment-relationship is still present.

We conclude that due to the small-scale within-site variability in helocrenic springs, niche mechanisms are present at both observational scales. In contrast, neutral mechanisms are determined by the springs' isolation as well as by the unidirectional flow of water, both leading to dispersal limitation.