Shifts in the importance of the species pool and environmental controls of epiphytic bryophyte richness across multiple scales.

Species richness is influenced by a nested set of environmental factors, but how do these factors interact across several scales? Our main aim is to disentangle the relative importance of environmental filters and the species pool on the richness of epiphytic bryophytes across spatial scales. To do so, we sampled epiphytic bryophytes in 43 oak forests across the northwest of the Iberian Peninsula. As predictors we used climate, descriptors of forest structure and micro-environment. We applied structural equation modeling to relate these variables with richness and cover at three scales: locality (forest), stand (three stands per forest), and sample (a quadrate in a tree). We assumed top-down relationships, so that large-scale variables influenced lower scale variables, and in which cover directly influenced richness. Richness at the next larger scale (locality to stand and stand to sample) is considered a surrogate of the species pool and included as a predictor of richness at the next smaller scale. Environmental variables explain locality richness, but as we decrease the spatial scale, its importance decreases and the dependence on species pool increases. In addition, we found unexpected bottom-up relationships (between micro-scale environment to locality richness). Our results point to the scale dependence of niche vs. neutral processes: niche processes are important at the locality (forest) scale, while neutral processes are significant at the small (sample) scale. We propose a modified conceptualization of the factors influencing biodiversity at different spatial scales by adding links across different scales (between micro-environment and locality-scale richness in our study).