Multi-scale sampling boosts inferences from beta diversity patterns in coastal forests of South Africa

Abstract

AIM : We used a hierarchical fractal-based sampling design to test how sampling scale influences i) beta diversity and ii) inferences on the modelled contribution of niche- versus dispersal-based assembly processes in structuring tree and bird assemblages. Location Coastal forest fragments, South Africa METHODS : We surveyed 103 tree and 267 bird points within eight forest fragments and partitioned beta diversity (βsor) into its turnover (βsim) and nestedness (βnes) components. We evaluated how sampling at fine, intermediate and coarse scales influenced beta diversity components and compared how tree and bird beta diversity respond to sampling grain variation. We then explored the relative contributions of niche- and dispersal based assembly processes in explaining spatial turnover as a function of sampling grain and/or study taxon by using multiple regression modelling on distance matrices and variance partitioning. RESULTS : Beta diversity (βsor) of trees and birds was mainly explained by spatial turnover (βsim) at all sampling scales. For both taxonomic groups, βsor and βsim decreased as sampling scale increased. Beta diversity differed among trees and birds at fine, but not at coarse sampling scales. Dispersalbased assembly processes were the best predictors of community assembly at fine scales, whereas niche-based assembly processes were the best predictors at coarse scales. Most of the variation in tree community composition was, however, explained at fine scales (by dispersal-based assembly processes), while most of the variation in bird community composition was explained at coarse scales (by niche-based assembly processes). MAIN CONCLUSIONS : Our study shows that inferences from beta diversity are scale dependent. By matching the grain of the data with the grain at which predictor variables and associated processes are likely to operate, multi-scale sampling approaches can improve biodiversity conservation and should be part of incentives directed at ecological sensible conservation plans.