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

AIM: We used a hierarchical fractal‐based sampling design to test how sampling grain influences (1) beta diversity of and (2) inferences from 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 plots 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 responded 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: The βₛₒᵣ of trees and birds was mainly explained by 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. Dispersal‐based assembly processes were the best predictors of community assembly at fine scales, whereas niche‐based assembly processes were the best predictors at coarse scales. However, most of the variation in tree community composition was 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 help improve planning for biodiversity conservation and should be part of initiatives aimed at ecological conservation plans.