Latitudinal differences in species abundance distributions, rather than spatial aggregation, explain beta-diversity along latitudinal gradients

Aim Variation in species composition among sites (beta-diversity) generally decreases with increasing latitude, but the underlying mechanisms are ambiguous. Although both local and large-scale processes may drive this pattern, they act all through influencing species abundance distribution (SAD) and spatial pattern of species. A null model incorporating SAD is often used to calculate expected beta-diversity, which accounts for most variation in beta-diversity. However, a recent study has shown that the deviation of observed beta-diversity from expected values (beta-deviation) increases with latitude. The latitudinal gradients in beta-deviation may be related to both latitudinal differences in SADs and the degrees of spatial aggregation. Our study aims to (1) investigate how beta-deviation varies with SAD and spatial aggregation, and (2) separate the contributions of SAD and aggregation in explaining latitudinal gradients in beta-deviation. Location Global. Methods 197 forest plots (each containing 10 subplots) distributed along latitudinal gradients were used. Two beta-diversity models were derived for communities with randomly and nonrandomly distributed species. The two models were used to simulate relationships of beta-deviation with SAD and aggregation, and to separate the contributions of these two factors in explaining latitudinal gradients in beta-deviation. Results beta-deviation increased with the degree of aggregation and peaked at intermediate species abundance. The fraction of beta-deviation linked to SAD increased with latitude in global and regional analyses, whereas the fraction of beta-deviation linked to aggregation was only significantly correlated with latitude in New World south. The degree of aggregation increased with latitude in New World south, but not in global extent and New World north. Main conclusions The latitudinal gradients in beta-deviation are primarily explained by latitudinal differences in SADs. Additionally, the expected beta-diversity is determined solely by SAD. Therefore, we conclude that latitude-beta-diversity gradients at local spatial scales appear to be explained by latitudinal differences in SADs.