Fig. 2: Hypotheses tested to illuminate the underlying drivers of bacterial β diversity patterns.
Scenario I: If regional species pools dominate β diversity pattern, then β diversity will vary depending on γ diversity and the distribution of species abundance. For example, with the increasing γ diversity along the latitudinal gradient, β diversity increases accordingly. In this study, γ diversity refers to regional species richness (a size of species pool), while regional species pool reflects both regional species richness and species abundance distribution. Scenario II: If local community assembly processes dominate β diversity pattern, the variation in β diversity mainly depends on local community assembly processes. For instance, β diversity is different due to different community assembly processes even when the observed γ diversity is consistent. β deviation is calculated as the difference between the observed and mean expected β diversity, divided by the standard deviation (SD) of expected values. Throughout the figure, letters represent hypothetical species. Large ovals represent regional species pools and small circles with letters represent local communities that comprise a subset of species from the regional species pool. The arrows going from the species pool to the local communities in scenario I reflect random sampling. The arrows passing through each respective condition in scenario II represent heterogeneous selection, homogeneous selection, stochastic processes, homogenizing dispersal and dispersal limitation, respectively. Here, stochastic processes refer to the ecological processes that give rise to β diversity patterns that are indistinguishable from random chance alone, such as ecological drift35. The shaded areas containing several hexagons represent environmental conditions, where hexagons with different colors represent environmental heterogeneity; a hexagon of a single color represents homogeneity. Dash lines perpendicular to the direction of arrows in the shaded area represent the dispersal limitation; dash lines parallel to the direction of arrows represent homogenizing dispersal.
