The applicability of the species pool hypothesis to community diversity in the Inner Mongolia grassland along a mean annual precipitation gradient

Abstract

Exploring the underlying mechanisms of community diversity is a key issue in ecology and conservation biology. Community diversity studies typically focus on local processes, but these factors cannot completely explain community diversity. Many previous studies have shown that species richness can be quite different among communities with similar habitats. Therefore, the importance of regional processes has gradually been considered, and many hypotheses based on regional processes have been proposed. The species pool hypothesis developed by Zobel et al. is one of the most important theoretical developments in the field of community diversity. The species pool hypothesis suggests that community diversity is not only associated with contemporary environmental factors and local ecological processes (e.g., competition, predation, resources, spread, and interference), but is also limited by the regional species pool. The regional species pool is the set of species in a certain region that are capable of coexisting in a target community, which is shaped by historical (e.g., glaciation and geological age) and regional processes (e.g., speciation, immigration, dispersion, and extinction). To explore the applicability of the species pool hypothesis to community diversity in the Inner Mongolia grassland, we investigated the species diversity in the grassland region from late July to mid-August in 2012, when the grassland community biomass was at its peak. In this region, precipitation is considered the most important environmental factor affecting species diversity. Therefore, we established 192 field sites in the Inner Mongolia grassland along a gradient of mean annual precipitation. The position of each field site was located using GPS. At each site, an area of 10m×10m was delineated, and 10 plots of 1m×1m were randomly placed in the delineated area to survey all plant species. Based on these data, the relationships between regional diversity (gamma diversity) and community diversity (alpha diversity) were analyzed along seven mean annual precipitation (MAP) gradients. Gamma diversity is the total species richness at a site. Alpha diversity is defined as the mean species richness (number) for the 10 plots at a site. The correlation coefficient between these two diversity indices was used to verify the applicability of the species pool hypothesis. A few key results were obtained. (1) Both alpha diversity and gamma diversity increased significantly with MAP in the Inner Mongolia grassland. (2) Gamma diversity and alpha diversity showed a significant positive linear relationship under different gradients of MAP, which reveals that the species pool hypothesis adequately explained community diversity along different precipitation gradients. (3) The effect of the regional species pool on community species diversity weakened as the MAP increased, which explains the decrease in the applicability of the species pool hypothesis as MAP increased. (4) Exploring the relationship between gamma diversity and alpha diversity represents an effective method for determining the impact of the species pool on community diversity. This study contributes to the theory regarding the mechanisms that maintain community diversity. It also has practical applications for the protection of diversity in the Inner Mongolia grassland.