Rare Species Are Significant in Harsh Environments and Unstable Communities: Based on the Changes of Species Richness and Community Stability in Different Sub-Assemblages

To understand the contribution of different sub-assemblages (sub-communities) in the shrub and herb layers to the distribution patterns of community species richness and the stability of the Pinus massoniana Lamb. community, this study was carried out by using 160 shrub quadrats (5 m × 5 m) and 200 herb quadrats (1 m × 1 m). These quadrats were selected from 40 plots on six islands. In this study, common and rare species were classified according to the frequency, and “new communities” (sub-communities) were formed by adding or removing species. Then the changes of species richness and community stability in the “new communities” were analyzed. A redundancy analysis was also used to explore the factors affecting the size of the species richness in the understory of the Pinus massoniana community. The results showed the following: (1) The distribution patterns of both shrub and herb layer species frequencies in this area were plainly to the right, indicating a large proportion of non-common species (common species accounting for 37.87% in the shrub layer and 16.67% in the herb layer). (2) The higher the frequency of species, the greater their contribution to the pattern of species richness. Common species had a high frequency and were the most important contributors to the patterns of species richness in plant communities (64 common species and 41 most common species in the shrub layer and 10 common species in the herb layer each accounted for 95.72, 88.9, and 90.52%, respectively, of the species richness distribution pattern). However, rare species also made significant contributions to the species richness in regions with hard conditions (the (most) rare species in the herb layer explained more than 70% of the species richness distribution pattern, and the remaining species after removing the (most) common species explained more than 90%). (3) In relatively stable communities, rare species had relatively little influence on the stability of the community, which was mostly governed by the dominant species (common species (10 species) were more stable than rare species (38 species), Exc.-rare species (22 species) were more stable than except-common species (50 species), and Exc.-rarest species (35 species) were more stable than Exc.-most common species (55 species) in the herb layer). In less stable communities, the stability of the community gradually increased with the increase in species richness, which may be associated with the growth habit of the increased species (the stability of the herb layer was higher than that of the shrub layer, as shown by the Euclidean distance). The community stability was determined by not only the dominant species in the community but also the rare species that were important contributors to the stability of the communities. (4) The species richness of the shrub layer was considerably influenced ( p -value < 0.05) by the soil pH, soil organic matter, and wind speed, whereas the species richness of the herb layer was significantly influenced ( p -value < 0.05) by the soil pH. The greater the pH and wind speed, the greater the species richness in the island community. On islands, the soil stability was maintained in large part by the soil organic matter. The lack of soil organic matter can affect soil nutrients, destroy island habitats, and reduce species richness, all of which are harmful to the community stabilization.