Species-area relationships of plants and microbes are driven by different mechanisms in patchy grassland fragments

1. The island species-area relationship (SAR) describes how the number of species on an island varies with its area and provides important information about the maintenance of species diversity and how species might be lost as habitats are lost and fragmented. While the island SAR has been widely studied among macroorganisms, patterns for soil microbes have been less forthcoming, as have analyses of potential underlying mechanisms. 2. We examined the SAR patterns of plants, soil fungi and bacteria among remnant grassland fragments isolated by degraded habitat matrix in a steppe grassland. 3. As expected, we found that the total species richness of plants, fungi and bacteria in the entire fragment decreased with decreasing fragment area. However, the mechanisms differed among taxa. For plants, the lower diversity was most likely because of decreases in habitat quality and habitat heterogeneity on smaller fragments. For fungi, we found similar results, but also an important interaction with plant diversity, suggesting they were coupled via their functional associations. For bacteria, the lower diversity on smaller fragments primarily resulted from a sampling effect from the larger species pool, which outweighed the effect of higher habitat heterogeneity on smaller fragments. 4. Synthesis: Our study highlights that fragmentation-induced loss of diversity can have many different underlying mechanisms and that understanding these mechanisms can provide useful insights to project future biodiversity loss and conservation.