The theory of island biogeography, which predicts that species richness is a function of island area and isolation, is increasingly gaining attention in microbial ecology. Although the positive species–area and negative species–isolation relationships have been reported in various microorganisms, the underlying mechanisms remain unclear. We suggest that decomposing total richness per island (γ diversity) into the average richness per sample (α diversity) and the dissimilarities among samples within island (β diversity) can offer further insights. A positive relationship between α diversity and island area might indicate pure area effects, while a positive relationship between β diversity and area would indicate the importance of habitat diversity effects and/or dispersal limitation. Using land-bridge islands in a fragmented landscape as a model system, we examined the island biogeography patterns of soil bacteria and fungi. We collected 306 soil samples from 29 islands in the Thousand Island Lake, China, as well the nearby mainland, and identified the bacteria and fungi by Illumina MiSeq platform.
Results/Conclusions
The estimated total OTU richness per island (γ diversity) of bacteria and fungi both increased with island area, but the effects of isolation on γ diversity were not significant. However, the loss of bacterial and fungal richness in small islands was driven by different ecological mechanisms. For bacteria, average OUT richness per sample (α diversity) significantly increased with area, while the average dissimilarities among samples within islands (β diversity) remained constant. In contrast, for fungi, α diversity were not associated with island area, while β diversity significantly increased with area. Therefore, the loss of bacterial diversity in smaller islands was likely driven by pure area effect, while the loss of fungi diversity was likely driven by habitat homogenization and dispersal limitation. One possible explanation is that bacterial diversity was primary driven by environmental variables (e.g., soil property), while fungal diversity was more influenced by spatial factors. Together, these results suggest that habitat loss would increase the extinction risk of both soil bacteria and fungi through different mechanisms.