Island biogeography of soil bacteria and fungi: similar patterns, but different mechanisms

Microbes, similar to plants and animals, exhibit biogeographic patterns.
However, in contrast with the considerable knowledge on the island
biogeography of higher organisms, we know little about the distribution of
microorganisms within and among islands. Here, we explored insular soil
bacterial and fungal biogeography and underlying mechanisms, using soil
microbiota from a group of land-bridge islands as a model system. Similar
to island species-area relationships observed for many macroorganisms,
both island-scale bacterial and fungal diversity increased with island
area; neither diversity, however, was affected by island isolation. By
contrast, bacterial and fungal communities exhibited strikingly different
assembly patterns within islands. The loss of bacterial diversity on
smaller islands was driven primarily by the systematic decline of
diversity within samples, whereas the loss of fungal diversity on smaller
islands was driven primarily by the homogenization of community
composition among samples. Lower soil moisture limited within-sample
bacterial diversity, whereas smaller spatial distances among samples
restricted among-sample fungal diversity, on smaller islands. These
results indicate that among-island differences in habitat quality generate
the bacterial island species-area relationship, whereas within-island
dispersal limitation generates the fungal island species-area
relationship. Together, our study suggests that different mechanisms
underlie similar island biogeography patterns of soil bacteria and fungi.