Island Area and Remoteness Shape Plant and Soil Bacterial Diversity Through Land Use and Biological Invasion

1. Biodiversity is declining dramatically due to human-driven land use change and biological invasion, but our knowledge of how such drivers influence plant and heterotroph diversity on island ecosystems remains limited. Historically island biogeography theory has focused solely on direct effects of island size and remoteness on biodiversity, but these factors can also indirectly affect species gain and/or loss through impacting land use change and biological invasion. 2. We built the structural equation model to explore direct effects of island size and remoteness, and indirect effects of these factors via land use intensity and pinewood nematode invasion, on the diversity of plants and soil bacteria across 37 continental shelf islands in the largest land-bridge archipelago in eastern China. 3. As expected we found that increasing island area directly promoted plant diversity. However, land use intensity increased with island area which also promoted plant diversity, and loss of pine forest by the pinewood nematode invasion increased with island remoteness which reduced plant diversity. Island remoteness only indirectly reduced plant diversity through increasing pine forest loss. Soil bacterial diversity was directly negatively impacted by island remoteness, and indirectly negatively impacted by island remoteness through increased soil electrical conductivity likely caused by greater salinity from sea spray. Furthermore, soil bacterial diversity was indirectly promoted by island area through increased plant diversity and decreased soil electrical conductivity, and indirectly reduced by pine forest loss through decreased plant diversity. 4. Our findings highlight that island biogeography theory has relevance to understanding human impacts in the Anthropocene, and that there is a need to more explicitly recognizing how island size and remoteness affect biodiversity not only directly, but also indirectly via their effects on human-induced drivers of biodiversity, such as land use change and biological invasion.