Island area and remoteness shape plant and soil bacterial diversity through land use and biological invasion

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 the direct
effects of island size and remoteness on biodiversity, but these factors
can also indirectly affect species gain and/or loss by impacting land use
change and biological invasion. We built the structural equation model to
explore the 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. 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. 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 recognize 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.