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.

生物多样性正因人类驱动的土地利用变化与生物入侵而急剧下降，但目前我们对这些驱动因子如何影响岛屿生态系统中植物与异养生物多样性的认知仍十分有限。长期以来，岛屿生物地理学理论仅关注岛屿面积与隔离度对生物多样性的直接效应，但这两类因子还可通过影响土地利用变化与生物入侵，间接调控物种的获得与丧失。本研究构建了结构方程模型（Structural Equation Model, SEM），以探究中国东部最大的陆桥群岛内37个大陆架岛屿上，岛屿面积与隔离度的直接效应，以及这两类因子通过土地利用强度与松材线虫入侵产生的间接效应，对植物与土壤细菌多样性的影响。正如预期，岛屿面积增大可直接提升植物多样性。然而，土地利用强度随岛屿面积增大而提升，这同样促进了植物多样性；而松材线虫入侵导致的松林丧失程度随岛屿隔离度升高而加剧，进而降低了植物多样性。岛屿隔离度仅通过加剧松林丧失，间接降低植物多样性。岛屿隔离度会直接抑制土壤细菌多样性，还会通过提升土壤电导率间接产生负面影响——该电导率升高大概率由海水飞沫携带的更高盐度所导致。此外，岛屿面积可通过提升植物多样性、降低土壤电导率，间接促进土壤细菌多样性；而松林丧失则会通过降低植物多样性，间接减少土壤细菌多样性。本研究结果表明，岛屿生物地理学理论对理解人类世的人类活动影响具有参考价值，同时也提示我们需要更清晰地认识到：岛屿面积与隔离度不仅会直接影响生物多样性，还可通过作用于土地利用变化、生物入侵等人类诱导的生物多样性驱动因子，对其产生间接调控作用。