Human disturbances explain the small-island effect of large and medium-sized mammals in the largest archipelago of China

The small-island effect (SIE) is a widely accepted pattern in island biogeography where species richness varies independently of island area below an area threshold. Three main ecological mechanisms (the disturbance hypothesis, habitat hypothesis, and subsidized island biogeography hypothesis) have been proposed to explain the SIE resulting from natural drivers. However, extensive human disturbance has led to considerable changes in insular species richness worldwide. Here, we incorporated human disturbances into the disturbance hypothesis and explored the mechanism to explain the SIE of large and medium-sized mammals in the Zhoushan Archipelago, China. We used the camera trapping method to survey large and medium-sized mammals on 50 islands from May 2020 to April 2024. We then used piecewise regression, path analysis, and null models to determine the existence of the SIE. We applied iterative partial regressions to assess the relative importance of island area and other island characteristics (human disturbances, variability of normalized difference vegetation index (NDVI), habitat diversity, rocky shoreline proportion, and isolation) in determining species richness. We found strong evidence for the existence of SIE when using the island area, however, we did not detect it when using the available habitat area (uninhabited islands). Additionally, the fraction of variance in species richness explained exclusively by human population density and land use intensity was high on small islands and decreased as island size increased, while that explained by island area was low and increased as island size increased. Overall, our findings supported the human-driven disturbance hypothesis, which provides a new explanation for the SIE. For effective conservation of large and medium-sized mammals in this region, we should protect large islands beyond the larger area threshold of the SIE, and conservation priorities for small islands should focus on those islands where human disturbances are weak.