Abstract:

Aims The species-area relationship describes a fundamental pattern in community ecology, illustrating how species richness changes as the sampled area increases. It is a core issue in community ecology research. The island biogeography theory highlights a specific aspect of the species-area relationship known as the small island effect (SIE), which is a key component of the theoretical framework of island biogeography and biodiversity studies. Coastal islands represent a tangible and existing island system, yet there is relatively limited research on the SIE, especially concerning bryophytes.

Methods This study investigated the species composition of bryophytes on 43 coastal islands in the Xiangshan region of Zhejiang. We utilized four segmented regression models and two control models to examine the SIE across different bryophyte taxa. We compared the thresholds at which it occurs. Using generalized linear mixed-effect models and Mantel tests, we explored the impact of environmental factors, such as landscape characteristics and human disturbances, on bryophyte species richness on these maritime islands. Furthermore, we analyzed the response of different bryophyte taxa (liverworts, mosses, acrocarpous mosses, and pleurocarpous mosses) to environmental factors in these island environments.

Important findings Our study documented 209 bryophyte species belonging to 46 families and 85 genera across the 43 coastal islands in the Xiangshan region. All bryophytes taxa exhibited the SIE, with the double-breakpoint model identified as the optimal model. The area thresholds ranged from 0.098 to 4.419 km², with mosses showing higher thresholds than liverworts, and acrocarpous mosses higher than pleurocarpous mosses. Moreover, the richness of bryophytes increased with the island area, habitat richness, shape index, coastline length, population size, and traffic frequency, while it decreased with increasing isolation. Habitat richness was identified as the primary factor influencing bryophytes richness. Generally, taxa with lower resource requirements and higher dispersal abilities (such as mosses) exhibited lower SIE area thresholds. Larger islands with greater distances from the mainland, characterized by lower vegetation cover and habitat richness, were less suitable for the growth of pleurocarpous mosses, resulting in lower SIE area thresholds than acrocarpous mosses. Therefore, the conservation of habitat diversity is crucial for maintaining species diversity.

Key words: island biogeography, species-area relationship, small island effect, habitat heterogeneity, area threshold, generalized linear mixed models