Drivers of Moss Functional Richness on Subtropical Coastal Islands: Area‐Driven Environmental Changes and Trait Structure Shape Distinct Pathways

Abstract

Aim Although island attributes are known to influence functional richness (FRic), the mechanisms underlying these patterns remain unclear. Specifically, whether they are driven by area thresholds (the small‐island effect, SIE) or mediated by community assembly processes remains unclear. This study aims to disentangle the relative importance of island attributes versus climatic factors in shaping moss FRic and to evaluate how environmental scaling and trait structure contribute to its regulation in subtropical coastal islands. Location Coastal islands in Zhejiang Province, East China Sea, China. Taxon Mosses (404 species). Methods We analysed moss community data and environmental variables from 180 islands, classified into 110 small islands and 70 large islands (based on a detected SIE area threshold: 10 2.038 ha). FRic was calculated from 13 moss morphological traits, and the standardised effect size of FRic (SES.FRic) was derived via null models to represent community assembly outcomes. We used variation partitioning analysis (VPA) to quantify the relative importance of island attributes versus climatic factors. In addition, piecewise structural equation models (piecewise SEMs) were grouped by both the area and SES.FRic thresholds were used to evaluate the pathways linking island attributes to FRic. Results Variation partitioning and hierarchical partitioning consistently showed that island attributes explained more variation than climatic factors. On small islands, island characteristics were the main independent contributors ( R 2 = 0.051, p = 0.018), whereas on large islands, island area became dominant ( R 2 = 0.092, p = 0.002), with climatic variables showing comparatively weaker effects. Hierarchical partitioning revealed a similar shift, with island characteristics contributing most on small islands (57.2%) and island area increasing on large islands (46.2%). SEM analyses showed that area‐based grouping primarily altered the relationships between island attributes and mediating variables, whereas SES.FRic‐based grouping mainly affected the pathways linking mediating variables to FRic. Main Conclusions The regulation of moss functional richness reflects a pathway‐level differentiation in the mechanisms underlying FRic. Environmental scaling associated with island area structures the relationships between island attributes and mediating variables, whereas trait structure determines how these mediators are translated into functional diversity outcomes. Rather than representing alternative explanations, environmental scaling and trait structure operate at different stages of the ecological pathway underlying FRic. These findings extend and refine the view that community assembly uniformly mediates environment–diversity relationships and instead highlight a stage‐specific interaction between environmental constraints and trait structure in shaping FRic.

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