Evenness and Taylor's law scaling shape biodiversity-stability relationships in subtropical estuarine communities

Understanding the mechanisms that link biodiversity to ecological stability is crucial for predicting ecosystem responses to global change. Using three decades of standardized monitoring data from eight subtropical estuaries, we analyze diversity components (richness, evenness, dissimilarity, and variance-mean scaling) and interpret stability patterns through synchrony or portfolio mechanisms. Regional {gamma}-diversity increased steadily over time, reflecting sustained gains in fish and invertebrate assemblages. Community stability, defined here as community invariability, was strongly and consistently predicted by Shannon diversity index but not by species richness, underscoring the stabilizing role of evenness. Portfolio effects were robust, with community stability averaging approximately threefold higher than mean population stability, and the strength of this effect more than doubled with each unit increase in Shannon diversity. Structural equation models paired with a null model revealed that shared environmental forcing synchronizes estuarine populations. Shannon diversity generated a large portfolio effect that stabilized the community despite this environmental forcing, whereas richness effects were weak or absent. Taylors law scaling confirmed that abundant, persistent taxa such as blue crab (Callinectes sapidus), brown shrimp (Farfantepenaeus aztecus), and pinfish (Lagodon rhomboides) exhibited higher baseline invariability, contributing to community buffering, while rarer, more variable taxa introduced volatility. In contrast, compositional turnover strongly eroded stability, with high Bray-Curtis dissimilarity predicting reductions in community stability. Together, these results show that long-term estuarine community stability emerges from the interplay of portfolio averaging, demographic variance scaling of dominant species, and the persistence of community composition, highlighting the central role of evenness in biodiversity-stability relationships.