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

Abstract

Understanding the mechanisms that link biodiversity to ecological stability is crucial for predicting ecosystem responses to global change. Using three decades of standardized fish and invertebrate monitoring data from eight subtropical estuaries in Texas, we analyze diversity components (richness, evenness, dissimilarity, and variance-mean scaling) and interpret stability patterns through synchrony or portfolio mechanisms. Regional diversity ({gamma}-diversity) increased steadily over time, reflecting sustained gains in estuarine assemblages. Community stability, defined here as community invariability, was strongly and consistently predicted by Shannon diversity index but not by species richness, consistent with the stabilizing role of evenness. Portfolio effects were consistently strong, 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 showed that Shannon diversity reduced species synchrony less than expected from statistical averaging alone, consistent with shared environmental forcing, yet its direct effect on community stability exceeded the null expectation. Taylor's law scaling showed 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 combined effects of portfolio averaging, demographic variance scaling of dominant species, and the persistence of community composition, suggesting the central role of evenness-weighted diversity in biodiversity-stability relationships.