Seventy-one years of avian community dynamics on an island: functional homogenization among dynamic equilibrium

Long-term datasets are crucial for discerning directional ecological changes from natural variability and for testing foundational theories. This study analyzes 71 years (1949–2020) of breeding bird monitoring data from an island ecosystem to quantify long-term community dynamics and assess the applicability of the island biogeography equilibrium theory. We employed an integrated analytical framework combining quadrant classification (based on presence percentage and redistribution level), the DAISIE model (Dynamic Assembly of Island Species through Immigration, Extinction and Speciation), and temporal trend analysis. Our results reveal a marked restructuring of the avian assemblage: specialist species (e.g., Hydroprogne caspia) declined to local extinction, while generalist species (e.g., Phalacrocorax carbo) increased exponentially, driving a shift from a tern–gull to a cormorant–gull dominated system. Despite stable species richness, the community experienced significant functional homogenization, evidenced by a 47% decrease in Pielou's evenness (from 0.72 to 0.38) and a drop in the effective number of species from 2.8 to 1.7. The DAISIE model estimated a colonization rate (λ) of 0.18 species/year and an extinction rate (μ) of 0.12 species/year, predicting an equilibrium richness of 1.5 regularly breeding species, closely matching the observed long-term average. Annual species turnover averaged 18.3%, with colonization events clustering in discrete periods (1975–1985, 2015–2020), suggesting environmental windows for establishment. Quadrant classification successfully classified species into functional groups (successful colonizers, stable populations, new colonizers, rare/endangered), providing a robust diagnostic tool for conservation prioritization. We conclude that while the system operates near a dynamic equilibrium, as predicted by theory, it has undergone substantial functional simplification, underscoring the vulnerability of island ecosystems to biodiversity erosion even when species counts appear stable.