Abstract
Reef fish assemblages on oceanic islands offer valuable insights into ecological and evolutionary processes, yet their temporal dynamics remain poorly understood. This study evaluates the theory of island biogeography (TIB) with long‐term ecological monitoring to assess how island area, isolation from nearest reef and distance from mainland shape both species richness and stability across four southwestern Atlantic islands. Data were collected via underwater visual censuses from 2006 to 2011 and annually thereafter. A total of 3634 transects were conducted, recording over 150 fish species. Spatial β diversity was quantified using Sørensen and Bray–Curtis dissimilarity indices. Results indicate that larger islands, with greater habitat heterogeneity, support higher species richness and exhibit increased assemblage dissimilarity but reduced variability, suggesting a buffering effect against compositional shifts. Temporal analyses revealed that species turnover is the primary driver of annual changes in assemblage composition, with distinct patterns emerging according to island size. Isolation is significantly associated with lower temporal β diversity, whereas larger reef areas promote more dynamic shifts in assemblage structure. These findings provide empirical support for TIB, emphasizing that both island area and isolation are key factors influencing the richness and stability of reef fish assemblages. Long‐term monitoring, although scarce in the marine realm, is essential for capturing these complex dynamics and informing conservation strategies in the face of increasing anthropogenic pressures on unique insular marine ecosystems.