Spatiotemporal Patterns and Drivers of Fish Biodiversity on a Remote Freshwater Archipelago

ABSTRACT Aim Fish communities are shaped by environmental filters operating across different spatiotemporal scales. While many recent studies focus on anthropogenic gradients, few can isolate the role of natural drivers in community assembly. Isle Royale's inland lakes offer a rare opportunity to explore long‐term fish biodiversity dynamics with minimal confounding influences. We analysed fish communities in 23 lakes surveyed in the 1920s, 1990s, and 2020s to 1) assess temporal changes in richness and composition 2) identify lakes and species experiencing the greatest change, and 3) evaluate environmental drivers of community structure. Location Isle Royale National Park, USA. Taxon Freshwater fish. Methods We analysed historical and contemporary fish survey data using species‐area curves, PERMANOVA, and multivariate dispersion tests to quantify changes in richness and composition, temporal beta diversity indices and similarity percentages (SIMPER) to identify lakes and species changing most, and non‐metric multidimensional scaling (NMDS) to examine relationships with environmental factors. Results Mean richness and species‐area curve slopes declined slightly but non‐significantly over time. Community composition remained largely stable with no homogenization, although subtle shifts from cold‐water to cool‐water species were observed in larger lakes. Species gains and losses were associated with isostatic rebound rate. Fast‐rebounding lakes tended to gain species, while slower‐rebounding lakes tended to lose species. Richness and predator presence were structured along lake area and elevation gradients, respectively. Main Conclusions Fish biodiversity of Isle Royale's lakes has remained remarkably stable over the past century, reflecting minimal anthropogenic influence. Subtle shifts suggest continuous filtering by geological and environmental processes, however, and spatial gradients in composition highlight interactions between large‐ and small‐scale filters. This study provides a valuable baseline reference for understanding the processes of community assembly and environmental change in more impacted ecosystems.