Fish community survey data from 23 inland lakes of Isle Royale National Park (1929–2024)

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 analyzed 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. We
analyzed historical and contemporary fish survey data using species-area
curves, PERMANOVA, and multivariate dispersion tests to quantify changes
in richness and composition, temporal beta biodiversity indices and
similarity percentage (SIMPER) to identify lakes and species changing
most, and non-metric multidimensional scaling (NMDS) to examine
relationships with environmental factors. 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. 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.