Characterizing spatial and temporal trends in fish community diversity in the inland lakes of Isle Royale, MI, USA

Abstract

The inland lakes of Isle Royale, MI have remained relatively isolated from human impact, providing an opportunity to explore how fish diversity has natural shifted with minimal confounding effects. I analyzed fish assemblages of 23 lakes sampled in the 1920s, 1990s, and 2020s to 1) assess temporal shifts in species richness and composition 2) identify lakes and species experiencing the most change, 3) explore environmental factors driving community structure. I used species-area curves, PERMANOVA, and multivariate dispersion tests to quantify differences in diversity between surveys, temporal beta diversity indices and SIMPER to identify lakes and species changing most, and NMDS to examine lake factors influencing composition. Mean richness and slopes of species-area curves did not significantly differ between surveys, although both declined slightly. Overall community composition and spatial variability among lakes remained mostly stable, but larger lakes became more dominated by cool-water species. Species gains and losses were associated with isostatic rebound rate, where lakes with fast and slow rebound rates tended to gain and lose species, respectively. Sculpin and stickleback species were most commonly lost, while blackchin shiner and pumpkinseed were most commonly gained. Larger lakes hosted more diverse communities, and lakes with higher elevation lacked northern pike and yellow perch. Overall, these results suggest the inland lake fish communities of Isle Royale have not homogenized over time, but certain assemblages have gained cool-water and lost several cold-water species despite buffering effects of Lake Superior’s climate.