Macrophyte stand complexity explains the functional α and β diversity of fish in a tropical river-floodplain

Aquatic macrophytes increase habitat complexity and influence the structure of fish communities. We investigated relations between macrophyte stand complexity and functional alpha and beta diversity of fish. We sampled fish and plants in 30 macrophyte stands with differences in density and diversity in the Paraná River floodplain. The functional alpha diversity, measured as functional richness index (FRic), was calculated for each macrophyte stand. The functional beta diversity was examined using pairwise Jaccard dissimilarity as well as its turnover and nestedness components. We also partitioned functional beta diversity into contributions of single sites to overall beta diversity (LCBD) aiming to assess its relationship to macrophyte stand characteristics. We then used beta regressions and generalized dissimilarity modeling (GDM) to examine diversity patterns. As we expected, the greatest FRic occurred in stands with intermediate macrophyte density and high macrophyte diversity. The functional beta diversity responded mainly to the variation in macrophyte density, but the turnover component increased slowly at the beginning of the gradient (low density) and, after a certain point, it started to increase more rapidly. The stands that contributed the most to the functional beta diversity (higher LCBD values) were those with low and high FRic and, consequently, with lower and higher macrophyte density, as well as lowest macrophyte diversity. Our findings highlight the role of macrophytes as environmental filters that select the traits in fish communities, and the variation in fish traits is probably a result of factors such as food availability, shelter from predators and physical space for locomotion.