α-diversity often responds to habitat complexity as a unimodal function. In aquatic systems, increasing density of aquatic macrophytes creates more habitat complexity for fishes, but only up to a certain threshold, beyond which fish abundance and diversity are restricted by reduced space and hypoxia within macrophyte bed interstices. Species turnover and nestedness should be observed over habitat complexity gradients, and patterns of species co-occurrence may be influenced by associated environmental variation. We investigated the relationship of fish co-occurrence patterns, α-diversity, and β-diversity with gradients of habitat complexity created by macrophytes in the floodplain of Upper Paraná River.
Results/Conclusions
We collected a total of 1,832 fishes (24 species) along vegetation density gradients. α-diversity correlated with plant density according to a hump-shaped function, and also correlated negatively with dissolved oxygen concentration of the water. Variation in species turnover correlated with macrophyte density, whereas variation in species nestedness only correlated with dissolved oxygen. Species co-occurrence was influenced by macrophyte density as well as by dissolved oxygen concentration, with co-occurrence patterns positively influenced by these variables. Our results revealed that α-diversity peaked at intermediate levels of habitat complexity where interstitial space was greater and dissolved oxygen concentrations showed no limiting conditions. Low habitat complexity was associated with lower α-diversity, as commonly reported, and this may result from the influence of predation mortality or threat in areas with less physical structure providing refuge from predators or interference with predator lines of sight during prey detection and pursuit. High habitat complexity restricts fish size. Among large species, only small juveniles were collected from small interstitial spaces of dense macrophyte beds. Low dissolved oxygen in dense macrophyte beds also restricts species intolerant of aquatic hypoxia. Consequently, macrophyte density apparently functions as a strong environmental filter influencing spatial patterns of α- and β-diversity.