α and β diversity of fishes in relation to a gradient of habitat structural complexity supports the role of environmental filtering in community assembly

alpha-diversity often responds to habitat structural complexity as a unimodal function. In aquatic systems, increasing density of aquatic vegetation creates more habitat structural complexity for fishes, but only up to a certain threshold, beyond which fish abundance and diversity are restricted by reduced space. As a result, species turnover and nestedness should be observed over habitat structural complexity gradients, reflecting the sorting of species according to aspects of their environment. We investigated the relationship of fish alpha and beta diversity along gradients of habitat structural complexity created by aquatic vegetation in the floodplain of Upper Parana River. We collected a total of 1832 fishes (24 species) along vegetation density gradients. Our results revealed that alpha diversity peaked at intermediate levels of habitat structural complexity where interstitial spaces were numerous but no so small as to limit occupancy by most fishes. Low alpha diversity was associated with lower habitat structural complexity, as commonly reported, and this may result from the influence of predation mortality or threat where there is less physical structure that provides refuge from predators and interference with predator lines of sight for prey detection. Fish diversity is low in patches with high habitat structural complexity because small interstitial spaces restrict fish size and dissolved oxygen concentration sometimes is low. Aquatic vegetation density in floodplain habitats therefore functions as a strong environmental filter influencing spatial patterns of fish alpha and beta diversity.