Over- and Under- Representations of Predatory Fish Body Sizes Are Shaped by Habitat Architecture and Nutrient Loads in a Subtropical River Basin

Abstract

Body size distribution reflects the ecological and evolutionary mechanisms that shape biodiversity. Over- and under-representations -i.e., discontinuities- in this distribution reflect body sizes that could be favoured or limited by operating mechanisms. Although several non-exclusive hypotheses have been formulated to explain the discontinuities in body size distributions, few empirical studies have been able to advance on the determinants of these discontinuities along ecological gradients. By means of null models we assessed the existence of over-/under- representations in the body size distribution of 48 local assemblages of carnivorous fishes in the Rio Negro basin, Uruguay. Specifically, we identified the body size locations at which an over-representation and/or under-representation occurs in local communities compared to the null model distribution. Then, using Structural Equation Modelling (SEM), we explicitly connect these deviations with gradients in local conditions. Over-representations at the different sites ranged from sizes of 0.1 to 28.8 g, while under-representations ranged from 0.1 to 27.1 g. Our structural equation model identified a significant connection between substrate type and nutrient loads with the location of over-representations in the size spectra, while substrate type and depth were connected to the magnitude of the over-representation. The magnitude of the over-representation influenced the size and magnitude of the under-representation. Local environmental variables exhibited strong influence on the body size distribution, being favoured over the connectivity of communities. This suggests that local filtering has a preponderant role in explaining the dependence of species performance on body size in this system. The present study highlights the existence of discontinuities in the distribution of traits in general and body size in particular as a relevant feature of biodiversity organisation, seldom considered in empirical studies. Advancing on the local and regional determinants of these features emerges as a powerful and complementary approach for unravelling the mechanisms behind biodiversity assembly and functioning.