Historical and contemporary drivers of Amazonian stream fish community assembly

Predictive models considering the interplay between historical and ecological processes in community assembly are widely discussed in ecological theory, but scarcely assessed with empirical research. Here, we evaluate the role of these two major drivers in shaping local trait diversity and composition using a comprehensive Amazon stream fish dataset. Our analyses reveal that contemporary habitat characteristics, particularly channel depth and structural complexity, exert a strong positive effect on trait-space volume and the diversity of feeding and locomotor strategies: deeper streams harbour assemblages with broader ecomorphological breadth and enhanced microhabitat exploitation. In parallel, historical drivers leave a distinct signature on trait composition: sites located in regions exhibiting greater Quaternary climate stability (differences between the Last Glacial Maximum and the present) and those inundated by Miocene marine incursions (approximately 5 Ma) support communities dominated by large-bodied, highly dispersive species. These patterns indicate that past climate oscillations and extinction-recolonization processess are also important in defining trait structure. By explicitly comparing the magnitudes of historical versus ecological effects across multiple functional indices, our study demonstrates that predictive models of community assembly must integrate both deep-time and present-day processes.