Body size explains patterns of fish dominance in streams

Even after more than a century of research, the processes underlying species abundance distribution patterns are controversial. Here, we gathered abundance and size (standard length) data of fish species in 54 streams in the Midwest of Brazil to test whether subordinate species abundances (i.e., any species that is not dominant in a community) in each stream are correlated with the absolute size difference between dominant and subordinate species. A negative relationship between these variables would suggest a predominant role of environmental filtering because those species that differ more from the dominant species (the one with the optimum trait value) would become progressively less abundant. On the other hand, a positive relationship would suggest a limit to the similarity as the abundances of subordinate species that differ more from the dominant species would increase. Our results clearly indicated that subordinate species were those that most differed from the dominant species in terms of size. In addition, we found that the subordinate species were larger than the dominant species. Taken together, we infer that environmental filters favoring small body sizes (e.g., shallow water depth and scarcity of large shelters) are the main processes determining species abundance distributions in the streams we studied.