Trophic Trait Evolution Explains Variation in Nutrient Excretion Stoichiometry among Panamanian Armored Catfishes (Loricariidae)

Variation in nutrient excretion rates and stoichiometric ratios (e.g., nitrogen to phosphorus) by consumers can have substantial effects on aquatic ecosystem function. While phylogenetic signals within an assemblage often explain variation in nutrient recycling rates and stoichiometry, the phylogenetically conserved traits that underlie this phenomenon remain unclear. In particular, variation in nutrient excretion stoichiometry across a phylogeny might be driven by phylogenetic patterns in either diet or body stoichiometry. We examined the relative importance of these traits in explaining variation in nutrient recycling rates and stoichiometry in a diverse family of Neotropical-armored catfishes, Loricariidae, in Panamanian streams. We found significant variation in nutrient mineralization traits among species and subfamilies, but variation in nutrient excretion stoichiometry among species was best explained by trophic position rather than body stoichiometry. The variation in trophic position among Panamanian species was consistent with variation in the trophic niche of their genera across South America, suggesting that phylogenetic patterns underpin the evolution of trophic and nutrient excretion traits among these species. Such geographical variation in nutrient mineralization patterns among closely related species may be common, given that trophic variation in fish lineages occurs widely. These results suggest that information on trophic trait evolution within lineages will advance our understanding of the functional contribution of animals to biogeochemical cycling.