Intraspecific competition reduces the quantity of excreted nutrients in tadpoles

Abstract

Anuran larvae can form the largest animal biomass seasonally in freshwater environments, yet, they are still one of the least-studied taxa in terms of nutrient regeneration. The present study tested whether sympatric tadpoles regenerate nutrients at similar rates and ratios, and whether increased intraspecific competition (hereafter ‘competition’) alters patterns of excretion. We quantified rates and ratios of excretion (dissolved nitrogen (N) from ammonia, phosphorus (P) from total dissolved P, and N:P ratio) in three pond-dwelling Japanese tadpoles (Pelophylax nigromaculatus, Rhacophorus schlegelii, Hyla japonica), and tested the effect of competition on excretion in Pelophylax nigromaculatus and Rhacophorus arboreus. The three co-occurring species regenerated nutrients at different rates and ratios; H. japonica excreted nutrients and produced faecal pellets at the lowest rates. Inside field enclosures, increasing tadpole density reduced the quantity but not the quality of excretion by the tadpoles, suggesting higher nutrient sequestration, likely to maintain a stoichiometrically balanced growth under limited resources. Differences in rates and ratios of excretion have previously been shown to have various effects on community structure by affecting primary productivity, highlighting the importance of species identity and interactions on ecosystem function.