Contrasting effects of functionally distinct tadpole species on nutrient cycling and litter breakdown in a tropical rainforest stream

Abstract

Abstract There is growing interest in predicting how loss of species diversity and abundance affect the structure and functioning of ecosystems. Tadpole diversity and abundance can be exceptionally high in tropical streams, but, compared to other groups, relatively little is known about their functional roles. We assessed the trophic niches and investigated the contribution of tadpoles to nutrient recycling (excretion and egestion) and litter breakdown in streams. We used two ecomorphologically distinct tadpoles belonging to the two most dominant feeding guilds in Ranomafana, Madagascar: Boophis quasiboehmei (hereafter BQ, generalised mouthpart, grazer) and Mantidactylus melanopleura (hereafter MM, umbelliform funnel mouthpart, suspension feeder). We conducted field incubation experiments to quantify nutrients recycled from excretion and egestion, and set enclosures to analyse the effects of tadpoles on litter breakdown (leaves of Pauridiantha sp. and Chrysophyllum sp.) in nine treatments: control (no tadpoles), monospecific treatment (BQ or MM) at three densities (low, medium, high: 3, 6, 12 tadpoles/0.0625 m2, respectively) and mix‐species treatment (3 BQ + 3 MM, 6 BQ + 6 MM). BQ and MM occupied the same trophic level but tended to differ in their carbon isotopic signatures. MM excreted nitrogen (N) and phosphorus (P) at higher rates, but the two species had similar N:P molar excretion ratio. Nutrients recycled from faecal pellets (egestion) were not immediately available but needed to be mineralised: N increased with time in the water column and was more than 25 times higher in concentration at the end of the experiment (day 16), whereas P concentration showed a hump‐shaped pattern with a maximum value at day 4 of incubation. Effects of tadpoles on litter breakdown depended on tadpole identity, density and litter identity. Particularly, no breakdown occurred at low tadpole density, and effects were generally stronger on the softer texture leaves Pauridiantha sp. We found additive effects of tadpole diversity on litter breakdown. These findings indicate that (1) tadpoles with different mouthparts play different roles on the ecosystem; (2) tadpole faecal pellets are important latent sources of nutrients in freshwater environments; and (3) tadpole abundance exerted stronger effects than species diversity on litter breakdown, emphasising the importance of tadpole biomass and communities in tropical streams. Interspecific variation in nutrient recycling and differential effects on litter breakdown illustrate a strong linkage between species identity and ecosystem function.