Effects of biodiversity of larval amphibians on ecosystem functioning : a mesocosm approach

Abstract

Globally over 30% of amphibian species are endangered, and another quarter may be at risk. Their larvae are important components of freshwater ecosystems, and amphibians have different ecological roles before and after metamorphosis. Reductions in amphibian species richness or changes in their composition are likely to alter important ecosystem functions, including primary production, nutrient dynamics and decomposition. However, the relationship between amphibian biodiversity and ecosystem functioning (B-EF) remains unclear. I used an outdoor mesocosm-based approach in an attempt to understand how tadpole assemblages typical of tropical stream and wetland habitats influence ecosystem functioning with two sets of experiments. Each set of experiments comprised single and multispecies mixtures of tadpoles, comparing their effect on algal accrual (phytoplankton and periphyton), nitrogen and phosphorus dynamics, and leaf-litter breakdown in terms of leaf mass loss and fine particulate organic material (FPOM) generation. One set (comprising two experiments) involved two stream-dwelling tadpoles, a micro-particle filter feeder (Megophrys brachykolos: Megophryidae) and a grazer (Quasipaa exilispinosa: Dicroglossidae), with two mixed-species treatments (equal-biomass and equal-density) to account for the unequal body sizes between the two species. Another set (four experiments) involved four wetland tadpoles, two filter feeders (Microhyla fissipes and Kaloula pulchra: Microhylidae) and two grazers (Duttaphrynus melanostictus: Bufonidae; Polypedates megacephalus: Rhacophoridae). An experiment with only the two filter-feeders investigated B-EF-related density effects by incorporating two density treatments. Species identity of stream-dwelling tadpoles affected phosphate and nitrite dynamics, leaf mass loss and FPOM generation. In the first stream-mesocosm experiment, the presence of Quasipaa exilispinosa increased leaf mass loss and doubled FPOM generation, while the presence of Megophrys brachykolos increased nitrite concentrations but was associated with reduced levels of phosphate. Some minor effects of species identity of wetland tadpoles on leaf mass loss were observed in one experiment. However, changes in the species richness of tadpoles had very minor effects on ecosystem functioning and were evident only in phosphorus flux associated with the stream-dwelling species. Functional identity of wetland tadpoles affected both nitrite concentration and periphyton biomass in two of the experiments, in which the presence of grazers was associated with increased nitrite concentrations and decreased periphyton biomass. As only two species were included in the stream-mesocosm experiments, effects attributable to their functional identity could not be separated from their species identity, but the latter had a substantial influence on ecosystem functioning. The B-EF relationship was also mediated by tadpole density. In the first stream experiment, the equal-density mixed-species treatment had higher levels of leaf-litter breakdown than in the equal-biomass treatment, indicating different proportions of species in the same mixed-species groupings will differentially affect leaf-litter breakdown. In the wetland experiment, higher tadpole density altered nitrite flux in the mixed-species treatment. The study suggests that changes in tadpole diversity, especially functional identity, and density may affect stream and wetland ecosystem functioning, but the magnitude and direction of effects are context-specific. Future work should examine how tadpole abundance and warmer temperatures affect the B-EF relationship in the broader global context of amphibian population declines and climate change.