Disease-Driven Amphibian Declines Alter Ecosystem Processes in a Tropical Stream

Abstract

Predicting the ecological consequences of declining biodiversity is an urgent challenge, particularly in freshwater habitats where species declines and losses are among the highest. Small-scale experiments suggest potential ecosystem responses to losses of species, but definitive conclusions require verification at larger scales. We measured ecosystem metabolism and used whole-ecosystem stable isotope tracer additions to quantify nitrogen cycling in a tropical headwater stream before and after the sudden loss of amphibians to the fungal pathogen Batrachochytrium dendrobatidis. Tadpoles are normally dominant grazers in such streams, where greater than 18 species may co-occur and densities often exceed 50 individuals m⁻². Loss of 98% of tadpole biomass corresponded with greater than 2× increases in algae and fine detritus biomass in the stream and a greater than 50% reduction in nitrogen uptake rate. Nitrogen turnover rates in suspended and deposited organic sediments were also significantly lower after the decline. As a consequence, the stream cycled nitrogen less rapidly, and downstream exports of particulate N were reduced. Whole stream respiration was significantly lower following the decline, indicating less biological activity in the stream sediments. Contrary to our predictions, biomass of grazing invertebrates, or any invertebrate functional groups, did not increase over 2 years following loss of tadpoles. Thus, reductions in ecosystem processes linked to the amphibian decline were not compensated for by other, functionally redundant consumers. Declining animal biodiversity has ecosystem-level consequences that may not be offset by ecological redundancy, even in biologically diverse regions such as the Neotropics.