Effects of catastrophic disease-related tadpole declines on upland neotropical stream structure and function
Connelly, Scott J.
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Global declines of amphibian populations are well documented, yet effects of these declines on freshwater ecosystem structure and function are poorly understood. Here we examine responses of algal primary producers and decomposition processes to tadpole extirpation over differing spatial and temporal scales. We experimentally excluded tadpoles from artificial substrata within localized areas of two streams. One stream had an intact community of frogs (frog stream), and the other had recently experienced a catastrophic decline (frogless stream), leaving virtually no tadpoles. In the frog stream, there were significantly greater levels of chlorophyll a (+111 %, P = 0.009), AFDM (+163 %, P = 0.02), inorganic sediments (+114 %, P = 0.001), and higher mean algal cell biovolume in tadpole exclusion treatments than in the tadpole access treatments. Correspondingly, overall AFDM-specific net primary production increased by 38% (P = 0.001) and chlorophyll a-specific NPP increased by 29% (P = 0.001) in tadpole access.. Fifteen months after our experiments, a massive amphibian decline associated with a fungal pathogen occurred in the frog stream, resulting in the extirpation of over 90% of tadpoles. This extirpation was followed by significant increases in levels of chlorophyll a (269%, P = 0.001), AFDM (+220%, P < 0.001), and inorganic sediments (+140%, P = 0.001). Over the longer-term (3 years), the magnitude of this initial change dampened to increases of 1.9-fold in pools (P < 0.05) and 3.5-fold in riffles (P < 0.05) over pre-extirpation levels. We also experimentally excluded detritus-feeding tadpoles from localized areas. We found significantly higher mean levels of fungal biomass than in tadpole exclusion treatments (P<0.10; 56.9 vs 42.7 mg C g-1 AFDM), suggesting that Centrolenid tadpoles play a role in stimulating growth of stream fungal communities. Our experimental results, combined with algal monitoring at the reach scale, indicate that over the course of our study catastrophic amphibian losses have significant effects on stream ecosystem structure and function. Ecosystem-level impacts of tadpole extirpations were more dramatic than results from our small-scale, short-term experiments, which predicted the direction of change in response variables but underestimated the magnitude. However, the longer-term stream ecosystem responses remain unknown.