Ten years of TADS: Lessons learned from the long-term Tropical Amphibian Declines in Streams Project
Global declines in biodiversity are continuing at alarming rates, and there is growing evidence that current extinction rates are greater than those during the previous Big Five extinctions. A key question is: What effect do these ongoing biotic losses have on ecosystem structure and processes? This question is particularly relevant in light of changing amphibian communities. Amphibians are especially speciose and abundant in many terrestrial and aquatic tropical ecosystems, and are dominant grazers in many tropical streams. However, they are a being lost at unprecedented and disproportionally high rates. As part of the Tropical Amphibian Declines in Streams (TADS) Project, we characterized tadpole communities, primary producers, and macroinvertebrates in an upland Panamanian stream for two years pre- amphibian decline and ten years post-decline. Here, we present an overview of stream ecosystem response data collected over the ten year period following a widespread die-off of the stream breeding amphibians.
Tadpole densities dropped from initial levels of 8.2 individuals m-2 (pre-decline) to ~0 individuals m-2 ten years post-decline. During this period, levels of chlorophyll a in pool habitats increased by 2.8 fold from a pre-extirpation mean of 3.00 mg m-2 to 8.31 during the initial (5 month) post-extirpation period followed by a decrease to a mean of 5.70 ten years post-decline. Chlorophyll a in riffles followed a similar trend, increasing by 6.3 fold from a pre- tadpole extirpation mean of 1.07 mg m-2 to 6.74 initial post-extirpation, but decreasing to a mean of 3.7 during the ten year post decline period. Immediately following tadpole declines, total macroinvertebrate biomass and richness did not show a response. However, shift in community structure were evident at finer scales (the family- or genus-level) there were shifts in community structure (e.g. biomass and production), most notably in the grazer community, with declines in Psephenidae and Crambidae and increases in leptophlebiid and baetid mayflies. Longer-term response of algal-grazing insects showed diets shifted to include medium-sized diatoms, and predatory insect taxa consuming more diatoms and filamentous algae. Despite variability in trophic interactions at a population level, we found very little change in overall food web structure between pre- and post-decline. Our on-going study shows that tadpoles significantly alter stream ecosystem structure. Although the magnitude of their impact may change over time, our results suggest that stream food webs may be permanently altered.