Short-term sodium inputs attract microbi-detritivores and their predators

Understanding individual nutritional requirements can generate good predictions for how communities should be structured over gradients of nutrient availability. Sodium (Na) bioaccumulates from plants to predators: it is relatively unimportant for plants, which concentrate very little Na in their tissues, but critical for consumers, which concentrate Na well above plant levels. Thus, plant consumers are likely Na-limited but their predators, which consume salty prey, are likely not. From this framework, we can make predictions about how an entire community should respond to Na subsidies in Na-poor environments. We tested these predictions in an interior Amazonian brown (detrital) food web. Specifically, we quantified the responses of microbi-detritivores and their predators to experimental pulses of Na by adding 250 ml 0.5% NaCl solution that mimicked patchy urine deposition, or river water as controls, to 55 paired 0.25 m(2) plots every other day. We regularly harvested plots over a 19 day period. Consistent with the hypothesis of Na-limitation among plant consumers, the response (effect size) to Na addition by microbi-detritivores like termites was >6 times that of predators. Moreover, consistent with the bioaccumulation of Na, fewer predatory invertebrates increased on +NaCI plots, thus trophic position alone was a good predictor of response. Our results support the Na bioaccumulation hypothesis and suggest that patchy, short-term Na inputs (like urine) facilitate heterogeneity in these leaf litter communities. Further, this study demonstrates that combining principles from nutritional and community ecology can generate predictions about how communities should be structured over gradients of nutrient availability with good accuracy. (C) 2014 Elsevier Ltd. All rights reserved.