Nutrient availability and the structure and function of brown food webs
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Clay, N. A. 2013. Nutrient availability and the structure and function of brown food webs. University of Oklahoma, PhD Thesis.
Abstract:
Understanding individual nutritional requirements can generate good predictions
for how communities should be structured and how ecosystems function over gradients of nutrient availability. Aboveground consumers can shape belowground processes by serving as conduits for resources through excreta, and these nutrient pulses can profoundly impact ecosystem processes. Excreta like urine and feces is a frequently deposited source of nitrogen (N) and potassium (K), and sodium (Na) that can create enduring spatial heterogeneity in the forest floor by creating nutrient hotspots. Unlike N and K, Na is unique in that 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. This results in the trophic bioaccumulation of Na. Thus, plant consumers are likely Na-limited but their predators, which consume salty prey, are likely not. Omnivores are a common trophic group, eating both plant and animal tissue, that have Na tissue concentrations intermediate between herbivores and predators; but what determines the ratio of plants:prey is not well understood. Sodium is generally abundant near oceans, but its deposition in oceanic aerosols sharply decreases to zero moving inland away from coastlines. From this framework, I made predictions about how brown (detrital) food webs should be structured and function across Na gradients from tropical to boreal forests. I added Na subsidies to a Na-poor tropical forest and used stable isotope analysis to quantify the trophic positions of brown food web invertebrates and found that microbi-detritivores rapidly and predictably recruited to short-term Na subsidies but predators did not. In coastal Na-rich Panama I found that nutrient-rich excreta in the form of ant refuse from Azteca trigona, a dominant Neotropical canopy ant that feeds on honeydew and insects and rains refuse out of its hanging nests onto the leaf litter below, was a nutritional conduit between the canopy and forest floor. Refuse enhanced decomposition and supported larger brown food webs than leaf litter without refuse. I then used synthetic urine to separate the effects of Na from urine’s other nutrients and contrasted their roles in promoting decomposition and microbi-detritivore recruitment in both a Na-poor inland Ecuadorian and a Na-rich coastal Panamanian tropical forest. I found that urine was an important source of Na in the Na-poor but not Na-rich tropical forest that attracted termites and increased wood decomposition. Lastly, I tested whether omnivores become more carnivorous in Na- poor forests across a gradient of Na availability in ten paired (inland and coastal) lowland forests from Georgia to Maine, USA and also used laboratory Solenopsis invicta colonies. Omnivorous ants were up to an entire trophic level higher in Na-poor forests compared to conspecifics in Na-rich paired forests. Sodium gradients best predicted the difference in trophic position between conspecifics, and Na-starved S. invicta colonies increased Na and prey-seeking behavior 6-fold above control colonies. My results suggest that gradients of nutrient availability such as Na strongly impact brown food web structure and function. Across these gradients foraging animals facilitate heterogeneity in these leaf litter communities by acting as nutritional conduits between aboveground and belowground systems, but the impacts of these nutrient subsidies on brown food web structure and function depend on nutrient limitations of individual organisms. Lastly, my results suggest that there is a geography of omnivory that can be partially understood as a response to gradients in Na.