Natalie A. Clay

Sodium shortage as a constraint on the carbon cycle in an inland tropical rainforest

Sodium (Na) is uncommon in plants but essential to the metabolism of plant consumers, both decomposers and herbivores. One consequence, previously unexplored, is that as Na supplies decrease (e.g., from coastal to inland forests), ecosystem carbon should accumulate as detritus. Here, we show that adding NaCl solution to the leaf litter of an inland Amazon forest enhanced mass loss by 41%, decreased lignin concentrations by 7%, and enhanced decomposition of pure cellulose by up to 50%, compared with stream water alone. These effects emerged after 13–18 days. Termites, a common decomposer, increased 7-fold on +NaCl plots, suggesting an agent for the litter loss. Ants, a common predator, increased 2-fold, suggesting that NaCl effects cascade upward through the food web. Sodium, not chloride, was likely the driver of these patterns for two reasons: two compounds of Na (NaCl and NaPO4) resulted in equivalent cellulose loss, and ants in choice experiments underused Cl (as KCl, MgCl2, and CaCl2) relative to NaCl and three other Na compounds (NaNO3, Na3PO4, and Na2SO4). We provide experimental evidence that Na shortage slows the carbon cycle. Because 80% of global landmass lies >100 km inland, carbon stocks and consumer activity may frequently be regulated via Na limitation.

Thermal adaptation and phosphorus shape thermal performance in an assemblage of rainforest ants

We studied the Thermal Performance Curves (TPCs) of 87 species of rainforest ants and found support for both the Thermal Adaptation and Phosphorus-Tolerance hypotheses. TPCs relate a fitness proxy (here, worker speed) to environmental temperature. Thermal Adaptation posits that thermal generalists (ants with flatter, broader TPCs) are favored in the hotter, more variable tropical canopy compared to the cooler, less variable litter below. As predicted, species nesting in the forest canopy 1) had running speeds less sensitive to temperature; 2) ran over a greater range of temperatures; and 3) ran at lower maximum speeds. Tradeoffs between tolerance and maximum performance are often invoked for constraining the evolution of thermal generalists. There was no evidence that ant species traded off thermal tolerance for maximum speed, however. Phosphorus-Tolerance is a second mechanism for generating ectotherms able to tolerate thermal extremes. It posits that ants active at high temperatures invest in P-rich machinery to buffer their metabolism against thermal extremes. Phosphorus content in ant tissue varied three-fold, and as predicted, temperature sensitivity was lower and thermal range was higher in P-rich species. Combined, we show how the vertical distribution of hot and variable vs. cooler and stable microclimates in a single forest contribute to a diversity of TPCs and suggest that a widely varying P stoichiometry among these ants may drive some of these differences.

Arboreal substrates influence foraging in tropical ants.

1. Physically complex substrates impart significant costs on cursorial central‐place foragers in terms of time spent outside the nest and total distance travelled. Ants foraging in trees navigate varied surfaces to access patchy resources, thus providing an appropriate model system for examining interactions between foraging efficiency and substrates.

Manna from heaven: Refuse from an arboreal ant links aboveground and belowground processes in a lowland tropical forest

Aboveground consumers can shape belowground processes by serving as conduits for resources. Social insects dominate in terms of biomass in tropical forests, but compared to studies on large mammals, or aggregate solitary insects, we know relatively little about the role of social insects as nutrient conduits particularly in complex environments like tropical forests. Social insects like ants in the tropical forest canopy can connect aboveground and belowground food webs by producing a nutrient stream (excreta) from large, long-lived and stationary nests. The excreta, in turn, would create enduring spatial heterogeneity in the forest floor. Here we evaluate this scenario in a lowland Neotropical forest using Azteca trigona, a dominant canopy ant that feeds on honeydew and insects and rains refuse out of its hanging nests onto the leaf litter below. We investigate decomposition rates and detrital communities associated with areas near nests versus 10 m away. Further, we directly test refuses impact on decomposition and detrital communities in a common garden experiment. Relative to leaf litter, refuse is enriched 7-fold in P, 23-fold in K, and 3-fold in N, all elements shown to limit decomposition in this forest. Accordingly, both artificial substrates and natural leaf litter substrates decomposed over 1.5- and 1.2-fold faster respectively below A. trigona nests and areas under nests supported more invertebrate detritivores and predators compared to controls 10 m away. These decomposition results were replicated in a 6-wk common garden experiment, but the changes in detrital invertebrate composition were not. Canopy ants like A. trigona act as dependable nutritional conduits to patches of the forest floor, transferring significant quantities of aboveground exudates and necromass. The general capacity for such social insect colonies to generate ecosystem heterogeneity remains an open question.

Towards a geography of omnivory: Omnivores increase carnivory when sodium is limiting

Towards understanding the geography of omnivory, we tested three hypotheses that predict the proportion of animal tissue consumed: the sodium limitation hypothesis predicts that omnivores increase animal consumption in Na-poor environments because Na bioaccumulates from plants to predators; thus, heterotrophs are Na-rich sources. The nitrogen limitation and habitat productivity hypotheses use the same logic to predict more animal consumption in N-poor and productive environments respectively. Omnivory is a common trophic strategy, but what determines the balance of plant and animal tissue omnivores consume is relatively unexplored. Most of what we know comes from single populations at local scales. Here we quantitatively test these three hypotheses at a large geographic scale and across 20 species of omnivorous ants. We tested each hypothesis using N stable isotopes (δ15 N) to quantify the degree of carnivory in ant populations in 20 forests that span 12° latitude from Georgia to Maine, USA. We used the difference in δ15 N between 20 ant conspecifics in 10 genera between two paired forests (10 pairs of 20 forests) that consisted of a coastal and inland forests on the same latitude to determine if the proportion of animal tissue consumed could be predicted based on Na, N or net primary productivity. Sodium gradients accounted for 18% of the variation in δ15 N, 45% if one outlier ant species was omitted. In contrast, the nitrogen limitation and habitat productivity hypotheses, which predict more animal consumption in N-poor and more productive environments respectively, failed to vary with δ15 N. Our results reveal a geography of omnivory driven in part by access to Na.