Biogeochemistry and the structure of tropical brown food webs

Abstract

Litter invertebrates are notoriously patchy at small scales. Here we show that the abundance of 10 litter taxa also varies 100-fold at landscape and regional scales across 26 forest stands in Peru and Panama. We contrast three hypotheses that link gradients of abundance to ecosystem biogeochemistry. Of 14 factors considered (12 chemical elements, plus fiber and litter depth), four best predicted the abundance of litter invertebrates. In the Secondary Productivity Hypothesis, phosphorus limits abundance via the conversion of detritus to microbial biomass. Two of four microbivore taxa, collembola and isopods, increased with the percentage of P (%P) of decomposing litter. However, percentage of S (correlated with %P) best predicted the abundance of collembola, oribatids, and diplopods (r2 = 0.38, 0.33, 0.21, respectively). In the Structural Elements Hypotheses, N and Ca limit the abundance of silk-spinning and calcareous taxa, respectively. Mesostigmatids, pseudoscorpions, and spiders, all known to make silk, each increased with percentage of N of litter (r2 = 0.22, 0.31, 0.26, respectively). Calcareous isopods, but not diplopods, increased with percentage of Ca of litter (r2 = 0.59). In the Ecosystem Size Hypothesis, top predators are limited by available space. The abundance of the three remaining predators, chilopods, staphylinids, and ants, increased with litter depth (r2 = 0.31, 0.74, 0.69, respectively), and food webs from forests with deeper litter supported a higher ratio of predators to microbivores. These results suggest that biogeochemical gradients can provide a mechanism, through stoichiometry and trophic theory, shaping the geography of community structure.

Microbes are trophic analogs of animals

“…Further, such organisms must be examined while integrated within their respective communities (1). Assessing the trophic niches of microbial and animal species has too often been a theoretical endeavor (9), with macrofauna generally associated with plant-based food webs ("green food webs") and the microfauna relegated to the sphere of decomposition ("brown food webs") (9)(10)(11). It has proven difficult to merge these two spheres using a shared trophic metric, not only because of the difficulties in identifying microbial diversity (8), but also because of methodological obstacles in measuring the trophic position of a microbe.…”

“…Indeed, in terrestrial systems, the vast majority of primary production is not captured by herbivores; rather, it falls to the ground and is consumed by microbes and small invertebrate detritivores (7,12,14). Higher-order carnivores consume the detritivores, conjoining the upward flow of detritivore and herbivore biomass (9,11,15,16), but if the trophic positions in the basal layers of the food web cannot be accurately measured, the entire food web rests on a poorly known, tenuous platform (9).…”

“…Recent studies have documented evidence of feeding guilds among the animals of brown food webs (11,15,25), and this finding sets up questions as to the trophic positions of the resident microbiota. For example, when microbes feed on a resource, do they register trophically just as animals do?…”

Physicochemical Surface-soil Properties after Litter-removal Manipulation in a Cambodian Lowland Dry Evergreen Forest

“…Mass-basis soil C stock is used for the LR, and see Table 5 for soil C stock. Increased litterfall can actually drive net soil C losses to the atmosphere 41,43 , while standing leaf litter reflects the balance of litterfall and decomposition 17 . Remaining C stocks in Land F-layers corresponded to 29 ± 10 and 11 ± 9% of the annual leaf litter supply respectively, suggesting that ca.…”

“…Once hydrological conditions are altered 21,25,26 , and soil arthropod habitats are destroyed 17 , density recovery is seriously hindered. Moreover, fine and coarse roots are the primary source of belowground C and influence the physical, chemical, and biological properties of soil 15 .…”