In tropical rain forests, the density of arthropods in the detrital food web is predicted by the biomass of detritus and the abundance of limiting nutrients, usually P.  The trophic structure of this food web is likely to respond to spatial differences in biomass and nutrient inputs, and here we test competing models that may account for the trophic level of key occupants of the leaf litter food web in a Costa Rican rain forest.  Across eighteen sites spanning a threefold fertility gradient, we sampled live arthropods including collembola, oribatids, spiders and pseudoscorpions and evaluated their diet using stable isotopes of C and N, relative to the baseline composition of standing leaf litter.  
Results/Conclusions

For most litter taxa, the most parsimonious models indicate that the rate of litter decomposition and the concentration of Ca are predictive of the trophic level of litter arthropods.  These results suggest that Ca is a limiting nutrient that may determine the number of trophic steps early in the process of decomposition.