Scaling food web stoichiometry: A biogeographical comparison of habitat size constraints

Background/Question/Methods

Ecosystem size strongly affects key community properties, such as species richness, the abundance of individuals and food chain length. Further, ecosystem size may alter nutrient dynamics because of its effects on the structure of ecological communities. Understanding the role of habitat size on shaping pools of nutrients is critical, because consumers are sensitive to habitat size and they play key roles in the storage, processing, and cycling of nutrients within ecosystems. Therefore, we addressed the following questions: 1) What is the effect of ecosystem size on nutrient pools? 2) Does biogeography affect the structure and /or stoichiometry of the food webs, as well as the role of ecosystem size on nutrient pools? To answer these questions, we used aquatic insect food webs in bromeliads from Brazil and Costa Rica as a model system. Bromeliads trap both water and detritus; the detritus is consumed by larvae of many insect species, and these larva are consumed by aquatic predators. Bromeliads also vary substantially in their size. By principles of ecological stoichiometry and community data, we estimated the pools of nitrogen (N) and phosphorus (P) contained within detritivore and predator insects for each habitat size, is a key ecosystem function for these systems. 

Results/Conclusions

Pools of both N and P in the detritivore and predator biomass increase with increasing ecosystem size. The slope of the scaling relationships for N pools is different between Costa Rican and Brazilian food webs, and between detritivores and predators. Further, although both N and P pools scale with bromeliad size, N pools in both food webs showed an asymptotic relationship with ecosystem size. The P pools contained in the detritivore and predator compartments in both food webs scale linearly with ecosystem size.  Overall, predators store P faster (although N storage is slower) than detritivores as ecosystem size increases. Our results suggest that ecosystem size affects the body size structure of both predators and detritivores, with average predator individual body size increasing faster with bromeliad size than the average detritivore’s individual body size. Further, differences in the stoichiometry of detritivores and predators between Costa Rican and Brazilian food webs also influenced the patterns of N and P storage within these food webs. Thus, ecosystem size effects have consequences beyond food web structure, and these effects are mediated by fundamental constraints in consumer elemental stoichiometry.