Cornell University Ithaca, New York, United States
Background/Question/Methods
Animal-mediated nutrient dynamics are critical processes in ecosystems. Animals can dramatically alter the structure and function of ecosystems through nutrient deposition via mortality, excretion, and egestion, each of which are constrained by an organism's stoichiometric blueprint. Previous research in aquatic systems has shown that considering stoichiometric variation in aquatic vertebrates is critical to understanding ecosystem nutrient dynamics; however, little is known about the links between vertebrate stoichiometry and ecosystem functioning in terrestrial systems due to limited information on the body stoichiometry of organisms. Here, we examine variation in the body and waste stoichiometry of birds, ubiquitous and highly mobile animals found globally. Using inductively coupled plasma mass spectrometry (ICP-OES) and elemental analyzers (EA), we measured the concentration of 12 elements (C, N, P, Ca, K, Na, S, Si, Fe, Mg, Zn, Al) in the body of 25 different species (n = 50) diverse in size, life history, and taxonomy. Whole bodies were dissected of crop, stomach, and gizzard contents before being processed for analyses. We also compiled waste stoichiometry values from the literature (n=99, species = 66) through systematic literature review. We examined whether body mass, diet, morphology, or taxonomic identity were associated with elemental concentrations and elemental ratios across species.
Results/Conclusions
Our results show that nutrient content scales proportionally for C (m = 1, r2 = 0.99) and N (m = 1, r2 = 0.99), and slightly increases for P (m = 1.04, r2 = 0.99) due to allometric scaling of skeletal mass (m = 1.07). This leads to a relatively constant N:P ratio among birds (mean = 4.49 ± 0.87). Existing variation was best explained by a significant positive relationship between N:P and fat content. Tissue subsampling showed that the N:P content is disproportionally higher in fat relative to other tissue types (bone, muscle, organs), explaining this difference. Congruent with body stoichiometry results, we found little influence of allometry on the waste stoichiometry of birds. Instead, the N:P ratio of waste was best predicted by differences in diet. Vertebrate carnivores showed significantly lower average N:P ratios in waste (1.85 ± 1.75) compared to insectivores (6.24 ± 0.91) or herbivores (6.40 ± 2.78). This has important implications when considering the different functional roles of birds in biogeochemical cycling and highlights fundamental differences between birds and other organisms. Future research may apply this information to better characterize the role of birds in ecosystem functioning.
