Mass invariance of population nitrogen flux by terrestrial mammalian herbivores

Background/Question/Methods:
The physiological traits of individuals can interact with characteristics of populations to explain, in large part, how groups of organisms contribute to the functioning of ecosystems. One example is Damuth’s energetic equivalence rule, which evolved from the observation that individual energy use, E, and population density, D, of mammalian primary consumers scales approximately reciprocally with body mass, M, such that population energy flux, E_pop = E D, was independent of body mass, or E_pop = E D µ M^0.75M^-0.75 = M⁰.  Species energy use can be equated with carbon flux, and it has been suggested that population fluxes of other materials, such as nitrogen and phosphorus, might also be independent of body mass.  However, strong evidence for mass independence of population level elemental fluxes has been lacking for wild animals. We compiled data from the literature on individual nitrogen deposition rates (via faeces and urine) and average population densities of 26 species of mammalian herbivores.  We then calculated population nitrogen flux for the 26 species on varying diets as the product of average population density and individual nitrogen flux to test the assumption of elemental equivalence for nitrogen. 

Results/Conclusions :

Our dataset included placental and marsupial mammals; browsers and grazers; and ruminants, hindgut fermenters, and monogastric herbivores.  Individual masses ranged from 0.02 to 451.20 kg. Food nitrogen concentration ranged from 0.21 to 5.22 %.  Individual nitrogen flux ranged from 0.02 to 97.91 g N d^-1.We found that daily individual nitrogen flux (Q) is related to species body mass, food nitrogen concentration (N) and taxonomic affiliation (T) as Q = 0.43 M^0.77 N^0.84T (R² = 0.95), where T is 1.20 and 0.83 for eutherian and metatherian species respectively.  Average species densities ranged from 0.01 to 120 individuals per ha and is related to average species body mass as D = 1.56 M^-0.81 (R² = 0.73).  Population nitrogen flux (Q_pop) was not related to species body mass, food nitrogen concentration or taxonomic affiliation.  Thus, the amount of nitrogen fluxed by populations of mammalian herbivores is independent of species body mass. Population nitrogen flux for mammalian herbivores, regardless of average body mass, was approximately 3.22 g N ha^-1 d^-1.  Results from this analysis can be used to understand the influence of mammalian herbivore communities on nitrogen cycling in terrestrial ecosystems.