Nutritional constraints in terrestrial and freshwater food webs

Biological and environmental contrasts between aquatic and terrestrial syst ems have hindered analyses of community and ecosystem structure across Eart h's diverse habitats. Ecological stoichiometry(1,2) provides an integrative approach for such analyses, as all organisms are composed of the same majo r elements (C, N, P) whose balance affects production, nutrient cycling, an d food-web dynamics(3,4). Here we show both similarities and differences in the C:N:P ratios of primary producers (autotrophs) and invertebrate primar y consumers (herbivores) across habitats. Terrestrial food webs are built o n an extremely nutrient-poor autotroph base with C:P and C:N ratios higher than in lake particulate matter, although the N:P ratios are nearly identic al. Terrestrial herbivores (insects) and their freshwater counterparts (zoo plankton) are nutrient-rich and indistinguishable in C:N:P stoichiometry. I n both lakes and terrestrial systems, herbivores should have low growth eff iciencies (10-30%) when consuming autotrophs with typical carbon-to-nutrien t ratios. These stoichiometric constraints on herbivore growth appear to be qualitatively similar and widespread in both environments.