Animal stoichiometry influences critical processes from organismal physiology to biogeochemical cycles. However, it remains uncertain whether animal stoichiometry follows predictable scaling relationships with body mass and whether adaptation to terrestrial or aquatic environments constrains elemental allocation. We tested both interspecific and intraspecific body-mass scaling relationships for nitrogen (N), phosphorus (P), and N:P content using a subset of the StoichLife database, which includes 9,933 individual animals across 1,543 species spanning 10 orders of magnitude in body mass from terrestrial, freshwater, and marine realms. Our results show that body mass predicts intraspecific stoichiometric variation, accounting for 42-45% of the variation in 27% of vertebrate and 35% of invertebrate species. However, body mass was less effective at explaining interspecific variation, with taxonomic identity emerging as a more significant factor. Differences between aquatic and terrestrial organisms were observed only in invertebrate interspecific %N, suggesting that realm has a relatively minor influence on elemental allocation. Our study, based on the most comprehensive animal stoichiometry database to date, revealed that while body mass is a good predictor of intraspecific elemental content, it is less effective for interspecific patterns. This highlights the importance of evolutionary history and taxonomic identity over general scaling laws in explaining stoichiometric variation.

Primary consumers in aquatic ecosystems are frequently limited by the quality of their food, often expressed as phytoplankton elemental and biochemical composition. Effects of these food quality indicators vary across studies, and the relative importance of elemental (nitrogen and phosphorus) versus biochemical (fatty acid and sterol) limitation in aquatic food webs has been debated. Here we present results of a meta-analysis using >100 experimental studies, which confirms that limitation by N, P, essential fatty acids, and sterols all have significant negative effects on zooplankton performance. However, effects varied by grazer response (growth versus reproduction), specific manipulation, and across taxa. P limitation had greater effects on zooplankton growth than fatty acids, but P and fatty acid limitation had equal effects on reproduction. Furthermore, we show that nutrient co-limitation in zooplankton occurs, that indirect effects induced by P limitation exceed direct effects of mineral P limitation, that effects of nutrient amendments using laboratory phytoplankton isolates exceed those using natural field communities, and that algal physiology mediates zooplankton responses to nutrient limitation. Our meta-analysis reconciles contrasting views about the role of various food quality indicators, and their interactions, for zooplankton performance, and provides a mechanistic understanding of how environmental change affects trophic transfer.