Abstract
The recycling of nutrients by animals is important at many levels of ecological organization. At the level of the individual, the rates at which animals recycle nutrients (egestion of solids and excretion of dissolved molecules) are important because they can be used to explore theories related to Metabolic Ecology (Gillooly et al. 2001; Brown et al. 2004) and Ecological Stoichiometry (ES, Sterner and Elser 2002). Metabolic Ecology predicts that biological rates of individuals (for example, nutrient excretion rates) are power functions of body size (Gillooly et al. 2001). Body size dependence is usually captured in the formula B = B0Mb, where B is individual metabolic rate (e.g., oxygen consumed, or nitrogen excreted, per individual per unit time), B0 is a ‘normalization constant,’ M is organism body mass, and b is the ‘scaling coefficient.’ A prominent version of ME, the Metabolic Theory Ecology (MTE), predicts that b is ~0.75 for most biological rates (West et al. 1997; Gillooly et al. 2001), although there is controversy regarding the theory and empirical evidence for this value (White and Seymour 2005; Glazier 2010; Isaac and Carbone 2010). The MTE framework also recognizes the importance of temperature; most biological rates increase exponentially with temperature over most of the thermal tolerance range of an organism (Gillooly et al. 2001, Clarke 2004).
Ecological Stoichiometry theory (ES) predicts that nutrient excretion rates and ratios of consumers are functions of the imbalance between the nutrient content of the organism’s body versus that of its food source. ES usually focuses on nitrogen (N) and phosphorus (P), and their ratio (N:P; Sterner and Elser 2002). For example, ES predicts that an animal with a high concentration of P in its body (i.e., low body N:P) will sequester more dietary P to grow, compared to a counterpart with low body P (high body N:P). As a consequence, the animal with low body N:P will release wastes at a higher N:P than its counterpart with high body N:P. More generally, across individuals or species, body N:P and waste N:P should be negative correlated (Sterner 1990; Sterner and Elser 2002). Differences among animals in body N:P are often driven by differences in the allocation of P-rich structures such as RNA and bone (Elser et al. 1996; Vanni et al. 2002). ES also recognizes the importance of dietary nutrients in driving nutrient excretion rates and ratios; specifically, consumers whose diet is rich in a particular element should release that element at higher rates than a counterpart consuming a diet that is deficient in that element, given similar body elemental compositions (Sterner 1990; Sterner and Elser 2002). Thus, ES also predicts that food N:P will be positively correlated with the N:P of wastes.
Animals can be important agents of nutrient cycling at the ecosystem level. In some ecosystems, the release of potentially limiting nutrients by animals can sustain a substantial proportion of primary production (McNaughton et al. 1997; Vanni 2002; McIntyre et al. 2008). However, the importance of animals in nutrient cycling varies greatly among species and ecosystems (Vanni 2002). For example, across aquatic ecosystems, nutrient excretion by animal assemblages can support anywhere from <5% to >80% of algal primary production, depending on the ecosystem. Furthermore, the relative importance of different animal groups (e.g., zooplankton, benthic invertebrates, fish) varies greatly among ecosystems (Taylor et al. 2015).
Given the potential ecosystem-level importance of animal-mediated nutrient cycling, and the potential value of data on animal nutrient recycling rates for testing predictions of Metabolic Ecology and Ecological Stoichiometry, a comprehensive compilation of animal nutrient excretion rates can be of great use to the ecological community. The number of studies of animal mediated nutrient cycling has increased greatly in recent decades (Taylor et al. 2015). However, a comprehensive compilation of excretion rates has not been available. Here, we combine published and unpublished data to create a dataset that includes 10,534 observations of N or P excretion rates of animals from freshwater and marine ecosystems worldwide. This data set was used recently to test predictions of MTE and ES, as described in Vanni and McIntyre (2016).