Individual growth models support the quantification of isotope incorporation rate, trophic discrimination and their interactions

Two large but independent bodies of literature exist on two essential components of the dynamics of isotopic incorporation: the isotopic incorporation rate (λ) and the trophic discrimination factor (Δ). Understanding the magnitude of these two parameters and the factors that shape them is fundamental to interpret the results of ecological studies that rely on stable isotopes. λ scales allometrically with body mass among species and depends on growth within species. Both are often assumed to be constant and independent of each other but evidence accumulates that might be linked and to vary with growth. We built and analyzed a model (IsoDyn) that connects individual growth and isotopic incorporation of nitrogen into whole body and muscle tissues. The model can assume a variety of individual growth patterns including exponential or asymptotic growths. λ depends on the rate of body mass gains which scales allometrically with body mass. Δ is a dynamic response variable that depends partly on the ratio between fluxes of gains and losses and covaries negatively with λ. The model can be parameterized either from existing large databases of animal growth models or directly from experimental results. The model was applied to experimental results on three ectotherms and one endotherm and compared to the results of the simpler and widely used time model. IsoDyn model gave a better fit with relatively little calibration. IsoDyn clarifies and expands the interpretation of isotopic incorporation data. ### Competing Interest Statement The authors have declared no competing interest.