Fick’s law describes many rate processes in environmental physics, including diffusive fluxes, conduction, and water flow. Many biological fluxes are biochemical rather than biophysical and require formulations other than Fick’s law. For example, the rate of photosynthesis increases with higher irradiance and higher CO2 concentration. The rate of carbon loss during respiration increases with higher temperature. Rates of plant productivity and soil organic matter decomposition vary with temperature, soil moisture, and other factors. Common formulations for these processes are the following: the Michaelis-Menten equation for a biochemical reaction; the Arrhenius equation to describe the temperature dependence of a biochemical reaction; minimum, multiplicative, and co-limiting rate multipliers; and first-order, linear differential equations to describe mass transfers within an ecosystem. In addition, mathematical principles of optimization provide a formal method to frame many ecological processes.