Statistical decomposition of passive and active phenotypic plasticity in traits under homeostatic regulation

Phenotypic plasticity enables organisms to maintain an optimal phenotype in the face of environmental change. Phenotypic plasticity is often viewed as being either passive or active. Yet many traits exhibit what we term mixed plasticity, where both passive effects of the environment and active physiological regulation shape trait dynamics. Indeed, this is a key characteristic of traits that are under homeostatic regulation, where organisms attempt to reverse environmental disturbance of the phenotype. Here, we develop a novel model that decomposes the temporal dynamics of homeostatic or mixed plasticity traits into parameters that describe the passive effect of the environment, the rate at which active regulation is adjusted (i.e., rate of plasticity), and the asymptotic magnitude of active regulation (i.e., capacity for plasticity). We apply this model to a dataset comprising 653 experiments documenting time-course changes in ion concentrations and osmolality in aquatic organisms following salinity shifts.