Where does occupancy collide?

In pharmacology, occupancy is the fraction of a tissue's receptors that are bound by a drug or ligand at a given concentration, the central quantity of classical receptor theory. Clark's 1926 The reaction between acetyl choline and muscle cells argued that response is a graded function of how many receptors a drug occupies, following a law-of-mass-action binding curve, and Nickerson's 1956 Receptor occupancy and tissue response showed the link between occupancy and effect is rarely one-to-one. The referent is a proportion of bound binding sites, from zero to one, fixed by drug concentration and binding affinity, and it is a different thing from a filled quantum state or an inhabited habitat patch.[1][2]

Unit

The fraction of receptors bound, from zero to full occupancy, at a given ligand concentration

Recovery means

Not applicable

Measured by

Estimated from binding curves (radioligand or functional assays) and the dissociation constant, or in vivo by receptor-occupancy imaging such as PET

In ecology, site occupancy is the probability that a given site or patch is occupied by a species, a core parameter of distribution and metapopulation studies. MacKenzie and colleagues' 2002 Estimating site occupancy rates when detection probabilities are less than one showed how to estimate it from repeated surveys while correcting for the fact that a present species is often missed, so raw presence counts understate true occupancy. The referent is a proportion of sites holding the species, a probability between zero and one estimated from detection histories, which shares the filled-fraction image of bound receptors or filled quantum states but is about organisms on a landscape.[3]

Unit

The probability (proportion of sites) that a species is present

Recovery means

Not applicable

Measured by

Estimated from repeated-visit detection/non-detection data with occupancy models that separate true presence from detection probability