Scale should not be confused with hierarchy.
Hierarchical descriptions can provide useful ways of representing and investigating biological phenomena. But differences between molecular, cellular, physiological, organismal, ecological, or other domains do not by themselves establish a universal hierarchy of causal or explanatory levels.
Biological systems exhibit materially real differences of spatial and temporal extent, as well as part–whole, nested, and other organisational relations. These relations belong to the biological organisation being investigated. They should not be converted into independently acting scales.
In APS, Scale is the analytic projection concerned with where persistence is organised across spatial and temporal extents.
Scale therefore identifies the extent required for a specified explanation. It does not itself organise, coordinate, couple, constrain, or cause biological activity.
Scale should also be distinguished from resolution.
Scale concerns the spatial and temporal extent relevant to the organisation being explained.
Resolution concerns the granularity at which that organisation is described, measured, modelled, or analysed.
A change in resolution does not necessarily change the explanatory extent required by the explanandum, and a broader explanatory extent does not automatically require a coarser description.
APS therefore does not require biological explanation to move across multiple scales. For some explananda, a relatively bounded spatial and temporal extent may be sufficient. For others, materially realised processes, constraints, interactions, or organisational relations extending beyond a narrower boundary may make an additional explanatory difference.
A broader spatial or temporal extent is required only in the latter case.
Key Point: Scale is not a hierarchy of biological levels. It is the analytic projection that identifies the spatial and temporal extent relevant to explaining organised persistence. Differences of extent do not by themselves establish causal rank, explanatory priority, or a requirement for multiextent explanation.