|Modified State Factors and Interactive Controls Diagram to account for external supplies of resources.|
Terry Chapin and others modified the state factors approach to understanding ecosystems to include interactive controls of ecosystem properties. Interactive factors are not independent of the ecosystem properties, but sit somewhere in between. For example, the macroclimate is independent of what species are present at a site, but the microclimate can be influenced by species composition. These interactive controls include disturbances, microclimate, resource availability, and species composition.
With these state factors and interactive controls, one can have a better framework for understanding how ecosystems are structured and function. For example, the amount of biomass in a forest is not just a function of the state factors, but is influenced by interactive controls such as disturbance and the species that are present in a stand, which is influenced but not determined solely by state factors.
The state factor/interactive control approach is a big improvement in our conceptual framework, but it's generally left out external supplies of resources. Atmospheric CO2 concentrations, dust inputs, and nitrogen deposition can have profound influences on ecosystem properties, but are not adequately included in the current conceptual framework. These factors are somewhat influenced by ecosystem properties. The amount of dry deposition is influenced by canopy structure, for example. Yet, it's probably better to consider these state factors. N deposition plumes are wide and more similar to climate than disturbance regimes.
It's a minor tweak in many respects, but a likely necessary improvement to a long-tenured conceptual framework.