When valuing ES we must remember that ecosystems are dynamic, nothing can be held constant (an important assumption in most economic models). Put simply, when one aspect of an ecosystem experiences a change, it can be felt throughout the entire system.
This is why aggregating the values of a system is more complicated than simply adding all of the values together. When different aspects of an ecosystem interact with each other, they may not influence each other in ways that are additive. Meaning that the total value of ES provided by an ecosystem is more than the sum of its parts; this total value is also impacted by the interactions of ES with other ES.Â
It is imperative that intra-ecosystem interactions are also measured and accounted for, and that researchers understand that changes in one aspect of a system can influence other aspects of the ecosystem in ways that are not linear. These interactions are complicated to model and require the expertise of an ecologist who understands the ecosystem to create interaction models, and an economist to assign value to them.