In the summer of 2011, the Willamette Partnership organized a series of training sessions on how to calculate the number of credits a particular environmental restoration project should receive. During this workshop, two wetland ecologists engaged in a protracted debate about whether beavers were present on a site, and if so, how likely they were to prevent certain species of trees from growing. While this was only a training session, it was obvious that different answers to these questions would profoundly impact the number of credits a restoration project on this site could generate. Tree growth is the key determinant in whether water-cooling shadows are produced, but the Shadea-a-Lator does not include the presence of beavers in its calculation. Even a single pair of beavers can quickly reduce the number of standing trees, and therefore the amount of shade, in a riparian zone.

This conversation highlights common uncertainties associated with environmental restoration work, but it also reveals a more specific problem the developers of spreadsheets like the Shade-a-Lator have to overcome. For an ESM to function, its proponents have to produce compelling evidence that newly created ecosystem services in one place are equivalent to or greater than a negative environmental impact elsewhere. Each effort to create an ESM therefore needs to develop or select a way to calculate the number of credits a landowner receives for a restoration project. Relevant tools include spreadsheet-based calculators, but also elaborate rules about site visits to visually confirm model outcomes and training programs to ensure that capable experts conduct those site visits. I use the phrase “measurement systems” to describe this complex of models, protocols and practices, which together produce the quantity of credits a participant in an ESM needs to buy or is able to sell. These measurement systems are at the heart of the second challenge for ESMs, namely that of producing equivalence.

It is easy to disagree about a calculation of how many trees need to be planted to compensate for a specific quantity of discharged warm water in a different location. Two experts can argue, at length, about whether beavers are present on a site, and how their presence would affect the survival rate of newly planted trees.