"While human experts may be able to rationally defend drawing a particular borderline, it may be difficult for them to describe or elucidate the method that they used to place it at that precise location. An inability to fully circumscribe the model being used means that drawing the last ecological region border does little to help with the placement of the next, particularly if the same human expert is not available."

Hargrove and Hoffman (2004), Potential of Multivariate Quantitative Methods for Delineation and Visualization of Ecoregions. Environmental Management Vol. 34, Supplement 1, special issue on Ecoregionalization