ADDENDUM. It was recently drawn to our attention that during the production of our paper another publication was concurrently being completed detailing concepts that closely correspond with our own. It was an unintentional oversight that conference proceedings in which S. Cristancho and J. Vining discussed this concept were not included in our paper. The publication, "Culturally Defined Keystone Species", which synthesizes this work was published in Human Ecology Review (Vol. 11, No.2, 2004).