Social-ecological system theory draws upon concepts established within the discipline of ecology, and applies them to a more holistic view of a human-in-nature system. We incorporated the keystone concept into social-ecological system theory, and used the quantum co-evolution unit (QCU) to quantify biocultural elements as either keystone components or redundant components of social-ecological systems. This is done by identifying specific elements of biocultural diversity, and then determining dominance within biocultural functional groups. The “Hawaiian social-ecological system” was selected as the model of study to test this concept because it has been recognized as a model of human biocomplexity and social-ecological systems. Based on both quantified and qualified assessments, the conclusions of this research support the notion that taro cultivation is a keystone component of the Hawaiian social-ecological system. It further indicates that sweet potato cultivation was a successional social-ecological keystone in regions too arid to sustain large-scale taro cultivation, and thus facilitated the existence of an “alternative regime state” in the same social-ecological system. Such conclusions suggest that these biocultural practices should be a focal point of biocultural restoration efforts in the 21st century, many of which aim to restore cultural landscapes.