A Conservation Interest Index (CI) was designed to numerically assess the natural quality or value of a given terrestrial area. This CI has been applied along the Golako River Watershed (Biosphere Reserve, Basque Country, Spain). The area, although benefiting from some protection, is strongly influenced by human activities (forestry and cattle breeding). The CI is based on both available cartographic information about vegetation and fieldwork, the later needed to provide estimations for the various descriptors included in this index: in this way, a particular vegetation fragment received a final score on a scale from 0 to 1000. A set of 9 Vegetation Units has been defined to analyze the current vegetation profile and, a ten plot (500 x 500 m) uniformly distributed sampling design has been implemented. Landscape homogenization is high where main land use relies on timbering, contrasting with the more heterogeneous and fragmented profile related to rural activities. At a height of more than 150 m forest appears to be dominant while human occupation becomes patchy, whereas abruptness restraints farms to locations below 100 m. Concerning the index performance, gradual differences have been displayed by the forest, which appears as the only vegetation unit attaining values above 500 (50% in the index scale), mature forest ranking highest (860), followed closely by the riparian forest. We have developed a formula to translate environmental value into economic benefit to promote conservation work at private property level, imitating the initial work undergone in Central American countries, where environmental services are rewarded irrespective of their conservation status.