although nrps may lack enforcement power defining such goals and tools should certainly bring in more coherence and help nrps achieve their complex goals a wide stream of this literature emphasises the key role of institutions in the efficiency of collective action ostrom underlines that the effects of the size of the protected area and the heterogeneity of appropriators on the outcomes of the protected area are unclear there is a need to evaluate nrps on these important elements to know how they could be improved natural regional parks and environmental zoning cont the stated goals of a natural park may be conflictual to specific stakeholders interests but they may also be contradictory or at least mitigated by other policies such as local uiban planning instruments or other environmental measures