This strategy features in the framework of Agenda 21 as formulated at UNCED (Rio de Janeiro, 1992), now coordinated by the United Nations Commission on Sustainable Development and implemented through national and local authorities. Agenda 21 recommends undertaking country studies or other methods to: identify components of biological diversity important for its conservation and for sustainable use of biological resources; ascribe values to biological and genetic resources; identify processes and activities with significant impact on biological diversity; evaluate potential economic implications of conserving biological diversity and the sustainable use of biological and genetic resources; and suggest priority action. Caring for the Earth: A Strategy for Sustainable Living published in 1991, is a follow-up to the World Conservation Strategy. It formulates principles for sustainable living and the actions to put the principles into practice with the aim of integrating conservation and development. For conserving biological diversity every country should: establish a comprehensive national system of protected areas (with different categories), which is the core of other programmes that seek to maintain the diversity of ecosystems, species and wild genetic resources; for the diversity of species the protected-area system should ensure that protected areas are linked to other areas by corridors of suitable habitats along which species can disperse (Action 4.9); improve conservation of wild plants and animals (Action 4.10); improve knowledge and understanding of species and ecosystems (Action 4.11); use a combination of in situ and ex situ conservation to maintain species and genetic resources (Action 4.11); harvest wild resources sustainably (Action 4.13); support management of wild renewable resources by local communities, and increase incentives to conserve biodiversity (Action 4.14). The Strategy distinguishes 10 categories of protected areas, each providing distinctive benefits (Annex 4).