As suggested by Arun Agrawal and Nicolas Perrin, depending on how risks are reduced or avoided, five classes of adaptation strategies can be identified. Mobility avoids risks across space. Storage reduces risks experienced over time. Diversification reduces risks across assets owned by households or collectives. Communal pooling involves joint ownership of assets and resources; sharing of wealth, labor, or incomes from particular activities across households or mobilization and use of resources held collectively during time of scarcity. Exchange can substitute for the first four classes of adaptation strategies.