Sustainability is customarily looked upon as a socially and environmentally responsible action to achieve goals of human well-being and environmental health. Human well-being is predicated on sustaining environmental services provided by species within ecosystems. Many services derive from functional interdependences among species. Sustainability thus requires sound stewardship that balances trade-offs between exploitation of species and the services they provide vs. protection of species' functional interdependences. How such stewardship comes about, however, depends on the conception of a sustainable system and the definition of sustainability. Using conceptual thinking from the ecosystem sciences, this chapter elaborates on the minimal ingredients needed for a system to be sustainable. This discussion is followed by elaboration of three definitions of sustainability: persistence, reliability, and resilience. This chapter shows how the different definitions lead to different conclusions about sustainability and even raises the possibility that goals of human well-being and environmental health cannot be achieved under certain conditions of system sustainability.
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