6. Conclusion
The increase of human population and its consumption of animal products push livestock production systems toward more complex and intensive use of both human and environmental resources. The threat to livestock systems can be understood in terms of increasing scarcity of these resources (resource sufficiency) or in terms of the increasing fragility of systems interactions (functional integrity). In either case, the achievement of sustainable animal production becomes more pressing every year. This urgency notwithstanding, there are still divergent opinions about how to define and research sustainability. Animal science should develop more in depth approaches to the issue of sustainability in strict co-operation with other disciplines.