Protected areas cannot be unlimited, being thus necessary to rationalise their constitution. For a long time nature conservation was based on the protection of outstanding endangered species. Later on, the interest expanded to the remaining organisms (which status was generally unknown) and to the entire ecosystem processes. The application of the 'representativity' criterion is an emergency solution to safeguard insufficiently known organisms (the great majority). However, the application of this representatively criterion requires an integrated typification of ecosystems. Some examples applied to the case of the steppe wetlands are suggested here.