Reallocation is a technique applied to natural ecosystems that have reached the irreversible threshold (degraded or desertified ecosystems) at which restoration or rehabilitation cannot rebuild the original ecosystem. This technique is based on the introduction of allochthonous species, and generally leads to a new ecosystem different from the pre-existing one, known as a "constructed ecosystem". In the arid and desert zones of Tunisia, under the arid Mediterranean and Saharan bioclimate, the degradation of natural ecosystems has reached the irreversibility threshold, and restoration or rehabilitation techniques can no longer return these ecosystems to their initial state. The bioclimatic limit for successful rehabilitation of natural ecosystems in Tunisia's arid and desert zones is the lower understorey of the Mediterranean arid bioclimate. The climatic limit is an annual rainfall of at least 150 mm.