 Johns however concluded that "The hypothesis of an 'Abbasid agricultural revolution is challenging and may well prove useful".The historian Eliyahu Ashtor wrote in 1976 that agricultural production declined in the period immediately after the Arab conquest in areas of Mesopotamia and Egypt, on the limited basis of records of taxes collected on cultivated areas.In a 2012 paper focusing on the Sawād area of Iraq, Michele Campopiano concluded that Iraqi agricultural output declined in the 7th to 10th century; he attributed this decline to "competition of the different ruling groups to gain access to land surplus".