Kemal [Ataturk], because if they go to Athens they will overthrow everything". However, increasing grievances of the refugees caused some of the immigrants to shift their allegiance to the Communist Party and contributed to its increasing strength. The impoverished slum districts of Thessaloniki where the refugees were concentrated became strongholds of the Greek Communist Party in the Great Depression together with the rural areas of Macedonia where tobacco farming was the main industry. In May 1936, a strike of the tobacco farmers in Macedonia organized by the Communists led to a rebellion that saw the government lose control of Thessaloniki for a time. Prime Minister Ioannis Metaxas, with the support of the King, responded to the communists by establishing an authoritarian regime in 1936, the 4th of August Regime. In these ways, the population exchange indirectly facilitated changes in the political regimes of Greece and Turkey during the interwar period. Many immigrants died of epidemic illnesses during the voyage and brutal waiting for boats for transportation. The death rate during the immigration was four times higher than the birth rate. In the first years after arrival, the immigrants from Greece were inefficient in economic production, having only brought with them agricultural skills in tobacco production. This created considerable economic loss in Anatolia for the new Turkish republic. On the other hand, the Greek populations that left were skilled workers who engaged in transnational trade and business, as per previous capitulations policies of the Ottoman Empire. Effect on other ethnic populations While current scholarship defines the Greek-Turkish population exchange in terms of religious identity, the population exchange was much more complex than this. Indeed, the population exchange, embodied in the Convention Concerning the Exchange of Greek and Turkish Populations at the Lausanne Conference of January 30, 1923, was based on ethnic identity. The population exchange made it legally possible for both Turkey and Greece to cleanse their ethnic minorities in the formation of the nation-state. Nonetheless, religion was utilized as a legitimizing factor or a "safe criterion" in marking ethnic groups as Turkish or as Greek in the population exchange. As a result, the Greek-Turkish population exchange did exchange the Greek Orthodox population of Anatolia, Turkey and the Muslim population of Greece. However, due to the heterogeneous nature of these former Ottoman lands, many other ethnic groups posed social and legal challenges to the terms of the agreement for years after