What is the cause of the decline of the Greek city-states in the Northern Black Sea region?

Firstly, the decline of the Hellenic settlements is associated with the devastating wars in which they were drawn because of the invasions of the barbarians and the Roman Empire. This resulted in significant land loss. Secondly, arable farming decreases, due to the depletion of land suitable for cultivation. And this led to a decrease in population, which negatively affected the development of cities. Third, the destruction of trade ties with the Mediterranean.