Interaction with the Balkan populationPrior to the advent of Roman rule, a number of native or autochthonous populations had lived in the Balkans since ancient times. South of the Jireček line were the Greeks. To the north, there were Illyrians in the western portion (Illyricum), Thracians in Thrace (modern Bulgaria and eastern Macedonia), and Dacians in Moesia (northern Bulgaria and northeastern Serbia) and Dacia (modern Romania). They were mainly tribalistic and generally lacked awareness of any greater ethno-political affiliations. Over the classical ages, they were at times invaded, conquered and influenced by Celts, Greeks and Romans. Roman influence, however, was initially limited to cities later concentrated along the Dalmatian coast, later spreading to a few scattered cities inside the Balkan interior particularly along the river Danube (Sirmium, Belgrade, Niš). Roman citizens from throughout the empire settled in these cities and in the adjacent countryside. The vast hinterland was still populated by indigenous peoples who likely retained their own tribalistic character.
Following the fall of Rome and numerous barbarian raids, the population in the Balkans dropped, as did commerce and general standards of living. Many people were killed, or taken prisoner by invaders. This demographic decline was particularly attributed to a drop in the number of indigenous peasants living in rural areas. They were the most vulnerable to raids and were also hardest hit by the financial crises that plagued the falling empire. However, the Balkans were not desolate; considerable numbers of indigenous people simply remained. Only certain areas tended to be affected by the raids (e.g. lands around major land routes, such as the Morava corridor).[
The pre-Slavic inhabitants sought refuge inside fortified cities and islands, whilst others fled to remote mountains and forests, joining their non-Romanized kin and adopting a transhumant pastoral lifestyle. The larger cities were able to persevere, even flourish, through the hard times. Archaeological evidence suggests that the culture in the cities changed whereby Roman-style forums and large public buildings were abandoned and cities were modified (i.e. built on top of hills or cliff-tops and fortified by walls). The centerpiece of such cities was the church. This transformation from a Roman culture to a Byzantine culture was paralleled by a rise of a new ruling class: the old land-owning aristocracy gave way to rule by military elites and the clergy.
In addition to the autochthons, there were remnants of previous invaders such as "Huns" 
and various Germanic peoples when the Slavs arrived. Sarmatian tribes (such as the Iazyges) are recorded to have still lived in the Banat region of the Danube.
As the Slavs spread south into the Balkans, they interacted with the numerous peoples and cultures already there. Since their lifestyle revolved around agriculture, they preferentially settled rural lands along the major highway networks which they moved along. Whilst they could not take the larger fortified towns, they looted the countryside and captured many prisoners. In his Strategikon, Pseudo-Maurice noted that it was commonplace for Slavs to accept newly acquired prisoners into their ranks. Despite Byzantine accounts of "pillaging" and "looting", it is possible that
many indigenous peoples voluntarily assimilated with the Slavs.
The Slavs lacked an organised, centrally ruled organisation which actually hastened the process of willful Slavicisation.
The strongest
evidence for such a co-existence is from archaeological remains along the Danube and Dacia known as the Ipoteşti-Cândeşti culture. Here, the villages dating back to the 6th century represent a continuity with the earlier Slavic Pen'kovka culture; modified by
admixture with Daco-Getic, Daco-Roman and/or Byzantine elements within the same village. Such interactions awarded the pre-Slavic populace protection within the ranks of a dominant, new tribe. In return, they contributed to the genetic and cultural development the South Slavs. This phenomenon ultimately led to an exchange of various loan-words. For example, the Slavic name for "Greeks", Grci, is derived from the Latin Graecus presumably encountered through the local Romanised populace. Conversely, the
Vlachs borrowed many Slavic words, especially pertaining to agricultural terms. Whether any of the original Thracian or Illyrian culture and language remained by the time Slavs arrived is a matter of debate. It is a difficult issue to analyse because of the overriding Greek and Roman influence in the region. However,
what is certain is that the Thracian and Illyrian identities disappear from history during this period.
Over time, due to the larger number of Slavs, the descendants of most of the indigenous populations of the Balkans were Slavicized,
an exception being Greece,
where the smaller number Slavs scattered there came to be Hellenized over succeeding centuries (aided in time by more Greeks returning to Greece in the 9th century and the role of the church and administration). The Romance speakers within the fortified Dalmatian cities managed to retain their culture and language for a long time, as Dalmatian Romance was spoken until the high Middle Ages. However, they too were eventually assimilated into the body of Slavs.
In contrast, the Romano-Dacians in Wallachia managed to maintain their Latin-based language, despite much Slavic influence. After centuries of peaceful co-existence, the groups fused to form the Romanians.
https://en.wikipedia.org/wiki/South_Sla ... population 
