Basically, we don't know precisely. Slave records are frustratingly rare, incomplete, and vague. I say this as someone who has been attempting to identify slaves belonging to one family in one decade in one state since January; I think we're up to six names (out of ninety or so slaves). For the most part, the documentation just ain't there to make quantitative comparisons.

In a broader sense, the mortality rate was higher than replacement levels during the first few decades of English slavery in Virginia, necessitating bringing in a continual stream of slaves to replenish those dead of disease and overwork, but it had significantly dropped off by the late 17th and early 18th centuries. I haven't read up on South Carolina in depth, but as the other major entry point for slaves into the colonial South, it probably experienced a similar trend. Certainly by 1808 - when the international slave trade was outlawed by the United States - slaves were reproducing at beyond a replacement rate, and with foreign importation cut off, a thriving domestic slave trade developed, whereby settlers in the Deep South (the cotton belt effectively extended from East Texas through Georgia) purchased excess slaves from the Upper South, where slave agriculture had been declining in profitability for decades, mostly due to falling tobacco prices as well as the diversification of agriculture and the economy more generally.

I don't want to get into the morality of slave ownership, but I will say that a paternal aspect was often present in slave owners, at least those I have studied. Many of them genuinely thought that they were looking out for the best interest of their slaves, whom they assumed to be incapable of caring for themselves. But purely from an economic standpoint, due to the constricted supply, slaves were generally quite expensive, and slave owners were, as a rule, intensely concerned with turning a profit. For one or both reasons, slave owners generally made a basic effort to keep slaves alive and moderately content. They bought clothes for them; they purchased their garden produce for small sums of cash; and they hired doctors when slaves were sick.