I reject the idea that that shows a "need" for slavery at all. In fact Scandinavia is a counter-example - as you actually point out!!! The small population was the result of the land's low carrying capacity with the agricultural techniques the Scandinavians used. Bringing in more people did not make the land more efficiently used - on the contrary, it created more mouths to feed on poor land. The slaves were brought in not at all out of need, but entirely out of greed - greed for their free labour, and from the fact that the Scandinavians could, and did, underfeed them and kill them when they weren't helpful, which wasn't as viable with other Scandinavians.
Let's be clear: the Vikings, glorified wildly by history, were violent, greedy people who operated by stealing from others, and helped set the stage for the worst excesses of later colonialism. Indeed with were "colonizers" themselves, and of a very unpleasant kind. They didn't need slaves any more than they "needed" the gold from Lindesfarne or wherever. Honestly you can make a better moral/ethical (or rational) case of justifying their colonialism in the British Isles (due to the crapness of the land they inhabited) than you can their slaving.