I've been reading a lot about the Viking invasion of England/Wessex. I was looking into the population at this time and it seems that the population was a lot lower then than it was during the Roman occupation... I've tried to find out why this is specifically but can't seem to find anything.
Populations contracted sharply just about everywhere in the western Mediterranean following the "fall" of the Roman system. Simply put, the Roman Empire was a globalized distribution economy that maximized production and distribution on a regional and supra-regional scale. Producers of fish sauce in Spain were sending their goods to Syria and the forts along the Danube frontier. Vegetables from Londinium were being crated and shipped across the Channel to markets in Gaul. Fine pottery from northern Italy was being purchased to be used as tableware in Sicca Veneria, in Tunisia. Grain from all over Gaul was being sent to Barbegal to be milled into flour on an industrial scale.
The majority of these long-distance connections contracted or disappeared in the centuries following the "fall." Pace to medievalists, I did not say all of them, and yes, some remnants of the old economy survived, on a much smaller scale. But for the majority of post-Roman populations, the outlook became hyper-local. Produce was sold and consumed within a few miles of its origin, and there was no longer any need for Farmer Otto to grow 3,000 pumpkins, because he could not sell that many and his local community could not eat that many. The means of distribution had broken down. The end result of this process is that larger populations cannot be sustained. Rome was receiving daily grain shipments at the port to feed its hundreds of thousands, and when the networks behind those shipments ceased to function, it was physically impossible for hundreds of thousands to continue to inhabit that small space among the seven hills on the banks of the Tiber. The population dispersed, died, and/or was not replenished due to lack of surplus. This happened at every population center in the Roman west, to a greater or lesser degree depending on the local (or in rare cases the regional) ability of the area to support itself.
For a good, relatively recent source for the end of Roman Britain, see A. S. Edmonde Cleary, The Ending of Roman Britain (London/NY: Routledge, 1989).
For a very basic (but generally good) survey of the "fall of the Roman west" (and why that is a problematic phrase), see Neil Christie, The fall of the Western Roman Empire : an archaeological and historical perspective (London/NY: Bloomsbury Academic, 2011).
How do you know that the population of England in the 11th century was lower than the population of Roman Britain?