event in 540. Michael Jones suggests that declining agricultural production from land that was already fully exploited had considerable demographic consequences. Population fluctuation Theories for reduction Background Slaves were important in the economy and the army in the Roman Empire. Estimates for the prevalence of slavery in the Roman Empire vary: some estimate that around 30% of the population of the Empire in the 1st century was enslaved. A more recent study suggests 10-15% even for the early empire "as any greater estimate would require implausible levels of transformation in a pre-modern context." The difference the lower percentage in the Later Roman Empire can be attributed to fewer slaves in sub-elite households and agricultural estates (replaced by a great expansion in various types of tenancy). The Germanic region was one of the main sources of slaves. It was mainly wholesale dealers, who followed the Roman armies, who sold slaves. After the Empire expanded, there were fewer places to obtain slaves. Around 210, piracy increased around the North Sea and boosted the supply, taken from villages in that area, along with those captured for ransom. Britain was not easily defensible. It did not pay completely the costs of occupation. Nevertheless, the Romans were forced to keep three or four legions, 30,000 to 40,000 men with auxiliary units in place to defend it. They managed fairly well until the collapse of Roman authority after the garrison was reduced in size by Magnus Maximus in 388 and Stilicho in 401. It seems that after 350 the Roman government was having more difficulty in recruiting soldiers. In an effort to remedy the situation it resorted to payment instead of provision of recruit, the aurum tironicum. Landowners could pay a set fee to prevent any of their tenants from being pressed into the army (slaves were rarely resorted to even at critical moments in exchange for their freedom). Not enough men wanted to enter military service. The gold from the tax led to a greater use of Germanic or other tribal groups who did not need to be expensively equipped, housed, and paid pensions, as the tax was used to recruit mercenaries as foederati, but it also drained the treasury. Previously foreigners were put into units, the auxilia, officered by Romans. Roman army units, the smaller Later Roman legions, continued to exist but gradually disappeared in the 5th century leaving defense of the Empire to