and Danube frontiers, raids by barbarians killed Romans and disrupted commerce. Raids also forced Romans into walled towns and cities furthering the spread of pathogens and increasing the rate of depopulation in the West. A low population and weak economy forced Rome to use barbarians in the Roman Army to defend against other barbarians. Environmental degradation Another theory is that gradual environmental degradation caused population and economic decline. Deforestation and excessive grazing led to erosion of meadows and cropland. Increased irrigation without suitable drainage caused salinization, especially in North Africa. These human activities resulted in fertile land becoming nonproductive and eventually increased desertification in some regions. Many animal species became extinct. The recent research of Tainter stated that "deforestation did not cause the Roman collapse", although it could be a minor contributing factor. Also, high taxes and heavy slavery are another reason for decline, as they forced small farmers out of business and into the cities, which became overpopulated. Roman cities were only designed to hold a certain number of people, and once they passed that, disease, water shortage and food shortage became common. Lead poisoning Publishing several articles in the 1960s, the sociologist S. Colum Gilfillan advanced the argument that lead poisoning was a significant factor in the decline of the Roman Empire. Later, a posthumously published book elaborated on Gilfillan's work on this topic. Jerome Nriagu, a geochemist, argued in a 1983 book that "lead poisoning contributed to the decline of the Roman empire." His work centred on the level to which the ancient Romans, who had few sweeteners besides honey, would boil must in lead pots to produce a reduced sugar syrup called defrutum, concentrated again into sapa. This syrup was used to some degree to sweeten wine and food. If acidic must is boiled within lead vessels the sweet syrup it yields will contain a quantity of Pb(C2H3O2)2 or lead(II) acetate. Lead was also leached from the glazes on amphorae and other pottery, from pewter drinking vessels and cookware, and from lead piping used for municipal water supplies and baths. The role and importance of lead poisoning in contributing to the fall of the Roman Empire is the subject of controversy, and its importance and validity is discounted by many historians. John Scarborough, a pharmacologist and classicist, criticized Nriagu's book as "so full of false evidence, miscitations, typographical errors, and a blatant flippancy regarding primary sources