Crossing the thresholds: human ecology and historical patterns of landscape degradation

in discussions of landscape sensitivity, human activities have generally be en regarded as external forces contributing to landscape change, with a foc us on the impacts of cultivation methods, fertiliser practices, grazing pre ssures and atmospheric pollution. However, there has been comparatively lit tle study undertaken that integrates physical and social systems in a histo ric context to explain the basis of human activity in sensitive landscapes. Where such attempts have been made, the manner of common land management h as figured prominently, with 'tragedy of the commons' concepts used to expl ain land degradation and to provide a foundation for policy response. This has also been the case in Southern Iceland and in this paper we assess the extent to which common land domestic grazing pressures were the primary ext ernal force causing soil erosion and land degradation during the period of occupation from ca. 874 AD. We first provide field observation of soil eros ion, temporally defined by tephrochronology, to highlight the extent of lan d degradation during this period. The 'tragedy of the commons' explanation of degradation is then assessed by evaluating historic documentary sources, and by environmental reconstruction and modeling of historic grazing press ures. These analyses indicate that regulatory mechanisms were in place to p revent overgrazing from at least the 1200s AD and suggest that there was su fficient biomass to support the numbers of domestic livestock indicated fro m historic sources. We suggest that failure to remove domestic livestock be fore the end of the growing season and an absence of shepherding were more likely to contribute to land degradation than absolute numbers. Lack of app ropriate regulation of domestic livestock on common grazing areas can be at tributed to limited cultural knowledge of changing and rapidly fluctuating environmental conditions. (C) 2001 Elsevier Science B.V. All rights reserve d.