Climate and Collapse: Agro-Ecological Perspectives on the Decline of the Tiwanaku State

After approximately 700 years of growth and colonial expansion, the Tiwanaku state disintegrated as a regional political force in the south-central Andes between c. AD 1000-1100. This paper examines the collapse of the state of Tiwanaku through the lens of its agricultural history. We argue that the proximate cause of Tiwanaku's decline as a politically integrated, expansive state society was the deterioration and ultimate abandonment of its regional-scale agricultural systems, both in its core area in the Andean altiplano and in its economic colonies in the lower-altitude yungas zones. We present evidence that the collapse of Tiwanaku intensive agriculture was triggered by regional change in climatic conditions recorded in highly-resolved palaeoenvironmental data derived from the Quelccaya ice cap of southern Peru and in sediment cores from Lake Titicaca. Our analysis of the Quelccaya data documents a radical climate change in the south-central Andes during the post-AD 1000 era in the form of a statistically significant decrease in mean precipitation level that persisted until c. AD 1400. By defining vulnerability classes for various agricultural technologies used in different regions of the Tiwanaku state, we then relate this palaeoclimatic data to performance of specific core area and colonial Tiwanaku agricultural systems. We demonstrate that chronic drought conditions led to sequential collapse of these distinct agricultural systems: irrigation-based agriculture in Tiwanaku colonies at lower altitude failed first, followed by groundwater-dependent raised-field systems in the altiplano. Agricultural and settlement pattern changes in the post-AD 1000 period are correlated with the palaeoenvironmental data to present an integrated view of the coupling of climate and cultural process. The full implications of the agro-ecological collapse model presented here reach well beyond an explanation for the decline of Tiwanaku alone.