Four thousand years of environmental change and human activity in the Cochabamba Basin, Bolivia Joseph J. Williams a, ⁎, William D. Gosling a , Angela L. Coe a , Stephen J. Brooks b , Pauline Gulliver c a Department of Earth and Environmental Sciences, Centre for Earth, Planetary, Space and Astronomical Research (CEPSAR), The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK b Department of Entomology, Natural History Museum, Cromwell Road, London, SW7 5BD, UK c NERC Radiocarbon Laboratory, Scottish Enterprise Technology Park, East Kilbride, Glasgow, G75 0QF, UK abstract article info Article history: Received 26 April 2010 Available online 17 April 2011 Keywords: Charcoal El Niño/Southern Oscillation Fire Fossil pollen Holocene Human impact Inca Polylepis Sporormiella Tiwanaku The Cochabamba Basin (Bolivia) is on the ancient road network connecting Andean and lowland areas. Little is known about the longevity of this trade route or how people responded to past environmental changes. The eastern end of the Cochabamba valley system constricts at the Vacas Lake District, constraining the road network and providing an ideal location in which to examine past human–environmental interactions. Multi- proxy analysis of sediment from Lake Challacaba has allowed a c. 4000 year environmental history to be reconstructed. Fluctuations in drought tolerant pollen taxa and calcium carbonate indicate two periods of reduced moisture availability (c. 4000–3370 and c. 2190–1020 cal yr BP) compared to adjacent wetter episodes (c. 3370–2190 and c. 1020 cal yr BP–present). The moisture fluctuations broadly correlate to El Niño/ Southern Oscillation variations reported elsewhere. High charcoal abundance from c. 4000 to 2000 yr ago indicates continuous use of the ancient road network. A decline in charcoal and an increase in dung fungus (Sporormiella) c. 1340–1210 cal yr BP, suggests that cultural changes were a major factor in shaping the modern landscape. Despite undisputable impacts of human populations on the Polylepis woodlands today, we see no evidence of woodland clearance in the Challacaba record. © 2011 University of Washington. Published by Elsevier Inc. All rights reserved. Introduction The highly diverse ecosystems of the elevated tropical Andes are vulnerable to change from increasing human occupation of the area and predicted future climate change (Cincotta et al., 2000; Myers et al., 2000; Malcolm et al., 2006). To develop an effective policy to conserve and manage this ecosystem, a robust understanding of the region's natural history is required (Godwin, 1956; Willis and Birks, 2006). Today, uncultivated sections of the high Central Andes are principally grasslands within which are pockets of woodlands dominated by the tree genus Polylepis (Rosaceae). Kessler (2002) has implied that the relationship between grassland and woodland ecosystems is strongly influenced by human practices and an intensive land use at high elevations has severely limited Polylepis woodland distribution today (Ellenberg, 1958; Fjeldså and Kessler, 1996). However, the nature and degree to which human factors ultimately govern the ecology of this region remains unclear and recent research has highlighted the vulnerability of these Andean woodlands solely to climatic lead changes (Gosling et al., 2009). The main Cochabamba Basin (eastern Andes, Bolivia; Fig. 1) is, today, a major region for food production. The topography favors agriculture because it is: i) on an easily accessible trade route between upland and lowland areas and, ii) provides a flat area of land upon fertile Quaternary deposits, that largely experience a temperate seasonal climate. Limited archeological evidence from the region suggests a long human history around Cochabamba with agricultural practices from at least 3000 yr before the present (cal yr BP) (Hensen, 2002). By the time of the Tiwanaku state (c. 1500–900 cal yr BP), the warm fertile main Cochabamba Valley had become a center for farming, and frontier for trade into the subtropical Mizque Valley (Higueras, 1996; Janusek, 2008). The collapse of the Tiwanaku state, around 1000–900 cal yr BP, has been linked to a prolonged drought on the Altiplano as indicated by the low water levels of Lake Titicaca (Abbott et al., 1997; Binford et al., 1997) and an elevated dust signal from the nearby Quelccaya (Peru) ice core record (Thompson et al., 1985). Elsewhere in the eastern Central Andes (i.e. Marcacocha), the impacts of human occupation on the landscape, identified from lake sediment records, are correlated to changes in agricultural practices, herbivore domestication and cultural adjustments over the last 4000 yr (Chepstow-Lusty et al., 1998, 2007). Regardless of whether climatic events were coupled with a social transformation (Janusek, 2008), the transition from Tiwanaku fragmentation and collapse (1000–900 cal yr BP), towards rule by the Inca Empire (c. 650–550 cal yr BP, 1400–1533 AD), is certain to Quaternary Research 76 (2011) 58–68 ⁎ Corresponding author. Fax: + 44 1908 655151. E-mail address: J.J.Williams@open.ac.uk (J.J. Williams). 0033-5894/$ – see front matter © 2011 University of Washington. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.yqres.2011.03.004 Contents lists available at ScienceDirect Quaternary Research journal homepage: www.elsevier.com/locate/yqres