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A New Anthropogenic Bioindicator

Alex Chepstow-Lusty, from Montpellier's CBAE,1 has focused his research on past human activity. This has taken him to the Andes, where traditional burning is still used to prepare land for agriculture. But while he was analyzing some lake sediments looking for charcoal, he accidentally came across exoskeletons of oribatid mites. These tiny organisms eat vegetable detritus, including livestock excrement, a particularly rich food source. Given these eating habits, the abundance of oribatid mites can be used to indicate historical grazing patterns, as recent work by Chepstow-Lusty and co-workers shows.2,3 This makes them novel bioindicators for human activity, especially for periods lacking written records.

The scientists tested this hypothesis by analyzing sediments from a place that is particularly well-suited to the study, Lake Marcacocha, at an altitude of 3350 meters near Cuzco (Peru), the ancient capital of the Inca empire. “Ideally located, and surrounded by rich pastureland, it is adjacent to a traditional trade route which was being used by llama caravans during the time of the Inca empire,” Chepstow-Lusty explains. With his team, he dated the layers of sediment and counted the mite exoskeletons present. This process revealed the existence of four periods, which correspond to well-known social and economic changes. The first period, during which the mites were extremely abundant, was when the empire was at its peak, between the early 15th century and the arrival of the Spanish (1532). During the second period, a fall in the number of mites is observed, corresponding directly to the disappearance of the llama caravans, as well as most of the indigenous people. “Many diseases were imported, and historical documents record the death of nearly two thirds of the llamas in the Cuzco region in Peru,” adds Chepstow-Lusty. With the third period, the number of mites starts to increase again. At that time, under Spanish influence, the use of European livestock (sheep, cows, and horses) was becoming increasingly widespread. Finally, at the beginning of the 18th century, mite numbers fell once more as an epidemic struck the city of Cuzco: According to historical records, the herdsmen in the valley of Lake Marcacocha were wiped out, which led to the pastureland falling into disuse. “Mites appear to be a good indicator for herding and human activities in the environment,” Chepstow-Lusty concludes. The team then looked even further back in time, and observed that the number of mites also peaked 900 years ago. One possible explanation is that at that time, rising temperatures enabled herdsmen to use higher mountain pastures. The researcher is now going to count mites in samples from other lakes in the Andes to confirm the validity of the method, one which could well become a valuable tool for reconstructing the history of social and cultural change in the Andes and elsewhere.


Jean-François Haït

Notes :

1. Centre de bio-archéologie et d'écologie (CNRS / EPHE / Université Montpellier-II).
2. A. Chepstow-Lusty et al., “Evaluating socio-economic changes in the Andes using oribatid mites abundances as indicators of domestic animal densities,” Journal of Archaeological Science. 34: 1178-1186. 2007.
3. Including A. Gioda (IRD, Montpellier), M. Frogley (University of Sussex, UK) and Brian Bauer (University of Illinois in Chicago, USA).

Contacts :

Alex Chepstow-Lusty
CBAE, Montpellier.


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