Journal Club

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Pollen frozen in a glacier reveals the legacy of human impacts in the heart of the Inca Empire

Ice cores from the Illimani glacier in Bolivia suggest that at least some portions of the Inca empire had limited impacts on their environment compared to later societies. Image credit: Margit Schwikowski

Ice cores from the Illimani glacier in Bolivia suggest that at least some portions of the Inca Empire had limited impacts on their environment compared to later societies. Image credit: Margit Schwikowski

The Illimani glacier sits between the urban sprawl of La Paz, Bolivia, on one side, and the roads and plantations of the Amazon basin on the other. It was no doubt a very different landscape some 550 years ago, when the glacier towered over the heart of the Inca Empire. Indeed, researchers have long debated the extent to which the Inca and other indigenous peoples changed their surroundings.

In hopes of finding out, paleoecologist Sandra Brügger used grains of pollen frozen in Illimani’s layers of ice to reconstruct thousands of years of vegetation change in the habitats below the glacier. Her findings, recently published in Quaternary Science Reviews, suggest that the Inca had only moderate impacts on the vegetation. Major ecological changes in the region’s high plains and Bolivian Amazon began with European colonists. The Inca did cultivate crops and maintained agroforests, possibly for harvesting timber or food, notes Brügger, who conducted the new work as a doctorate student at the University of Bern in Switzerland. “But on a large scale, they didn’t turn everything upside down,” she says. “When Europeans come is when we have large-scale shifts.”

This new study is among the first to reconstruct ecology using pollen from glacier ice cores, says chemist and coauthor Margit Schwikowski of Switzerland’s Paul Scherrer Institute in Villigen, and the University of Bern. Pollen records are much more commonly analyzed from lake sediments, researchers say, because lakes trap higher concentrations of pollen than high-altitude ice, making it easier to find and count in the lab. Brügger ensured that she obtained plenty of pollen from the ice via a novel method that froze the bottom of her samples where the pollen accumulated, hence preventing it from washing away as she removed excess ice water.

The advantage of pollen analysis in ice, Schwikowski explains, is that ice cores can be very precisely dated. The one Brügger used could be dated within about two to five years in its upper layers corresponding to the last two centuries, and within roughly 20 years between 1800 and 1250. Clear timelines have been lacking, and Illimani’s proximity to the core of the Inca Empire makes its ice records especially useful to track ecosystem change with the rise and fall of a huge indigenous society.

To carry out the new work, Brügger analyzed pollen, spores, microscopic charcoal, and tiny spherical fossil fuel byproducts from two ice cores drilled out of Illimani in 1999 and 2015, representing about 12,000 years of history. She isolated these products from small slices of the ice, each corresponding to time bands, then counted the pollen, charcoal, and fossil fuel byproducts under a light microscope. By tracking the types of pollen over time, Brügger could infer how the set of local plants had changed, and when human land use might have played a role.

Changing concentrations of microscopic charcoal particles in the ice suggested the frequency of past fires; a count of fossil fuel byproducts indicated industrial air pollution levels. From its high elevation, Illimani catches windborne pollen from up to 300 km away, and fluttering charcoal and fossil fuel byproducts from an even larger catchment.

Looking at pollen and spores over time, Brügger found mostly herbaceous plants, with some evidence of crops like maize, quinoa, and amaranth, in the 10,000 years before the colonial period, suggesting that the Inca grew some foods, but didn’t inflict wholesale change on their environment. It wasn’t until around 1740, two centuries after the Spanish invasion and fall of the Inca Empire, that the pollen changed dramatically to pasture grasses and weeds, indicating established colonial settlements expanded their cattle grazing, she explains. Exotic trees such as pine and eucalyptus also appeared after 1740 and spread after 1950, giving rise to the widespread timber plantations in the area today. The pollen record indicates a relatively stable ecosystem before 1740, followed by rapid and drastic change. Fire activity tells a similar story, tracking climatic dry and wet periods rather than human activity, and signatures of fossil fuels only turning up frequently after 1820.

These findings suggest that the Inca may have used sustainable land management practices, at least near the core of their civilization, Brügger says. Previous studies using sediment pollen analysis and other tools have found larger environmental footprints. These new findings don’t necessarily contradict past work, says paleoecologist Encarni Montoya, at the Institute of Earth Sciences Jaume Almera in Barcelona, Spain. The Inca Empire was huge, and could have easily held pockets of aggressive development and zones of sustainable management, she says.

Not all pollen rides the wind up a mountainside, though. Only well-dispersed grains get to high altitudes and land in snow, which could bias the snapshot captured in ice, explains paleoclimatologist Broxton Bird, of Indiana University–Purdue University joint campus in Indianapolis. But while certain plants might end up over- or underrepresented, any bias would be constant, and the trends captured in the ice still hold.

By introducing exotic species, degrading soils, and radically changing the ecosystem, settlers began a change that may have hamstrung the environment’s ability to adapt. Studies like this show what ecosystems may have been like before dramatic human impacts. Such a baseline could guide conservation measures to help forests become more resilient. Says Brügger, “The best window to the future is sometimes the past.”

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