In the Eastern Cordillera of Bolivia, pollen grains travel from near and far to become sandwiched in layers of snow in the Andean mountaintops, ultimately becoming trapped as the layers turn to ice. Such is the case on the Illimani Glacier, which towers approximately 2,500 meters over Lake Titicaca. The lake sits at an altitude of 3,800 meters above sea level in what was the heart of the ancient Incan Empire.
University of Bern paleoecologist Sandra Brugger headed a team of researchers from various European universities to investigate the vegetative history of the Andean region. Their findings, published in Quaternary Scientific Reviews, indicate that the Inca used sustainable land use practices and that large scale ecological changes did not occur until 1740, long after the Spanish invasion and fall the Inca. The study is one of the first to reconstruct past ecology using pollen grains pulled from glacial ice.
The goal of Brugger’s study was to determine the resilience potential of the Andean mountain-forest ecosystem to a varying intensity of anthropogenic land-use practices. The researchers constructed a timeline of vegetation from 10,000 BC through to the present day. Of particular interest were the years following 1438, which represented the transition from the rise to the demise of the ancient Inca, which was then followed by the the reign of their Spanish conquerors. The degree to which the indigenous peoples altered their environment is a topic still deeply debated amongst researchers.
Much like tree rings, glacial ice accumulates in distinct annual layers; therefore, scientists can date ice core samples by ring counting, analyzing the layer’s isotopic signature, or by finding evidence of volcanic eruptions that have been well-dated throughout history. These methods are extremely accurate. Ice from the uppermost layers, which correspond to the last two-hundred years, can be dated within two to five years, while the ice corresponding to the time period of the Incas can be pinpointed to within two decades of accuracy.
The methods for extracting ice cores are actually quite challenging, Brugger said. An experienced team is required to extract samples from high altitudes because conditions become increasingly treacherous with elevation. Moreover, they must ensure that samples remain frozen throughout the delivery process—in this case, from Bolivia to Switzerland. “If they melt, samples are no good,” said Brugger.
The team of Margit Schwikowski at the Paul Scherrer Institute in Switzerland undertook these dangerous drillings, climbing to an elevation of approximately 6,000 meters above sea level. Additionally, they analyzed the chronology and measured many chemical species in the ice cores. Two core samples from the Illimani Glacier were extracted: one in 1999 and another in 2015.
Once in the lab, Brugger applied a series of evaporative and chemical-processing techniques to isolate pollen grains from samples corresponding to specific time periods. Each of the samples held approximately 500 pollen grains. “A good sample took me two to three hours to identify,” she said. A bad sample, she added, could take an entire day. The whole process took about three months.
The trickiest part, according to Brugger, was the patience required to identify the pollen. Not only is the catchment area of Illimani large, but the Amazon basin is also one of the most biodiverse regions on the planet, so many different species of pollen were represented in the samples. Undoubtedly, the identification process was painstaking work that required long days behind a microscope at a lab bench – far from the charm of the Bolivian Glacier.
Much of the previous research on Andean vegetation was constructed using pollen grains from lake sediments, noted Brugger, which ultimately captures more of a local signal from vegetation directly surrounding the lake. In what was the heart of the Incan Empire near Lake Titicaca, archaeological records suggest that pre-European cultures were highly advanced, domesticating llamas and alpacas, harvesting a wide variety of crops, and practicing metallurgy. Together, these practices could have brought about significant land-use impacts.
Digging deeper, researchers found that llama dung was an important maize fertilizer for the indigenous Andeans.
The switch to agricultural reliance allowed the Inca to abandon traditional hunting and gathering methods and supported the growth of society. An article recently published in the Journal of Archaeological Science details how oribatid mites that once dined on llama feces have been found in sediment cores from wetlands such as Lake Marcacocha, high in the Andes. As merchants passed through these areas with their llamas and maize yields, they boosted the oribatid mite population of the wetlands. This population boom strongly correlates with the time period dominated by the Inca (1483-1533), and the mites’ eventual decline corresponds to the arrival of the Spanish conquistadors, who wiped out the Inca and replaced their llamas with cows, horses, and sheep.
Interestingly, a study published in Applied Animal Behaviour Science suggests that llamas are not as impactful on the landscape as the Old World animals brought over by the Spanish. While llamas graze evenly among the various plant types, cows and sheep appear to be more scrupulous in their dietary decisions. Llamas do not eat plants down to their roots and have padded feet that are less environmentally destructive than hooves. Additionally, explained Brugger, while the native Puna grasses declined around 1740, the population of nutrient-loving weedy species escalated due most likely to the increase European cattle grazing activity. Therefore, the Incan llama grazed the land in a way that was sustainable to the Andean ecosystem, while their European counterparts decimated the land.
Unlike lakes, glaciers trap pollen on a larger scale, as particles drift in from a catchment area of approximately 200-300 kilometers in each direction. Brugger’s research suggests that, on a large scale, the Incan people did not change the Andean forest composition. It is important to note that local versus regional pollen collection methods do not necessarily contradict one another, said Brugger. Instead, they reveal that pockets of disturbance may have occurred closer to the lake where paths and roads were constructed, but overall, the Incan empire did not leave a significant ecological footprint.
The team identified vegetation that dates as far back as 10,000 BC, establishing an ecological baseline of plant diversity prior to human intervention in the landscape. The baseline served as the control for which human-induced vegetation change over time could be compared.
Brugger found small signs of maize, quinoa, and amaranth, after AD 1, suggesting that the Incas, as well as the indigenous populations before them, grew agricultural crops. Despite signs of human impact, Puna composition did not deviate from previous centuries.
Likewise, the expansion of Polylepis and Alnus after the year 800 followed a warming climate trend. Although Alnus, commonly know as alder, was favored for agroforestry, its range did not dissipate during the Incan regime. According to the book An Environmental History of Latin America, the Incan emperor himself maintained a sustained population of Alder and inflicted harsh punishments for unauthorized logging. In an area naturally defined by so little trees, the alder’s continued existence suggests strict environmental regulation. Its population soon declined with the arrival of Europeans.
According to Brugger’s data, changes in the mountain forest composition didn’t occur until around 1740 (two hundred years after the fall of the Incan Empire), implying a long transitional period before the Spanish were able to establish a stable land-use system. After 1740, the pollen record showed a rapid increase in dry grasses and nutrient-loving, weedy species, typical of pasturelands. Then, around 1950, signs of eucalyptus and pine appear in the pollen record, a result of the Bolivian land reform that promoted timber plantations.
Brugger is now stationed at the Desert Research Institute in Reno, Nevada, analyzing pollen and charcoal in ice cores from Central Greenland in order to reconstruct the response of sensitive Arctic ecosystems to past climate change. “It was a sensation that the approach actually worked,” said Brugger, “as the site was extremely remote from any plants — and pollen.” The prestudy to the project is published in The Holocene.
Glaciers provide an incredible glimpse into the past because they safeguard microscopic clues that allow researchers to uncover our most ancient secrets. For instance, Brugger’s study suggests that the Incan people, though large in number, were able to form a society that peacefully coexisted with its environment. Modern society has largely degraded the Bolivian ecosystem, but might learn a thing or two by studying ancient Incan methods of sustainable agriculture and agroforestry. Brugger’s research is part of a larger project that examines glacial cores from around the world to explain our past. As the project gains momentum, scientists can begin to unravel other fascinating mysteries trapped within glacial ice.