A palaeoecological perspective on the transformation of the tropical Andes by early human activity

Abstract

Palaeoecological records suggest that humans have been in the Andes since at least 14 000 years ago. Early human impacts on Andean ecosystems included an increase in fire activity and the extinction of the Pleistocene megafauna. These changes in Andean ecosystems coincided with rapid climate change as species were migrating upslope in response to deglacial warming. Microrefugia probably played a vital role in the speed and genetic composition of that migration. The period from ca 14 500 to 12 500 years ago was when novel combinations of plant species appeared to form no-analogue assemblages in the Andes. By 12 000 years ago most areas in what are today the Andean grasslands were being burned and modified by human activity. As the vegetation of these highland settings has been modified by human activity for the entirety of the Holocene, they should be regarded as long-term manufactutred landscapes. The sharp tree lines separating Andean forests from grasslands that we see today were probably also created by repeated burning and owe their position more to human-induced fire than climatic constraints. In areas that were readly penetrated by humans on the forested slopes of the Andes, substantial modification and settlement had occurred by the mid-Holocene. In hard-to-reach areas, however, the amount of human modification may always have been minimal, and these slopes can be considered as being close to natural in their vegetation. This article is part of the theme issue 'Tropical forests in the deep human past'.

Keywords: fire; manufactured landscapes; megafauna; microrefugia; migration; tree line.

Figure 1.

Schematic diagram showing the difference in ice limit and glacial extent relative to vegetation in the deglacial period. (a) Full glacial conditions in which tree line is lowered to approximately 2200 m elevation. Icons represent trees of slightly different niche requirements, also shown as population abundance on side panels. Microrefugia exist where trees survive above the tree line. Genetic differentiation takes place within microrefugia. (b) Upslope expansion of populations and contraction of grasslands. Microrefugial populations are important in establishing the genetic composition of tree line community. When species migrations are fastest, unusual combinations of lowland and highland species create assemblages without modern analogue. Fire may signify human presence, especially in wet locations. (c) Near-modern system established in the early Holocene. (Online version in colour.)

Figure 2.

Map showing the location of archaeological sites and sources of palaeoecological records mentioned in text. (Online version in colour.)

Figure 3.

Diagram of proxy indicators for system wetness, megafaunal abundance and fire. Isotopic δ¹⁸O data from Santiago cave [56] have more negative values as the system is wetter. Diatom data from Pacucha [57] indicate lake shallowing as benthic diatom abundances increase. Sporormiella spore occurrence is expressed as concentrations of spores per cm³, and per cent of pollen sum is contrasted with charcoal concentration cm² per cm³ at four Andean sites (Pacucha 3104 m elevation, Caserococha 3997 m elevation, Llaviucu 3150 m elevation and Chochos 3215 m elevation). VPBD, Vienna Peedee belemnite.

Figure 4.

A compiled history of megafauna, fire, Polylepis cover, confirmed human occupation and climate change from the High Andes from 20–8 kcal BP. Resampled means of Sporomiella (a) and charcoal (b) abundances expressed as relative proportions. For sites contributing data and methods of calculation for curves in figure 4a,b,d, see electronic supplementary material and electronic supplementary material, figures S1–S3. (c) A probability density function (pdf) of ¹⁴C ages (N = 83) associated with human occupation (data source [62]). (d) Importance values for Polylepis, a tree that can grow above the tree line. (e) The El Condor [63] and Santiago δ¹⁸O [56] data provide a proxy for precipitation with more negative values indicating wetter conditions. (g) The clastic sediment flux is a compilation of 13 high Andean records [64]. Dotted lines indicate 1 s.d. CI. (Online version in colour.)

Figure 5.

Suggested changes to the tree line in the presence of fire. Humans produce a sharp tree line through burning. Occasional fires penetrate the tree line and cause disturbance behind the tree line, but not enough for the tree line to retreat. The unburned upper forest limit may have been less abrupt upslope, giving way to a grassy, shrubland, with denser forest near the upper limit than when fire is present. (Online version in colour.)