A changing climate doesn't just affect the weather. Entire ecosystems are forced to change, often leading to dramatic shifts in local environments, which can be challenging - even fatal - for species that depend on those ecosystems.
But the hunter-gatherer humans who lived in Patagonia between 6,500 and 2,500 years ago rolled with the punches. New research shows how they adapted their hunting strategies and eating habits to survive changing conditions - both seasonal and long-term.
Patagonia and the archipelago of Tierra del Fuego comprise the southernmost tip of South America. During the middle and late Holocene, a number of groups populated these regions, remaining unusually stable and homogeneous for thousands of years.
Some, like the Kawésqar and the Yaghan, were nomadic seafarers. They took advantage of the rich marine life of the region, supplied and replenished by the Humboldt Current that sweeps up the West coast of South America, cycling nutrient-rich water from the cool depths to replace the warmer surface waters.
Thanks to middens, the heaps these societies used to dispose of their garbage, we have a pretty good handle on what they ate. Their diet included a lot of seafood - fish and shellfish - and other animals that came to feast on the rich bounty, like seabirds and sea lions.
What was less clear was whether that diet varied at all from season to season, as dramatic changes altered ecosystems. Seasonal glacier melt would lower ocean salinity, temperature and nutrients, all of which would impact the phytoplankton at the base of the food chain. In turn, that could impact the availability of the species on which human societies relied.
So a team of researchers led by archaeologist Jimena Torres of the University of Magallanes in Chile decided to take a closer look at four middens, one from the Middle Holocene (6,500 to 5,000 years ago) and three from the Late-Middle Holocene (3,500 to 2,500 years ago).
In particular, they were looking for the bones of a fish abundant in Patagonian coastal waters - the tadpole codling (Salilota australis), and looking at when in the year the fish were caught.
This can be gauged by counting growth rings called annuli in the fishes' bones. Much like trees, these are laid down periodically, with thinner rings for leaner seasons (winter) and thicker for times of plenty.
The team collected 255 archaeological samples. In addition, they collected 69 modern samples over the course of a year with which to compare the midden bones, to make sure their observations were as accurate as possible.
They found that, at the Middle Holocene site, the tadpole codling bones were likely from fish caught during the warmer months. This was supported by analysis of the other contents of the midden, particularly the bones of juvenile seabirds that wouldn't have been present during the colder parts of the year.
On the later sites, however, the bones suggest tadpole codling was caught year-round, or during colder months, suggesting a shift in strategy - which could have been influenced by a changing climate.
"Oceanographic and ecological changes during the Holocene could have influenced the accessibility and abundance of different fishes exploited by hunter-fisher-gatherer societies," the researchers wrote in their paper.
They note that, while sea surface temperatures remained relatively high during the Middle Holocene, some studies have found these temperatures started to decline around 6,000 years ago, and there may have been a neoglacial period between 5,500 and 4,500 years ago.
"Sea surface temperature records for the Late Holocene show that in the fjords and inland seas, as well as in the offshore areas of the Chilean continental margin, there was a decrease in sea surface temperature," the researchers wrote.
"Similarly, palaeoceanographic proxies of primary productivity from the central basin of the Strait of Magellan display a sharp decline between 3,000 and 2,200 years ago, with a strong reduction in salinity due to a period of intense freshwater supply, probably caused by higher rainfall and glacial advance."
The year-round fishing activity by the Late-Middle Holocene communities could have been the result of an abundance of fish, the researchers note. And they believe that changing environmental conditions were likely drivers of this shift.
It's also possible that these communities had seasonal hunting camps in other locations in order to take advantage of the seasonal migrations of different animals, consistent with a working relationship with the natural environment that enabled long-term survival.
The research has been published in The Journal of Island and Coastal Archaeology.