Thursday, June 7, 2018

Rats can open a window on ancient island environments


It may be hard to think of something rats are good for, but a team of scientists has figured out a unique way to look into the history of Pacific islands—using rats.
(Image: A Pacific rat, Rattus exulans, in a New Zealand government photo.)

They looked into the bones of rats from ancient archaeological sites, and it turned out that they could extract information from the marrow of rat bones. They were able to figure out what the rats were eating over time, and from that, make assessments about the changing island environment.

A new study on this work was just published in the Proceedings of the National Academy of Sciences. The authors are Jillian Swift, Patrick Roberts and Nicole Boivin of the Max Planck Institute for the Science of Human History, and Patrick Kirch of the Univeristy of California-Berkeley Department of Anthropology. The paper is entitled "Restructuring of nutrient flows in island ecosystems following human colonization evidenced by isotopic analysis of commensal rats."

Some definitions: Isotopes are forms of the same element that have different numbers of neutrons in the nuclei. Isotopic analysis in this instance is studying changes over time in the mix of isotopes of certain elements—in this case carbon and nitrogen. And commensal means living with something, without causing it any particular help or harm—in this case rats living among humans.

When early Polynesians arrived on Pacific islands, they almost always brought Pacific rats with them. People argue over whether that was intentional or not, but it almost always happened. Pacific rats, or Polynesian rats, are Rattus exulans.

And those rats quickly went out on their own, foraging initially in the virgin environment, and later in the environment that humans had created through bringing new plants, slash and burn agriculture and so forth.

"Stable isotope analysis of commensal animals provides an effective proxy for local human environments because these species are closely associated with human activities without being under direct human management," the authors write.

They studied rat bones from archaeological sites in three Polynesian island areas: Mangareva, Ua Huka in the Marquesas and Tikopia far to the west.

They found that their study of rats was able to track dramatic changes in local environments, demonstrating "human land use, intensification and faunal community restructuring."

More specifically, "Our results highlight the large-scale restructuring of nutrient flows in island ecosystems that resulted from human colonization and ecosystem engineering activities on Pacific islands."

This is complicated stuff, and I won't go into details on how the lab work was done, but you can look it up yourself here.

This is data that the authors say is hard to get from standard analysis of early archaeological remains, and it suggests that this kind of stable isotope analysis can help shed more light on how and how quickly humans changed their environments after arriving on Pacific islands.

It is also true that the rats themselves did a lot of that modification of ecosystems—eating birds and insect and seeds and seedlings. But that's all part of the story, since it was humans who brought the rats to these new environments.

© Jan TenBruggencate 2018

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