This project studied the effects of feedbacks between climate change and increasing human impacts as an intensifier of extinction.  The study focused on the major extinction event of large mammals that took place in South America near the end of the last ice age (e.g., the late Quaternary, near the Pleistocene-Holocene transition, around 12,000 years ago). This provided an ideal natural experiment with which to untangle the relative contributions of human population growth, climate change, and interactions between the two as a multiplier of extinction. 

The study traced human population growth from Homo sapiens’ first entry into South America some 15,000 years ago, and compared that with the timing of extinction of large mammal species such as horses, llamas, giant ground sloths, and proboscideans (elephant relatives), and with the timing of climate and environmental changes as inferred from paleontological and geological records such as fossil pollen, charcoal, geomorphology, and isotopic data.

The chronology of extinction was refined through radiocarbon dating of fossil specimens, including experimenting with and applying a new radiocarbon-dating technique (XAD) that improved upon the previously accepted state-of-the art for radiocarbon dating (ultrafiltration).  Approximately 100 specimens that were provided by collaborators were analyzed for radiocarbon dating, and the peer-reviewed scientific literature was mined to extract published radiocarbon dates that were vetted for reliability.  Statistical techniques were used to estimate timing of human arrival and large-mammal extinction with respect to climate and vegetation changes, and a new quantitative technique was developed to clarify the relative contributions of climate and environmental change, human impacts, and interactions between the two as a potential multiplier of large-mammal extinction.

Results: The study revealed that human population growth after first entry into South America followed the trajectory expected from an invasive species, and that the pattern of large-mammal extinction is best explained by a combination of increasing human pressures and climate change. Regionally within South America the extinction pattern is consistent with an interplay between resource use by humans and climate-driven environmental changes. Comparison of extinction patterns in South America and North America strongly supports that removal of some megafauna, notably proboscideans, can initiate lasting ecological state changes that include altering vegetation structure and diminishing biodiversity.

Results have been disseminated through a book, 9 peer-reviewed journal articles (including publications in Nature and the Proceedings of the National Academy of Sciences), 18 national and international conference presentations and abstracts, 2 Ph. D. dissertations, and 3 blogs.

Intellectual Merit: Many large mammals are presently at risk of extinction from a variety of human impacts.  This study demonstrates that two drivers operating today—climate change and growing human population size—combined to cause the major extinction that took place near the end of the last ice age, when half of Earth’s large-bodied mammal species died out, and that extinction of some large mammals induces collateral changes that transform ecosystems dramatically. Such knowledge is critical to conservation biologists and global change analysts who are attempting to develop solutions to the current biodiversity crisis, now widely recognized as having the potential to become a mass extinction more intense than any that have occurred since 66 million years ago, when an asteroid delivered the coup-de-grace to the dinosaurs.

Broader Impacts: The project involved more than 30 collaborators drawn from most countries in South America as well as North America. Collaborators participated in international workshops, contributed data, and jointly authored scientific publications. The resulting scientific network strengthened intellectual exchange among the involved countries.

The project contributed substantially to developing human resources through STEM training, including: a female Ph.D. student from Chile who will return to Chile and incorporate in the academic culture of that country the cutting edge techniques learned during the project; a female postdoctoral student who as a result of training was offered and accepted a tenure-track position; two female and one male Ph.D. students with degrees in progress; and four female and one male undergraduate students.

Information has been disseminated to the general public through a book explaining the extinction crisis (Dodging Extinction, published 2014, UC Press).  Outreach also includes an educational “comic book” that explains the scientific method employed by paleontologists who study extinctions and relevance to the current extinction crisis, to be published in English, Spanish, and Portuguese and to be served through the University of California Museum of Paleontology (UCMP) Understanding Global Change website. The UCMP will also incorporate information arising from this research into materials for teacher training about extinction and global change.

The new and previously published, vetted radiocarbon dates will be available to the general public through the ANTIGUA database that was built as part of this study and will be served through the NSF-sponsored NEOTOMA data repository. 

Last Modified: 08/04/2016
Modified by: Anthony D Barnosky