Life and Extinction of Megafauna in the Ice-Age Arctic
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Life and extinction of megafauna in the ice-age Arctic Daniel H. Manna,1, Pamela Grovesb, Richard E. Reanierc, Benjamin V. Gagliotid, Michael L. Kunze, and Beth Shapirof,1 aGeosciences Department, University of Alaska, Fairbanks, AK 99775; bInstitute of Arctic Biology, University of Alaska, Fairbanks, AK 99775; cReanier & Associates, Inc., Seattle, WA 98166; dWater and Environmental Research Center, University of Alaska, Fairbanks, AK 99775; eCooperative Extension and Resources, University of Alaska, Fairbanks, AK 99775; and fDepartment of Ecology and Evolutionary Biology and University of California, Santa Cruz Genomics Institute, University of California, Santa Cruz, CA 95064 Edited by Richard G. Klein, Stanford University, Stanford, CA, and approved September 23, 2015 (received for review June 29, 2015) Understanding the population dynamics of megafauna that in- The study area is Alaska’s North Slope, the tundra region habited the mammoth steppe provides insights into the causes of bordered to the south by the Brooks Range and to the north by extinctions during both the terminal Pleistocene and today. Our the Arctic Ocean (Fig. 1). The North Slope is a particularly in- study area is Alaska’s North Slope, a place where humans were teresting place to study end-Pleistocene extinctions for several rare when these extinctions occurred. After developing a statisti- reasons. First, its ice-age megafauna included iconic species like cal approach to remove the age artifacts caused by radiocarbon woolly mammoth (Mammuthus primigenius), steppe bison (Bison priscus), and cave lion (Panthera spelaea) (14). Second, the local calibration from a large series of dated megafaunal bones, we ’ compare the temporal patterns of bone abundance with climate extinctions of megafauna on Alaska s North Slope occurred at a records. Megafaunal abundance tracked ice age climate, peaking time when archaeological remains are rare, suggesting that people seldom ventured there (15, 16). Third, bone preservation in arctic during transitions from cold to warm periods. These results sug- environments tends to be excellent because of the presence of gest that a defining characteristic of the mammoth steppe was its permafrost (perennially frozen ground), which makes it possible temporal instability and imply that regional extinctions followed to 14C date large numbers of bones from many different species by population reestablishment from distant refugia were charac- (SI Appendix,TableS1). Our record of dated bones provides key teristic features of ice-age biogeography at high latitudes. It fol- insights into the temporal dynamics and biogeographical character- lows that long-distance dispersal was crucial for the long-term istics of the mammoth steppe, a biome that was unique to the ice ages persistence of megafaunal species living in the Arctic. Such dis- and the exact nature of which has been long debated (17). persal was only possible when their rapidly shifting range lands SCIENCES were geographically interconnected. The end of the last ice age Background ENVIRONMENTAL was fatally unique because the geographic ranges of arctic mega- Mammoth Steppe. Episodically during the late Pleistocene, the fauna became permanently fragmented after stable, interglacial mammoth steppe extended from Europe to northwestern Canada climate engendered the spread of peatlands at the same time that (18). Its soils were relatively dry, warm, and fertile compared with rising sea level severed former dispersal routes. those of the present day (19, 20) (SI Appendix, SI Text), and its vegetation supported large herds of grazing mammals in species- ice age | megafauna | extinction | paleoecology | mammoth steppe rich communities (14, 17, 18). The biomasses and diversities of these ice-age communities contrast starkly with the impoverished megafaunal communities living in the same regions today (10, 21). EARTH, ATMOSPHERIC, ne of the most intriguing examples of mass extinction and AND PLANETARY SCIENCES The climate supporting the mammoth steppe was more continental Othe most accessible in terms of its geological record occurred – than today (18) and, as detailed below, it was much more change- around the end of the Wisconsin ice age ca. 10 45 calendar ka B.P. able at millennial and centennial timescales. (10,000–45,000 calendar y ago) when some 65% of terrestrial > The nature and, for some authors, even the veracity of the mam- megafauna genera (animals weighing 45 kg) became globally ex- moth steppe remain controversial (14, 17). Some paleobotanists tinct (1). Based on what we know about recent species extinctions, the causes of extinction are usually synergistic, often species-specific, Significance and therefore, complex, which implies that there is no universal explanation for end-Pleistocene extinctions (2, 3). Globally and specifically in the Arctic (3–10), megafaunal extinctions have been Understanding species extinction is a major concern today, and variously blamed on overhunting, rapid climate change, habitat loss, past extinctions provide valuable lessons. Numerous mammal and introduced diseases (3–10). Further complicating a clear un- species became extinct in the Arctic at the end of the ice age, but derstanding of the causes of ice-age extinctions is that the magni- it is unclear why. By comparing numbers of dated bones with tude and tempo of environmental change during the last 100,000 y climate records, we find that megafaunal species, like mam- of the Pleistocene were fundamentally different than during the moth, horse, and bison, experienced boom and bust cycles dur- Holocene (11), and these differences had far-reaching implications ing the ice age as they tracked rapid climate changes. For these for community structure, evolution, and extinction causes (12). species to persist, long-distance dispersal was necessary. Their A recent survey comparing the extinction dates of circum- extinction on the North Slope occurred as the ice age ended, boreal megafauna with ice-age climate suggests that extinctions because rising sea level severed dispersal routes and spreading and genetic turnover were most frequent during warm, in- peat simultaneously degraded range quality. This finding sug- terstadial events (13). However, the mechanisms for these ex- gests that arctic mammals can be resilient to environmental tinctions remain unclear, partly because this previous study changes but only if their habitats remain widely interconnected. considered multiple taxa living in many different ecosystems. Here, we focus on five megafaunal species that coinhabited a Author contributions: D.H.M., M.L.K., and B.S. designed research; D.H.M., P.G., B.V.G., M.L.K., region of the Arctic with an ecological setting that is relatively and B.S. performed research; D.H.M., P.G., and R.E.R. contributed new analytic tools; P.G. well-understood. To avoid the methodological problems involved and R.E.R. analyzed data; and D.H.M., P.G., R.E.R., B.V.G., and B.S. wrote the paper. in pinpointing extinction dates (13), we infer population dy- The authors declare no conflict of interest. namics from changes in the relative abundance of megafauna This article is a PNAS Direct Submission. over time. Using a uniquely large dataset of dated megafaunal Freely available online through the PNAS open access option. bones from one particular area, we test a specific paleoecological 1To whom correspondence may be addressed. Email: [email protected] or dhmann@ hypothesis relating rapid climate change to population dynamics— alaska.edu. namely, that transitions from cold to warm intervals were briefly This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. optimal for grazing megafauna. 1073/pnas.1516573112/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1516573112 PNAS Early Edition | 1of6 are heavily defended with antiherbivory compounds (SI Appendix, SI Text). Peat also cools the underlying ground (34), which allows perma- frost to rise nearer the surface and it perches the water table there. The resulting combination of increased moisture and colder temper- ature retards decomposition, reduces nutrient availability, and en- courages more peat to form (35). Many regions in northern Eurasia and northwestern North America that supported mammoth steppe during the ice age are today blanketed by peat-rich plant communities (36) incapable of supporting large biomasses of grazing mammals. Warm Transitions Were Briefly Optimal Hypothesis. Guthrie (37) identified the Pleistocene–Holocene transition (12–16 calendar ka B.P.) as a highly favorable time for megafauna (Fig. 2). Guthrie (37) speculated that this was the case because the Pleistocene– Holocene transition was a period of ecological disequilibrium, Fig. 1. The North Slope is the tundra region between the Brooks Range and during which the climate had become warmer and wetter but the the Arctic Ocean. The light blue area shows the extent of the Bering Land Bridge vegetation cover had not yet had time to equilibrate with these during the last glacial maximum (LGM) ca. 19,000 calendar y B.P. Glacier extent changes. During this lag period, the edible graminoids and forbs (gray) during the LGM is based on the works by Dyke (64) and Brigham-Grette that had dominated the mammoth steppe became more productive et al. (71). The timing of the opening of the ice-free corridor is still uncertain. and could support more animals. Today, grasses and forbs flourish at tundra sites where soils and vegetation have been disturbed (38, 39) but only