Late Pleistocene climate change, nutrient cycling, and the megafaunal extinctions in North America

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Abstract

This study proposes an ecological mechanism for the terminal Pleistocene population collapse and subsequent extinction of North American megafauna. Observations of modern ecosystems indicate that feedback mechanisms between plant nutrient content, nitrogen cycling, and herbivore–plant interactions can vary between a nutrient accelerating mode favoring increased herbivore biomass and a nutrient decelerating mode characterized by reduced herbivore biomass. These alternate modes are determined largely by plant nitrogen content. Plant nitrogen content is known to be influenced by atmospheric CO2 concentrations, temperature, and precipitation. It is argued that Lateglacial climate change, particularly increases in atmospheric CO2, shifted herbivore–ecosystem dynamics from a nutrient accelerating mode to a nutrient decelerating mode at the end of the Pleistocene, leading to reduced megafaunal population densities. An examination of Sporormiella records – a proxy for megaherbivore biomass – indicates that megafaunal populations collapsed first in the east and later in the west, possibly reflecting regional differences in precipitation or vegetation structure. The fortuitous intersection of the climatically driven nitrogen sink, followed by any one or combination of subsequent anthropogenic, environmental, or extra-terrestrial mechanisms could explain why extinctions took place at the end of the Pleistocene rather than during previous glacial–interglacial cycles.

Highlights

► An ecological mechanism for North American late Pleistocene extinctions is proposed. ► Late Pleistocene Sporormiella records in North America are reviewed. ► Feedback between herbivores, plants, and nitrogen cycling is reviewed. ► Increased CO2 during the Lateglacial could cause a megafaunal population collapse.

Introduction

The late Pleistocene of North America is characterized by the extinction of 36 genera of large mammals (Grayson, 1991, Grayson, 2011, Barnosky et al., 2004, Grayson, 2007, Faith and Surovell, 2009). Six of these genera survived elsewhere, although within them some species were lost, and the majority became globally extinct. The losses were most severe for the largest taxa, with extinctions taking place across all genera larger than 1000 kg (e.g., Mammuthus, Mammut, Glyptotherium) and over half the genera between 32 and 1000 kg (Lyons et al., 2004a, Koch and Barnosky, 2006). Due to the limitations of the fossil record, the timing of the extinctions is debated (Grayson, 2001, Grayson, 2007, Grayson and Meltzer, 2003, Fiedel and Haynes, 2004, Faith and Surovell, 2009, Fiedel, 2009). At least 16 genera are unequivocally known from the very latest Pleistocene (Grayson, 2007, Faith and Surovell, 2009, Fiedel, 2009), although the chronology is consistent with the synchronous loss of all 36 genera at the very end of the Pleistocene, between 12,000 and 10,000 14C yrs BP (∼13,800 to 11,400 cal yrs BP) (Faith and Surovell, 2009).

At the same time that extinctions were taking place, there is evidence for dramatic shifts in the geographic range and composition of plant (Williams et al., 2001, Williams et al., 2004, Gill et al., 2009) and animal communities (FAUNMAP, 1994, FAUNMAP, 1996, Stafford et al., 1999), which includes non-analog communities (Williams et al., 2001, Gill et al., 2009), and increased biomass burning (Robinson et al., 2005, Gill et al., 2009, Marlon et al., 2009). This time period also encompasses the Younger Dryas cold interval (Alley, 2000, Rasmussen et al., 2006), the arrival of Clovis hunter–gatherers in North America (Meltzer, 2004, Waters and Stafford, 2007) and a proposed – although highly contested (Paquay et al., 2009, Surovell et al., 2009, Daulton et al., 2010, Haynes et al., 2010, Holliday and Meltzer, 2010, Scott et al., 2010) – extra-terrestrial impact event (Firestone et al., 2007, Kennett et al., 2008, Kennett et al., 2009, Melott et al., 2010).

To account for the massive loss of North America’s Pleistocene megafauna (animals > 44 kg), a number of hypothesis have been forwarded. These include human hunting pressure (Martin, 1967, Martin, 1984, Martin, 2005, Martin and Steadman, 1999, Alroy, 2001, Lyons et al., 2004a), environmental change (Graham and Lundelius, 1984, Guthrie, 1984, King and Saunders, 1984), or some combination of both (Barnosky et al., 2004). Alternative explanations include an extra-terrestrial impact (Firestone et al., 2007, Kennett et al., 2008, Kennett et al., 2009, Paquay et al., 2009, Surovell et al., 2009, Daulton et al., 2010, Haynes et al., 2010, Holliday and Meltzer, 2010, Melott et al., 2010, Scott et al., 2010), disease (MacPhee and Marx, 1997, Lyons et al., 2004b), or vegetation change stemming from the loss of keystone mega-herbivores (Owen-Smith, 1987). This study discusses how the interplay between Lateglacial climate change, nutrient cycling, and plant–herbivore interactions could have played a deciding role in the demise of North America’s Pleistocene megafauna.

Section snippets

The late Pleistocene megafaunal population collapse

Davis (1987) demonstrated that Sporormiella dung fungal spores track herbivore abundances on the landscape. Sporormiella is a coprophilous fungus found only on the dung of herbivores (Ahmed and Cain, 1972). The spores are common on the dung of domestic herbivores and mega-herbivores (Ebersohn and Eicker, 1997) and are also known from the dung and gut contents of Pleistocene Mammuthus (Davis et al., 1984, Mead et al., 1986, van Geel et al., 2008). In pollen cores, Sporormiella abundances <2% of

Nutrient cycling in modern ecosystems & Lateglacial climate change

Modern ecosystems are characterized by dynamic feedback mechanisms between herbivores, plant tissue chemistry, and nutrient (nitrogen) cycling (Pastor et al., 2006). As reviewed by Pastor et al. (2006), ecosystems are broadly characterized by two primary modes of herbivore–ecosystem interactions, termed nutrient accelerating or nutrient decelerating modes (Fig. 4) (Ritchie et al., 1998). Theoretical and empirical evidence suggests that the distinction between these modes depends on forage

Conclusion

This study proposes an ecological explanation for the population collapse and subsequent extinction of North American Pleistocene megafauna. Drawing upon observations of contemporary herbivore–ecosystem dynamics, it is argued that Lateglacial environmental change, including rising CO2 concentrations and possibly elevated temperatures and precipitation, prompted a transition from a nutrient accelerating mode to a nutrient decelerating mode in North America. This transition would have been

Acknowledgments

I thank Kay Behrensmeyer, Don Grayson, and John Pastor (reviewer) for their comments on previous versions of this manuscript and helpful discussions on the topic. I also thank Mark Ritchie for pointing out useful references at the onset of this research. This research was funded by a NSF IGERT fellowship.

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