Prehistoric Megaherbivores, Woody Plants, and the American Buffalo: Lessons from the Late Pleistocene Extinction Event

Past research on paleontological data gathered in northeastern Iowa indicates the Iowan prairie only supplanted hardwood forests between 6,500 and 5,500 years ago with oak savanna following a few thousand years later.  At the time, this shift was attributed to fires due to the lack of pronounced changes in climate preceding the transition.  Yet even as fire plays an undeniable role in the maintenance Iowa’s tallgrass prairies and oak savannas, the action of Iowa’ large herbivores, such as the American bison, may have played an instrumental role in the creation and maintenance of these habitats.

In a recent paper, Bakker et al. reviewed the ecological relationship between the presence of megaherbivores – mainly mammalian herbivores weighing 1,000 kg or more – and woody vegetation.  In this review, the authors focused on the impact of the loss of many megaherbivores during the mast extinction, occurring 50 to 10 thousand years ago, at the end of the Pleistocene Epoch.  They were particularly interested in this subject as they viewed the present, anthropogenic reductions in many large herbivores populations as functional extinctions as far as their impact on woody species was concerned.  While the review focused mainly on extinct Pleistocene and modern African megaherbivores, the authors’ general conclusions are applicable to other extant large herbivores.

Unfortunately, the record provided by Pleistocene megaherbivore fossils was to disjointed to be of much use in answering ecological questions the authors were interested in.  However, microorganisms preserved in lake sediment deposited during the Pleistocene provide a more continuous ecological record. Using spores produced by Sporormiella, a fungus that grows in herbivore dung, were able to estimate past abundances of megaherbivores.  The fossilized pollen from the same sediment layers could then be used to approximate the abundance and species composition of woody plants from same time period.  This paleoecological record provides evidence of an increase in hardwood deciduous trees shortly following the mass extinction, indicating the extinct megaherbivores suppressed woody plant abundance.

While extinct megaherbivores – such as Mastodons, Mammoths, and Giant Sloths – cannot be resurrected for experimentation (or at least not in the near future given the current progress toward mammoth cloning), many African, Pleistocene megaherbivores have survived into the present.  The authors review contemporary experiments on these extant megaherbivores as they have contributed too much of the understanding of the ecological role Pleistocene megaherbivores.  To evaluate the impact of a herbivore on a community’s vegetation, barriers or exclosures can be erected to exclude the animal from part of the community being studied, allowing researchers to observe the growth of vegetation in the presence and absence of herbivores.  In exclosure studies, the abundance of woody species within the exclosure often dramatically exceeds the area outside the exclosure that is accessible to a target megaherbivore, demonstrating the degree to which megaherbivore populations can supress woody species (Figure 1).  While browsers directly kill woody plants by stripping them of bark and leaves, grazers, like bison, indirectly kill woody plants through trampling.  An animal of considerable bulk can also reduce woody vegetation growth and abundance by various other means including uprooting, debarking, and excessive browsing.  Claiming large herbivores caused the shift from hardwood forest to prairie and savanna requires more evidence, such as an influx in Sporormiella spores preceding the decline in woody species’ pollen, yet the previously discussed paleoecological record and exclosure studies indicate it may have been possible.

Figure 1:  Exclosures – human-constructed and naturally occurring – demonstrate the increase in woody species when large herbivores are excluded from part of their usual range. (a) Kruger National Park, South Africa, (b) temperate wetland grasslands, The Netherlands, (c) logged Boreal forest, Quebec, Canada, (d) Borkener Paradies, Germany, and (e) temperate sagebrush and grassland, Yellowstone National Park, United States

Although large herbivores reduce the overall abundance of woody plants, their depredations afflicted woody plants unequally and conferred a competitive advantage on certain woody species.  Enclosure experiments indicate large herbivores cause the greatest reduction in the tastiest species, granting a competitive advantage to less palatable woody species resulting in a compositional shift towards these species within their communities. The influence of large herbivores similarly grants a competitive advantage to species that are more tolerant of browsing or who possess natural deterrents to herbivores.   The reduced shade resulting from a decrease in woody plant abundance can allow shade-intolerant species to supplant an area’s light-intolerant species.  North American exclosure studies have demonstrated that browsing can drive the transition from more dense deciduous forests to oak savannas.

Bakker et al. further discuss how geography modulates the impact of Pleistocene megaherbivores.   Since their impact is dependent on their density, their effect is greatest where geographic factors are conducive to more efficient foraging.  For example, modern herbivore communities achieve the greatest densities and diversity of species where the most fertile soil gives rise to higher quality vegetation.  Both Pleistocene and modern large herbivores are more common near bodies of water and in areas with more easily traversable terrain.  Based on the authors’ analysis, Iowa, with its flat to rolling terrain and rich soil, would be an ideal area to support a high density of bison.

One of the review’s particularly interesting aspects is its relevance to contemporary conservation efforts.  Currently there are many efforts being made to preserve and restore the habitats that were mostly destroyed by conversion to agricultural land. While these efforts often incorporate burning, an important disturbance in prairie ecology, such efforts usually ignore bison.  Yet the review’s description of the impact of megaherbivores, extinct and extant, on communities’ woody species indicates bison may have played a major role in the creation and maintenance of prairie and oak savanna in Iowa.  Given megaherbivores’ potential to influence the species composition of plant communities, it may be difficult to ever accurately replicate the structure of historical prairies and oak savannas without the use of bison or similar large herbivores such as cattle.

References:

Baker, R. G., E. A. Bettis III, D. P. Schwert, D. G. Horton, C. A. Chumbley, L. A. Gonzalez, M. K. Reagan. 1996. Holocene Paleoenvironments of Northeast Iowa. Ecological Monographs 66(2):203-234.

Bakker, E. S., J. L. Gill, C. N. Johnson, F. W. M. Vera, C. J. Sandom, G. P. Asmer, amd J.-C. Svenning. 2015. Combining paleo-data and modern exclosure experiments to assess the impact of megafauna extinctions on woody vegetation. Proceedings of the National Academy of Science doi: 10.1073/pnas.1502545112.

Knapp, A. K., J. M. Blair, J. M. Briggs, S. L. Collins, D. C. Hartnett, L. C. Johson, and E. G. Towne. 1999. The Keystone Role of Bison in North American Tallgrass Prairie. BioScience 49(1):39-50.