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.
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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
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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:
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.
