Photograph taken by
author at Neal Smith Wildlife Refuge
Imagine yourself wandering through the
Iowa tallgrass prairie, you admire the lush tranquility of the big bluestem and
compass plant, perhaps you stop to
pick a couple of wild blackberries while listening to the call of the red-headed
woodpecker. Suddenly, as if out of nowhere, you are confronted by a thunderous
sound that seems grow louder and louder and discover the source of the noise
appears to be the footsteps of a herd of large, four-legged animals. As these
great beasts approach, you see clearer and clearer a herd of bison. Nowadays, this
scene seems almost impossible but during the middle Holocene, 7,000 to 8,500 years
ago, bison ruled the prairie. This species served as a keystone
species: creating necessary
disturbances such as grazing and wallowing to continue the success of native
prairie plants. While ecologists now know that there were indeed large
populations of bison, little is known about the everyday habits of these
animals. What did they eat? Where did they drink? How far did they travel? All
of these questions become even more pertinent when considering that during the
middle Holocene, there was an increase in aridity and drought. Current research
suggests that bison respond to a drier climate by increasing their range
(Fortin et al. 2003), which might
change their diet and water source. So the question remains: how did the bison
population in the eastern Great Plains respond to climate changes in the middle
Holocene? Answering these questions will not only provide insight into bison
behavior but can also reveal the history of Iowa vegetation. Since the bison’s
diet consisted of the flora available, the content of their diet should reflect
the prairie’s composition.
A study conducted by Widga
et al. (2010) set out to answer
where the middle Holocene bison wandered and what they ate. Using fossils of
bison from five different populations located in tallgrass prairie and
prairie-forest border in Iowa, Minnesota and Nebraska, the researchers used
isotopic analysis to determine the diet, water source and seasonal migration
patterns of each herd. Isotopes
are a type of a specific element containing a different number of neutrons than
the typical version of the same element, making it heavier or lighter. Isotopes
are useful for ecologists because certain natural processes discriminate
between isotopes in their incorporation of elements. Thus, these natural
processes experience different isotopic ratios than would normally be expected.
Analyzing the ratios of isotopes can lend insight into the historical behaviors
of bison by comparing the different isotopic ratios in the fossils of bison
compared with the expected ratios in the sources of forage and water.
Map of Locations of
Bison Fossils Studied (Widga et al. 2010)
The bison populations sampled were
selected because there were at least 29 individuals in each herd, and their
deaths appeared to occur simultaneously. This study used tooth enamel as the source
of isotopes as tooth enamel can easily reveal age allowing for a comparison of
behaviors within the individual and between herd mates of the same age and
different ages.
13Carbon isotopes were
used to analyze the bison’s’ diet. Tallgrass prairie plants incorporate carbon
dioxide during photosynthesis in two distinct ways. The different pathways and
the plants that utilize them are referred to as C3
and C4 plants. Most grasses are C4 plants, while
woody vegetation is typically C3. The 13C to 12C
ratio is higher in C4 plants. Since bison eat these plants and
incorporate the carbon throughout their bodies including their teeth, investigating
the ratio of 13C to 12C in the bison teeth will lend
insight into the types of plants the middle Holocene bison ate. Widga et al. (2010) found that most of the
bison sampled had diets higher in C4 plants than C3
plants. In fact, the bison’s’ diets in all but one herd consisted of 60-85% C4
plants. This insight on the bison’s diet enables us to better understand the
composition of the tallgrass prairie during the middle Holocene, through the
assumption that the bison ate a diet representational of the prairie flora. Therefore,
we can conclude that the prairies in northwestern Iowa, northeastern Nebraska
and southern Minnesota were roughly composed of 60-85% C4 plants. These
findings add detail to a study conducted by Baker et al. (1996), which investigated vegetation composition in
northeast Iowa using a variety of methods including pollen fossils, vascular
plant macrofossils and isotopic evidence. Their study found that at this time
there were more mesic deciduous
forest species, C3 plants, than C4 grasses in northeastern Iowa. However using
the same methods, Baker et al. (1996)
found that 5,500-3,500 years ago (about 2,000 years after the bison
investigated by Widga et al. (2010)
died) northeast Iowa experienced an ecosystem transformation from forest to
prairie. Thus, the bison data reveals that prairie arrived in northwest Iowa
before spreading into northeast Iowa.
Widga et al. (2010) used 18Oxygen as a proxy to determine what
types of water sources bison utilized during this period. 18O is
heavier than 16O and because of the increase in mass, 18O
is less likely to be evaporated than 16O. In the tallgrass prairie,
there are a variety of water sources available to bison. Sources such as
ephemeral or upland lakes and ponds experience intense evaporation, especially
during the summer. At these water sources, there would be a high ratio of 18O
to 16O. In contrast, more stable water sources such as large lakes
or rivers have low ratios of 18O to 16O. Therefore, bison
were more likely to consume 18O in water sources prone to
evaporation. The researchers found high concentration of 18O in the
teeth sampled, revealing that these bison utilized water sources susceptible to
high levels of evaporation. The bison may not have found it necessary to travel
outside their normal water sources in the middle Holocene despite the increase
in drought conditions.
The last question investigated by
Widga et al. (2010) concerned the
movement of bison. Did the bison respond to the increase in temperature with an
expansion in movement and home ranges, or as with diet and water sources, did
the bison stick with similar patterns before the climate change? To answer this
question, researchers compared isotopic ratios of 86Strontium and 87Strontium
found in the bison’s teeth to the ratio found in the soil of different geologic
areas where the bison may have grazed. Different soil types naturally contain
different concentration of 86Sr and 87Sr, so one can
differentiate which substrate type a bison was grazing on by the concentration
found in the bison’s teeth. If the ratios of 87Sr and 86Sr
found in the bison teeth match the soil surface ratio, it would be expected
that these areas were utilized by the bison. Surprisingly, few bison of the
eastern Great Plains actually traveled across more than one examined area.
Further, every herd’s home ranges were smaller than expected, with a radius
less than 50 kilometers. These data reveal that the bison did not respond to
climatic pressures by searching for literally greener patches. This finding
additionally indicates that despite rising temperature, the bison had enough to
eat in these prairies.
It appears from this study that the
bison during the middle Holocene adopted a policy of if ain’t (that) broken,
don’t fix it. The bison did not change their diet, water source or mobility in
response to the increase in temperature and the expected limitation of
resources that accompanies low water availability. Surprisingly the habits of
the bison from the middle Holocene closely mimic the behaviors of bison today.
One explanation for this finding may be that the flora that Bison grazed in the
middle Holocene may be surprisingly similar to today’s reconstructed prairies
where the bison are currently confined due to the destruction of almost all of
C4 tallgrass prairie in Iowa. Importantly, these findings contradict
the belief that bison movement and grazing preferences were strongly influenced
by the structure of local resources. From Widga et al.’s (2010) findings, we can glean that bison roamed the
eastern Great Plains tallgrass prairie year round, creating a constant influence
on the behaviors and growth of tallgrass prairie species. While the ecosystem
and climate have changed and shifted over thousands of years in the eastern
Great Plains, bison activities concerning diet preferences, water source
utilization and migration patterns have stayed fairly constant. Hence, at least
for the middle Holocene eastern Great Plains bison, old habits truly die hard.
References
Baker, R. G.,
Bettis III, E.A., Schwert, D.P., Horton, D.G., Chumbley, C.A., Gonzalez,
L.A. and Reagan, M.K. 1996. Holocene
Paleoenvironments of Northeast Iowa.
Ecological Monographs 66(2):
203-234.
Fortin, D.,
Fryxell, J.M., O'Brodovich, L. and Frandsen, D. 2003. Foraging ecology of
bison at the landscape and plant
community levels: the applicability of energy
maximization principles. Oecologia 134: 219–237.
Widga, C.,
Walker, J.D. and Stockli, L.D. 2010. Middle Holocene bison diet and mobility
in the eastern Great Plains (USA)
based on 13C, 18O and 87Sr/86Sr analyses of
tooth enamel carbonate. Quaternary
Research 73 (2010) 449-460.
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