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THE EFFECTS OF WHITE-TAILED DEER POPUTALTION ON PLANT DIVERSITY AND
VEGATAION DENSITY
July 6, 2010
INTRODUCTION
Population ecology is the study of how a group of organisms relate to each other and their
environment. The carrying capacity is the maximum sustainable population for an area, and is
determined by a set of limiting factors, such as low food supply, disease, predators, and limited
space. Mortality, the death rate, and natality, the birth rate, affect the growth of populations.
When natality is slightly higher than
White-tailed deer (Odocoileus virginianus) gets their name from the underside of their
tail being covered in white hair, and when running holds it up so that the underside is visible.
The white-tailed deer is an herbivore and eats both woody and herbaceous plant species. The
normal habitat of the white-tailed deer is heavily wooded forests across the continent of North
America. Their mating season begins in September and lasts into late January.
When Europeans first came to America, white-tailed deer populations were free and
uncontrolled. They were a primary choice for hunters for their meat and fur. Trees were also
heavily cut down for lumber, and took away habitats for deer. Eventually, they were hunted so
much that there were fewer than 150,000 white-tailed deer across the country. With the
realization that the deer were dying out, hunting regulations were established, trees were
replanted and being conserved and local housing developments away from cites called suburbs
were created, giving white-tailed deer more habitat and protection from hunters. Also, major
predators of the deer were hunted and removed in order to increase the natality rate of deer and
decrease the mortality. Today, deer populations are higher than ever. In some places, there are
more than 140 per mile². There are more habitats for deer, like national parks, where hunting is
not allowed.
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Food selection by white-tailed deer is mainly a function of seasonal availability,
palatability, and nutritional factors. Palatability is distinction between food choices based on
taste. The white-tailed deer eat many herbaceous and woody stem species of plants, such as
clovers, berries, nuts, evergreen stems and leaves, and fruit. Besides palatability, white-tailed
deer choose food based on nutritional value as well. Sources of minerals like calcium, protein,
and energy. Protein is essential for proper growth, weight gain, appetite, and milk secretions.
Without proper protein intake, does run the risk of giving birth to an unhealthy fawn or not being
able to support the fawn once born, decreasing natality. Deer have a special need for high energy
foods during the breeding season and in cold weather. Many deer, especially fawns, die as a
result of energy deficiencies, increasing mortality.
High white-tailed deer populations cause depletion of favored vegetation and therefore
affect the diversity of ecosystems. Many forests have become overwhelmed with plants like heyscented fern and black cherry trees because they are the few plants that deer do not find
palatable. Hey-scented fern used to cover less than three percent of Pennsylvania forest floors.
Now, because deer devour its competition, this fern dominates more than a third of the forested
area in Pennsylvania and throughout the northern United States. The higher the deer populations,
the more of a competition for food there is between themselves and other wild animals. Bear and
squirrels are threatened by the white-tailed deer’s devouring of wild berries and nuts, along with
other plants that provided shelter for them. Not only do they affect other populations, but
themselves as well. The less food there is to eat, the more malnourished the white-tailed deer
become, increasing mortality. This also decreases natality because the does do not have enough
nutrients to support both a fetus and themselves, and have miscarriages.
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METHODS
Two sites were used to conduct the experiment: Catoctin mountain national park and
Frederick municipal forest. Both sites were located just outside of Frederick, Maryland. Catoctin
Mountain Park was our experimental site because of its high deer densities. Being a national
park, there is no hunting allowed at Catoctin national park. Also, its close proximity to Camp
David caused issues and a permit was needed in order to conduct the experiment. Frederick
municipal forest was used as our control site since it was especially set aside for environmental
reasons. Hunting is aloud there.
Data was collected on July 7, 2010. We measured the total stem abundance for
groundcover and understory levels. Plants that qualified as ground cover had to be less than
30cm tall. We counted all the plants that were inside our 1m² quadrats. All different species of
plants were recorded and then individually counted the number of stems for each species. Grass
species were counted in clumps instead of by stems because grass is a clumping species. Any
plants that were not identified during testing were brought back to the lab for further research
into their identification. Plants that qualified for understory had to be greater than 30cm tall but
less than 5cm in diameter at breast height (about 1.37m). We counted the understory in 4m²
quadrats. All species found were identified and recorded. Plants that were preferred by deer were
picked out and tested between the two study sites.
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We used the Shannon-wiener index to calculate species richness and species evenness.
We also used a robel pole to measure the cover density of the vegetation at each site. The robel is
1.5m in length and has 15 tape rings spread 10cm apart. The pole is placed in the middle of the
quadrat. While one person holds the pole, another person walks 4m out and crouches to about 1
m in height and counts the number of rings the can see on the pole. This is repeated 3 more
times, 1 for each cardinal direction. The four measurements were than averaged together for a
mean cover density for each quadrat.
RESULTS
We found that there was a significant difference in the preferred groundcover stem
abundance, cover pole density, preferred understory stem abundance, and understory Shannonwiener index data. At CMP we found that there was a total of 17 different plant species in the
groundcover and understory, while at FMF there was 26 different plant species.
We found that there was no significant difference in groundcover stem abundance
between CMP and FMF. The average stem abundance of each quadrat at CMP was 16.7 and the
standard deviation was 14.6. The average stem abundance of each quadrat at FMF was 20.5 and
the standard deviation was 8.6. We also found that there was no significant difference between
the understories between each of the study sites. The average stem abundance of each quadrat at
CMP was 2.3 and the standard deviation was 6.9. The average stem abundance at FMF was 8.2
and the standard deviation of 6.5.
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We found that there was a significant difference in the preferred stem abundance in
groundcover at FMF over CMP with FMF having an average preferred stem abundance of 6.7
and a standard deviation of 6.5, while CMP had an average preferred stem abundance of 1.1 and
a standard deviation of 1.3. We also found that there was a significant difference in preferred
stem abundance in understory at FMF over CMP with FMF having an average preferred stem
abundance of 1.9 and a standard deviation of 2.1, while CMP had an average preferred stem
abundance of 0.1 with a standard deviation of 0.3.
We found that there was a significant difference in Cover pole density at FMF over CMP
with FMF having an average cover pole density of 3.6 and a standard deviation of 3.7, while
CMP had an average density of 0.45 and a standard deviation of 1.4.
We found that there was no significant difference in the Shannon-wiener diversity in the
ground cover. FMF had an average of 1 and a standard deviation of 0.4 and CMP had an average
of 0.6 and a standard diversity of 0.5. We found there was a significant difference in understory
Shannon-wiener value at FMF over CMP with FMF having an average of 0.6 and a standard
deviation of 0.8 and CMP having an average of 0 and a standard deviation of 0.
CONCLUSIONS AND DISCUSSION
The results of our study show that we can reject our null hypothesis for ground cover
preferred stem abundance, understory preferred stem abundance, cover pole vegetation density,
and understory Shannon-Wiener diversity. We fail to reject all other of our hypothesis for
groundcover and understory preferred abundance, and groundcover Shannon-Wiener index.
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Therefore, our research shows that the white-tailed deer population does affect vegetation
density, preferred stem abundance, and diversity at the understory level, but does not affect the
overall stem abundance.
We observed a difference in understory stem abundance between CMP and FMF, but our
statistical test showed that it was not significant. This might have been caused by outliers in our
understory data at CMP. One of the test plots there was overly abundant og blueberry shrubs
(Vaccinium ovalifolium) at the understory level compared to the rest of our test plots there. This
one plot might have increased the mean stem abundance collected as well as the variance of the
data. With a higher mean and variance, the two test sites have a greater chance of not being
statistically different in regards to the t test. We found that the preferred abundance was
significantly different between our two test sites, but total stem abundance was not. We believe
this is because since deer eat palatable food over non-palatable food, the non-palatable food
might be taking over the preferred food once was. So, if the white-tailed deer eat all of the
palatable food, CMP and FMF will be left with dominantly non-palatable food. Our data also
showed that there was a significant difference for Shannon-wiener at the understory level but not
at groundcover. This could have been influenced by seeds sprouting in the groundcover that may
soon be eaten. Also, the understory shows the effect over time, meaning that it takes longer for
understory plants to grow than groundcover. So if the groundcover plants are eaten before they
have the possibility to grow into understory plants, then there will not be an understory.
These results can help show how deer are impacting the ecosystem. High deer densities
might change the composition of future forests by over eating groundcover plants that would
normally mature into shrubs and trees. The over story at each of our sites is mainly made up of
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oak trees, but while testing, we saw many maple tree saplings, showing that in the next fifty to
eighty years, we might expect a change in the over story at CMP and FMF, or lose it all together.
In order for there to be a change in understory, white-tailed deer densities will need to be
reduced. This will give plants and trees a better chance of surviving to adulthood. This would
also help restore plant species that have been extirpated by the deer. Extirpated means that a
plant species would be almost completely removed from one area. Plants like blueberry shrubs
and hay-scented fern are becoming more abundance because the deer do not eat them as much
because they do not find them palatable. With less plant sources available for deer and other
herbivores, there will in effect be less herbivores overall. Having a small herbivore population
will cause there to be a smaller carnivore population, since they will have limited food supplies.
Less plants sources will also cause there to be less cover for birds and squirrels, increasing their
own mortality. The high white-tailed deer populations will eventually affect themselves. Their
mortality will increase because less food sources will lead to starvation. Natality will decrease
because of malnutrition for does. Also, a lack of understory vegetation will lead to less cover for
fawns, increasing their likelihood of being killed by predators. The white-tailed deer might start
emigrating in order to find better food resources.
There are many problems and improvements that can be made for future studies. Some
plants found at CMP were not found at FMF and vice versa. This could have been caused by the
way the experiment was conducted and not actually on the composition of the forests. Ways to
improve future studies might be to enlarge the sample size and quadrat size, and to do more
transects in different directions. The greater the area tested, the more accurate our results will be
and the more representative of the entire forest it will be. Also, if there were twice as many
observers and recorders, there would be less of a probability for human error. Further questions
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regarding our research and related studies might be are the deer leaving permanently or just for
dinner? Are other herbivore populations affecting vegetation as well? Did the time of our study
affect our results?
TABLES AND GRAPHS
Understory Stem Abundance
20
15
10
5
Mean
0
CMP
FMF
-5
-10
Test sites
Understory Preferred Stem
Abundance
5
Axis Title
4
3
2
Mean
1
0
-1
Cmp
FMF
Study sites
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Understory Shannon-Wiener
Diversity
1.5
Axis Title
1
0.5
Mean
0
CMP
-0.5
FMF
Study Sites
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REFERENCES
Carson, W., Banta, J., Royo, A., Kirschbaum, C. (2005). Plant Communities Growing on
Boulders in the Allegheny National Forest: Evidence for Boulders as Refugia from Deer
and as a Bioassay of Overbrowsing. Natural Areas Journal, 25(1). 10-16.
Enger, E., Ross, F., Bailey, D. (2007). Concepts in Biology: Twelfth Edition. Boston: McGraw
Hill.
Fergus, C., Shope, Bill. (2007). White-tailed Deer. Wildlife Notes.
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Halls, L. (1984). What Do Deer Eat and Why. Wildlife Management Handbook. 11-16.
http://cnrit.tamu.edu/crgm/IRR2/1984/What%20Do%20Deer%20Eat%20And%20Why.p
df
Levy, Sharon. (2006). A Plague of Deer. BioScience, 56 (9). 718-721.
Liscinsky, Stephen A., et al. (2001). What Do Deer Eat? Pennsylvania Game News.
Miller, Scott G., Bratton, Susan P. (1992). Impacts of White-tailed Deer on Endangered and
Threatened Vascular Plants. Natural Areas Journal, 12(2). 67-74.
Porter, William F., Underwood, Brian H. (1999). Of Elephants and Blind Men: Deer
Management in the U.S. National Parks. Ecological Applications, 9(1). 3-9.
Sandt, Joshua L. 1997. A Brief History of Deer in Maryland and the Northeast. In Deer as Public
Goods and Public Nuisance: Issues and Policy Options in Maryland, ed. Bruce L.
Gardner, pp. 1-3. College Park, MD: Center for Agricultural and Natural Resource
Policy, October 27.
Toops, Coonie. (1999). By Leaps and Bounds. National Parks, 73(9). 30-33.
Waller, Donald M., Alverson, William S. (1997). The white-tailed deer: a keystone herbivore.
Wildlife Society Bulletin, 25(2). 217-226.
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