Determining the Population of Sciurus carolinensis on the
University of Mary Washington’s Campus
BIOL 322 Animal Ecology
November 20, 2012
by
Joseph W. Bryan
2
ABSTRACT
Knowing the relative abundance and density of a species is helpful in many
preservation methods. The purpose of this paper is to show the estimated population size,
absolute density and distribution of Sciurus carolinensis on the University of Mary
Washington’s campus. Counts were collected using the quadrant sampling method, the
data collected was analyzed, and several calculations were made to determine these
important ecological statistics. The data shows that this organism exhibits clumped
distribution and has an expected population size and absolute density for the ecosystem
of the campus.
INTRODUCTION
Precise and accurate population estimation is a vital skill a successful ecologist
possesses. This ability is valuable when designing species conservation methods or
analyzing a species’ effects on the environment. In this study, the population of Sciurus
carolinensis on the University of Mary Washington’s campus in Fredericksburg, Virginia
was estimated. S. carolinensis, the gray squirrel, is native to eastern North America
(UNEP, 2010). These squirrels exhibit home ranges of 1 to 2 or 1 to 4 acres for females
and males respectively (Fuller, 2004). They are most active either 2 hours after sunrise or
2 to 5 hours before sunset during summer months, or 2 to 4 hours before sunset during
winter months (Koprowski, 1994). Both the time of year and the weather conditions
affect the squirrel’s activity. They are typically less active during winter months and
during inclement weather, such as high winds or heavy rain (Koprowski, 1994).
Therefore, the best-suited time to count squirrels would be during the peak hours of
activity under dry and temperate weather conditions. Gray squirrels may exhibit varied
3
densities depending on the availability of tree seeds, a major food source. This indicates
that their densities will be highest in wooded landscapes. For example, the gray squirrel’s
typical density in small woodlots is approximately 6 squirrels/acre (Koprowski, 1994).
Because the University of Mary Washington’s campus contains woodlots as well as open
areas with buildings, the densities throughout the habitat will vary. The purpose of this
study is to estimate the population of grey squirrels on campus to determine their
distribution, density, and other ecologically significant information.
METHODS
This study was performed on the University of Mary Washington’s campus in
Fredericksburg, Virginia. The area is approximately 3209 feet in length and 1020 feet in
width, giving a total area of roughly 75.14 acres. This area was divided into 24
rectangular quadrants, each with dimensions 802 feet by 170 feet, or approximately 3.13
acres (Figure 1). This shape and size were chosen to reduce edge effect and accommodate
the squirrel’s home range, respectively. To determine the number of quadrants that
should be counted two-stage sampling was implemented, in which 3 quadrants were
chosen at random using a random numbers table. After quadrants 1, 6 and 18 (Figure 1)
were measured and marked off using flags, two others and I spread out equidistantly,
walked the length of each quadrant and counted the squirrels that were observed. Each
count took place during dry weather in the afternoon. With the preliminary data, the
following equation was used to determine the number of quadrants needed to be counted
to obtain a 45% level of accuracy: n =
t 2 s2
(Equation 1), where n is the number of
d2
quadrants that should be counted, t is the student’s t-value with 95% reliability and
4
2
infinite degrees of freedom representing a value of 1.96, s is the variance of the mean
number of squirrels observed from the three preliminary observations, and d2 is the mean
of the number of squirrels observed from the three preliminary observations times the
chosen level of accuracy. The counts were converted into an estimate of total population
^
size using the Area Density Method with the following equation: N =
A
´ å ni (Equation
a
^
2), where N is the estimated population size, A is the number of quadrants occupied by
the total population, a is the number of quadrants sampled, and ni is the number of
individuals counted in the ith sample area. The variance of the population was determined
é
2
é A2 - Aa ù ê aå ni - å ni
using V^ = ê
ú´
N
a ( a-1)
ë a û êê
ë
(
) ùú(Equation 3), where V
2
ú
úû
^
is the variance of the
N
estimated population size. To better report the fluctuation in the data gathered, a 95%
confidence interval (CI) was calculated using the following equation:
95% CI =
2s
(Equation 4), where s is the standard deviation of the samples collected and n is the
n
number of samples collected. The distribution was determined by dividing the sample
_
variance (s2) by the sample mean ( x ) and comparing that value to 1. A value equal to 1
indicates random distribution, a value significantly greater than 1 indicates clumped
distribution, and a value significantly less than 1 indicates regular distribution. The
significance was determined using the following chi square test: c 2 =
(n-1)s2
_
(Equation
x
5), where c 2 represents the calculated chi square value, n represents the number of
5
_
samples gathered, and s2 and x represent the above-mentioned values. The calculated chi
square value was then compared to a critical chi square value (Figure 2). Finally, the
density was determined by estimating the total population size and dividing that number
by the size of the campus to give a unit of squirrels per acre.
RESULTS
Preliminary counts in quadrants 1, 6, and 18 resulted in 5, 8, and 18 squirrels,
respectively. From this data and using Equation 1 it was determined that 9 quadrants
needed to be counted to obtain 45% accuracy. All counts were conducted according to
the methods described above and the results are listed in Table 1. Using Equation 2, the
total population was estimated to be 155 squirrels. From this value and Equation 3, the
variance of the estimated population size was calculated to be 1271. Equation 4 gives a
95% confidence interval of 3.76. Dividing the sample variance by the sample mean
results in a value of 4.93. To determine if this value is significantly greater than 1,
Equation 5 was implemented, resulting in a calculated chi square value of 39.45. This
value was greater than the value indicated on Figure 2, showing that 4.93 is significantly
greater than 1 and indicating clumped distribution. Finally, the density of the grey
squirrel population was calculated to be 2 squirrels/acre.
DISCUSSION
The results of this study were somewhat expected. First, the clumped distribution
was anticipated due to the scattered woodlands where squirrels would most likely inhabit
and the numerous buildings where squirrels would not be located. Next, the overall
density seems realistic as well. The University of Mary Washington’s campus is not
suitable to support a high density of squirrels. Only small areas of the campus are wooded
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areas where one would observe higher densities. Finally, the estimated population size,
though low, appears to fit the general environment of the campus. The low number may
have been due to the cold weather during which the counts took place.
LITERATURE CITED
Fuller, Jimmy. “Gray Squirrel.” Radford University. 8 April 2004.
http://www.radford.edu/~jfuller/graysquirrel.htm 7 October 2012.
Koprowski, John L. 1994. Sciurus carolinensis. Mammalian Species 480: 1-9.
UNEP-WCMC. 2010. Review of the Grey Squirrel Sciurus carolinensis. UNEP-WCMC,
Cambridge.
Table 1. Data collected for Sciurus carolinensis inhabiting the University of Mary
Washington’s campus in Fredericksburg, Virginia including sample number, quadrant
counted (Quad.), and number observed (Num. Obs.). Data was collected between October
26, 2012 and November 17, 2012.
Sample Number
Quad.
Numb. Obs.
1
1
5
2
2
11
3
3
0
4
6
8
5
7
1
6
9
2
7
15
8
8
16
5
9
18
18
Total:
58
Figure 1. University of Mary Washington’s campus divided into 24 equal quadrants.
Figure 2. Chi Square Table with the comparison value circled in red.
Preliminary = P
Final = F
P
Quadrant Counted
Number in Quadrant
1
5
P
6
8
P
18
18
F
2
11
F
3
0
F
7
1
F
9
2
F
15
8
F
16
5