Bryan
Hummel
Ecology 3434
Ribble;
8:30 TR
February
23, 1999.
Introduction:
An experiment was carried out to
test the predation risks on Fox Squirrels (Sciurus
niger) during forging on a college campus.
The results were expected to show that there was little risk of
predation on the campus.
Methods:
Each of 5 students made 4 feeders
with 100 grams of sand covering 50 grams of sunflower seeds. The feeders were placed in different areas
around campus. One tray was placed next
to the trunk of a tree (near) and the second was placed at a distance of 10
meters (far). The third and fourth were
done in a similar fashion, one near and the other far. The trays were then left alone for several
hours (2.5 hours to 5 hours). At the
end of this time, the trays were collected, the sand was screened out and the
remaining intact seeds were weighed to determine the amount of seeds consumed.
Results:
The seed consumption at each feeder
was recorded in Table 1. The total
amount of seeds consumed was 51.4 grams for the trays placed near the trees and
58.5 grams for the trays that were 10 meters away. Because the standard deviations were 4.8 and 4.9 respectively,
there is an overlap in the possible values.
Table 1 also shows that there were 4 sites where there were no seeds
consumed.
|
|
Weight of seeds left
(grams) |
Weight of seeds
consumed (grams) |
||||
|
|
Near |
Far |
Near |
Far |
|
|
|
Bryan |
42 |
45 |
8 |
5 |
|
|
|
|
44 |
44 |
6 |
6 |
|
|
|
Jennifer |
47.5 |
48 |
2.5 |
2 |
|
|
|
|
47 |
44 |
3 |
6 |
|
|
|
Matt |
38.1 |
37 |
11.9 |
13 |
|
|
|
|
36 |
40 |
14 |
10 |
|
|
|
Libby |
50 |
36.5 |
0 |
13.5 |
|
|
|
|
50 |
50 |
0 |
0 |
|
|
|
Vanessa |
46 |
50 |
4 |
0 |
|
|
|
|
48 |
47 |
2 |
3 |
|
|
|
TOTALS |
448.6 |
441.5 |
51.4 |
58.5 |
|
|
|
Average |
44.86 |
44.15 |
5.14 |
5.85 |
|
|
|
Standard Deviation |
|
4.815069 |
4.933164 |
|
||
Table 1.
Table 1 compiles the experimental
data into the first two columns. The
weight of seeds consumed was calculated, as well as the totals for all 4
columns. The average and standard deviation
were also calculated.
Discussion:
Evaluation of Table 1 shows us that
there was no significant difference between the feeders placed near the trees
and the trays placed at a distance.
Assuming that the seeds were eaten by squirrels and not by birds, this
confirms our hypothesis that the predation risk on the squirrels would be
minimal. At a college campus there are
several factors that would decrease the effects of predation. First, the campus is very well maintained
and always traversed with humans, which makes for an undesirable habitat for
most natural small animal predators.
The lack of predators over several generations of squirrels would
decrease the squirrels concern of predation.
The abundance of humans at all hours of the day and night would also
have an effect on the squirrels. The
squirrels would grow accustomed to people walking by, and thus they would not
feel threatened by the humans who walk by during their feeding.
There was a difference in the number
of grams consumed at the near feeders as compared to the farther feeders even
though the difference was deemed insignificant by the standard deviations. The standard deviations in this experiment
are not absolute because they could be changed with several factors such as
number of feeders or length of time the feeders were available. To explain the difference we can look at
other factors besides predation.
Predation is just one variable in the squirrels decision to stay and eat
or to leave in retreat. We can use an
equation that relates the rate of food harvest (H), the energetic costs of
finding and handling the food (E), the cost of missed opportunities (M), as
well as the risk of predation (P). This can be written as H > E + M + P,
meaning as long as the benefits of the harvest (H) outweigh the costs of the
harvest, the squirrel should stay and eat until the benefits lower or the costs
rise. The food consumption with the harvest and the missed opportunities are
constant because of the amount of food per tray is constant, thus we must look
at the energetic costs and predation.
Since predation is most likely of little importance on a college campus,
the focus will fall on the energetic costs of finding and handling the
food. Since squirrels spend a great
deal of their time in trees, it seems reasonable that they would find the
feeders close to the trunk before finding the ones further away from the
trunk. The handling time of actually
splitting and eating the seeds should be constant at either spot, so the
difference could be attributed to actually finding the feeders. There may still be an effect by predators
such as cats, but most of the cats are shy and they do not hang around during
the day. I would assume that the
difference in consumption between near and far is because of the energy used to
find the feeding trays more than predation, but if the experiment were
expanded, the data would surely change and allow for a better comparison.
Bryan Hummel
Ecology 3434
Ribble; 8:30 TR
February 23, 1999.