BIOL 2300 Laboratory #4
Erin Carter
7616892
March 16, 2011
1. Null Hypothesis (H0): There are no differences in the frequency of prey species
consumed by the fork predator in comparison to the spoon predator.
Prediction: There will be some differences between the frequency of prey species
consumed by the fork and spoon predator. This is primarily expected due to the fact that
forks are not adapted to eat styrofoam balls whereas the spoon predators can consume
this prey. Therefore, there will be at least one difference in the number of prey species
consumed by spoons and forks, being no ball species and some ball species consumed
respectively. Also since the styrofoam balls contain more energy units, (5 units versus
only 1 unit for any other prey type) we predict that the spoon predator will catch ball prey
at a high frequency initially to maximize their own net energy gain. Next, we expect that
the number of toothpick prey consumed will differ between the predators since the fork’s
mouth will be better adapted at catching this prey type compared to spoons. The forks are
said to be better adapted in this situation because their mouths can “lift” and catch
toothpicks much more easily and efficiently than spoons, that may not be able to catch
any toothpicks at all. These differences in adaptations to the limited resources of
toothpick, string, pasta, and ball prey types will most likely lead to differences in the
frequency of prey types in each predator’s diet.
2.
7
Number of Preadators
6
5
4
Fork
3
Spoon
2
1
0
0
1
2
3
4
5
6
7
8
9
10
Time (Trials)
Figure 1. The extinction curves for the fork and spoon predators (how the number of each
changed over time).
3.
140
120
100
80
Fork
60
Spoon
40
20
0
Toothpick
String
Pasta
Ball
Figure 2. The total number of prey species consumed by fork and spoon predators over all trials,
which shows the amount of dietary overlap.
4. Table 1. The proportion of each prey species consumed by each predator over all ten
trials combined.
Prey Species
Forks
Spoons
Toothpicks
0.4066
0
Strings
0.3765
0.3018
Pasta
0.2169
0.5858
Balls
0
0.1124
Sample Calculation:
Total toothpick prey consumed by forks = ∑ (Trial 1 consumption) + (Trial 2
consumption) +...
= 0 + 3 + 3 +2 + 9 + 16 + 28 +28 +26 + 20 = 135 toothpicks consumed by forks in total
Proportion of toothpick prey consumed by forks =
Total number of toothpicks consumed/ ∑ (All prey species consumed)
= 135/ (135 + 125 + 72 + 0) = 135/332 = 0.406626506 = 0.41
5. Morisita’s Index:
C = 2 ( ∑ pij pik )
∑ (pij2 + pik2)
= 2 [(0.4066 x 0) + (0.3765 x 0.3018) + (0.2169 x 0.5858) + (0 x 0.1124)]
[(0.4066)2 + (0)2] + [(0.3765)2 + (0.3018)2] + [(0.2169)2 + (0.5858)2] + [(0)2 + (0.1124)2]
= 0.4814/0.8009  C = 0.60
6. Table 2. Calculating and comparing the expected frequency (Fexpected) with the observed
frequency (Fobserved) of each prey type consumed. (using Fexpected = Ci x Ri / N).
Prey Types
Toothpicks
Fexpected
Strings
Fexpected
Pasta
Fexpected
Balls
Fexpected
Total
Fork
Spoon
Total
135
(332x135)/501 =
89.46
125
(332x176)/501 =
116.63
72
(332x171)/501 =
113.32
0
(332x19)/501 = 12.59
332 (C1)
0
135 (R1)
(169x135)/501 = 45.54
51
176 (R2)
(169x176)/501 = 59.37
99
171 (R3)
(169x171)/501 = 57.68
19
(169x19)/501 = 6.41
169 (C2)
19 (R4)
501 (N)
Chi-squared test: χ2 = ∑ (Fobserved – Fexpected)2 / Fexpected
Sample Calculation:
χ2 = ∑ [(135-89.46)2] / 89.46 + [(125-116.63)2] / 116.63 + [(72-113.32)2] / 113.32 +
[(0-12.59)2] / 12.59 + [(0-45.54)2] / 45.54 + [(51-59.37)2] / 59.37 + [(99-57.68)2] / 57.68 +
[(19-6.41)2] / 6.41 = 127.7598...
χ2 = 127.76
The large χ2 test statistic arises from large differences between expected and observed
frequencies of genotypes. This means that the frequency of prey species consumed changed
significantly between fork and spoon predators. In fact, by calculating the degrees of freedom
(found to be 3 in the lab report), a table of critical χ2 values for df=3 can be used to show
statistical significance. At a 5% level of uncertainty (95% certainty), the table gives a χ2 value of
7.815. When compared to our calculated value of 127.76, our value is clearly higher. This large
deviation between expected and observed frequencies are more than predicted if there had been
no change in the frequency of prey types consumed between forks and spoons. Therefore, our
results are statistically significant at the 95% level of certainty. Because of this, we reject our
null hypothesis with a certainty level of 95% and df=3.
7. a) Yes, competitive exclusion did occur in our experiment. The result of interspecific
competition between the fork and spoon predators lead to the elimination of the spoons
through competitve exclusion. This effect was driven by exploitation competition, where
the spoons and forks interacted indirectly. The spoons reacted to the decreased level of
the limited prey resources after they had been exploited by the forks, eventually leading
to competitive exclusion and elimination of the spoon species. This can be seen in Figure
1, where the number of spoon predators quickly decreases to zero by the end of the time
period. Therefore, we can determine that the two species cannot coexist in the same
environment over time.
b) By using Table 1, we can clearly see the proportion or how much of a certain prey
species is within the predators diet. First when looking at forks, the proportion of
toothpick, string, and pasta prey in its diet is relatively even (0.406, 0.3765, 0.2169).
Because of this, we could categorize forks as being generalists since they consume all
prey species at close amounts. The exception to this is the ball prey, but they were not
physically adapted to catch this prey for the purposes of this experiment. On the other
hand, the spoons could be categorized as more of specialists. This is because the
proportions of each prey varies more significantly in their diet (0, 0.3018, 0.5858,
0.1124). The spoons in this experiment exploited a narrower range of resources, with
only high amounts of string and pasta prey. In fact, the toothpick prey were not consumed
at all by spoons since they were outside their range of resources as specialists. Also,
while balls were preferred by spoons due to their high energy content and ability to
maximize their net enery gain, they were not caught at high frequencies since there were
few in the environment and few were needed to “fill up” a spoon.
c) There was a considerably high amount of dietary overlap between the spoon and fork
predator species. We say this because after calculating Morisita’s index of diet overlap, a
value of C = 0.60 was obtained. In the index, a value of 0 indicates no overlap and 1
being complete overlap. Therefore, a value of 0.60 would mean a considerable amount of
dietary overlap since it is greater than a half. Also, the amount of overlap in prey species
consumed between spoons and forks can be seen in Figure 2.
d) By using a chi-squared test, we were able to determine that yes, the frequency of prey
species consumed does differ significantly between the two predator species. In fact, the
χ2 = 127.76 was larger than the χ2 = 7.815 at 95% certainty and also the χ2 = 16.266 at
99.9% certainty. Because our χ2 was larger, we can reject our null hypothesis of no
difference between the frequencies of prey species consumed at 95% certainty (0.05
uncertainty) and even at 99.9% certainty (0.001 uncertainty).