Lab 6: Measuring diversity
Team Grey Van: Eric North, Andrew Testes, James Werner
Abstract:
Urban forest patches are classified as either remnant, emergent, or planted. In this
experiment we compared the tree species diversity of an emergent and a remnant patch.
For each patch a Simpson’s Index of diversity was calculated. The remnant stand was
found to have higher species richness with a value of six, one higher than the emergent
patch. The emergent patch had a higher Simpson’s index of diversity due to its greater
evenness.
Introduction:
Urban forest patches are generally classified as one of three patch types: remnant,
emergent, and planted. Remnant patches are characterized by being late successional,
uneven aged stands. Emergent patches are early successional stands, following
disturbances to the landscape. Mortality is more often due to competition and
environmental stress, not age as it is in remnant patches. Planted patches are simply
areas in which trees have been planted. They often include non-native species. In this
lab we looked at a remnant and an emergent patch and measured their species
composition in order to evaluate species richness and evenness. We expect the emergent
patch to have a higher diversity. This is because more sunlight reaches the forest floor,
and there is less pre-existing plant competition.
Methods:
We traveled to an emergent forest (early succession/ recent disturbance) and a
remnant forest (old growth) and tallied trees within a 1/20 acre plot. For our first plot we
set up a cruise line at the edge of each forest and walked a chain (66 ft.). We then placed
the second plot in the other direction perpendicular to the original line. And the third plot
was placed two chains from the edge of the forest forming a “T”. We used three existing
plots at site two. We then used the Simpson’s diversity index to calculate diversity of
each site. Simpson’s diversity index calculates the probability that if two individuals are
selected at a site that the two individuals will be of different species.
Results:
Figure 1
frequency of tree/acre
Species diversity between Emergent and Remnant forest
240
230
220
210
200
190
180
170
160
150
140
130
120
110
100
90
80
70
60
50
40
30
20
10
0
emergent
remnant
sugar maple
ash
dogwood
tree a
black cherry
tree species
black oak
white oak
red maple
hickory
Figure 2
Emergent
Simpson’s
index of
diversity
0.63856254
remnant
0.500717
The remnant forest has a greater variety of species while the emergent forest has a
greater number (richness) of species. The emergent forest has a higher Simpson diversity
index.
Discussion:
In a comparison of species richness we see that the remnant patch has six tree
species while the emergent has five. However, although the remnant patch has greater
species richness, it has less diversity according to the Simpson’s diversity index. The
indices show that it is nearly 14% more likely that two individuals chosen in the
emergent patch will be different species, compared to that chance in the remnant patch.
The emergent patch’s diversity index is higher because, while it has a lower species
index, the species are distributed more evenly. These results support our hypothesis that
the emergent stand would have higher diversity.
Conclusion:
The results show that emergent patches are more diverse due to a greater
evenness. Unlike in the remnant patch, there is no clear dominant species in the
emergent patch. However, by using a different measure of diversity different results may
be obtained. The Simpson’s index is weighted heavily by evenness. An index such as
Shannon-Weiner, which places more weight on species richness may, yield slightly
different results. But because richness only differed by one between the patches, it was
decided that evenness was the most important factor on which to focus the evaluation.