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Pitfall traps in ecotones and island
Elzonia Bezuidenhout, Frederick Anders, Stephen Loubster
Biodiversity and Conservation Biology
University of the Western Cape
Bellville
PO.Box
7535
Abstract
In a period of seven days a study was done to determine if latitude and vegetation type
influence insect sampling in the Cape Flats Nature Reserve. This study was aimed to
find ground dwelling insects and they were sampled through the method of pitfall
traps that was set out in different areas in the vegetation being studied. The containers
were filled half way up with glycerol and ethylene that will kill and preserve it.
Glycerol was used to attract the insects but it was still a random sampling method. A
hypothesis was set to estimate the chi-square distribution. The hypothesis was that the
number of species was equal to section ABC and to section EDF, but the null
hypothesis was rejected which favour the hypothesis1, that the numbers of species are
unequal in section ABC and in Section EDF. The area that was studied was not
disturbed by any factors therefore it was in a natural state. However, an animal
looking for food disturbed transect3 of B and C. The increasing habitat edges in
different latitudes bring about many changes that are different from the habitat interior
in wind humidity, radiation, predation, parasitism, and species interactions
(Diamond,1975; Yahner, 1988;Saunders et al., 1991; Murcia, 1995). However, as a
transition zone between adjacent habitats, habitat edge is a type of ecotone, which is
interpreted on the meso-spatial scale and on the community level (Holland et al.,
1991). This indicates that latitude play a vital role in insect communities and this will
determine the amount of insects sampled.
Introduction
In this study, vegetation differences with latitude were associated with arthropod
order and diversity. Climate shows a direct link in the abundance of insects in an area.
The vegetation cover in the two areas studied was different to each other. Trees
dominated the vegetation in the higher latitude and shrubs dominated the vegetation in
the lower latitude. Insects are distinguished from other members of the phylum
Arthopoda by having six legs, and not eight like spiders. They are the more diverse
group of organisms on earth and have a huge impact on our daily lives through the
food they supply (honey from honeybees), the food they destroy (agricultural and
stored product pests), the people they kill through spreading diseases, and the integral
part they play in the ecology of terrestrial ecosystems (museum). The order
Hymenoptera, was the most occurring order found in all transects.
Materials and methods
The experiment took place in dune slacks (nature reservation, University of Western
Cape), between Thursday, 18/10 and Monday, 22/10. 18 pitfall traps were prepared by
filling 1/3 of 20ml plastic beakers with 96% ethanol (doped with glycerol to prevent
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evaporation). Holes of the same size as the beaker were dug in the soil and the traps
were placed with the upper border at ground level. Three transects, 5 meters apart
were set up; in each transect six traps were disposed in line, separated from each other
by a five meters gap. A path was placed between the third and the fourth trap. This
way, 18 traps were disposed in a 250m2 area (see fig. 1).
The whole set of 18 traps was replaced twice, every 48 hours (first placement:
Thursday 18/10; first replacement: Saturday 20/10; second replacement: Monday
22/10). After removal of each trap, the content was sorted at the microscope and
identified using an insect key (Griffiths et al, 2002). The traps were marked from A to
F starting with A at the highest altitude. Altitude was measured with a GPS with 5m2
accuracy. In order to find out weather our results are in accordance with our
hypothesis we performed a Chi2 test (see table 4).
Fig 1.
Tr1
Tr2
Tr3
A
A
A
B
B
B
C
C
C
D
D
D
E
E
E
F
F
F
5m
5m
5m
5m
5m
5m
5m
Between point C and point D there is a 1,5m firebreak.
Results:
By calculating the total number of specimens in section ABC (higher altitude and
inside shrubs) to section DEF (lower altitude, no shrubs) we found a clear difference.
Altitudes, as meters over water-level is:
Table 1.
Number of specimens found
Day
1
2
3
SUM
Mean:
Number of destroyed traps
Due to weather/animals:
ABC
95
54
39
DEF
133
152
117
188
94
402
134
5
0
2
Mean per. Trap:
mean per. trap*number of
Destroyed traps:
Probable total nr. Of specimens
8.545454545
14.88889
42.72727273
0
Assuming no disruption:
230.73
402
Since five traps from section ABC were destroyed we made an estimate of probable
total number, based on the mean number of specimens per trap, to produce
comparable results. The dominant order (Hymenoptera) was found to cover 33% of
the total insect population (see table 2).
Table 2.
Total number of orders:
18
Dominant order:
Hymenoptera
Total count of dominant order:
195
Total spec:
590
Dominance %
33.05084746
As table 3 shows there were no statistical difference between number of species or
orders in the three transects, and also no notable difference from day to day.
Table 3.
Number of species
Tr1
10
14
10
Day
1
2
3
Mean:
11.33333333
Number of orders
tr 1
9
8
8
8.333333333
Day
1
2
3
Mean:
tr 2
11
9
12
tr 3
11
8
12
10.66667
10.33333
tr2
9
7
9
8.333333
tr3
8
6
9
7.666667
There was found no internal difference between the samples inside the sections ABC
and DEF (for example; no difference between A and C). The total number of species
found in the area was 21 where 15 species were found in section ABC and 11 in DEF
with a 5 species overlap.
Table 4
K
ABC
DEF
Observed
230.7
402
Expected
probability Expected No (O-E) (O-E)^2
0.5
316.35
-85.65 7335.923
0.5
316.35
85.65 7335.923
(O-E^2/E
23.18926
23.18926
3
SUM:
632.7
1
632.7
Hypothesis0: Number of specimens is equal in both sections.
Hypothesis1: Number of specimens is unequal in sections
K= 2-1
Critical value:
0
14671.85
23.18926
1
3.841
H0 is rejected by the Chi^2 test, because the (O-E)^2/E is bigger than the critical
value. This strengthens the H1 hypothesis.
Discussion:
Earlier research suggests that altitude, vegetation and topography will influence
species diversity and density in a given area.
Since section ABC and section DEF show variation in both all of these variables one
would expect that there would also be variation in Species diversity and density.
Table 1 verifies the difference in insect-density. And as mentioned earlier there is a
15/11 difference in species diversity (number of species found) between the areas.
Surprisingly, area ABC shows a higher number of species and a lower number of
specimens. This suggests that in this area there is no correlation between density and
diversity. A possible explanation for this result is that the dominant order
Hymenoptera (shows only 2 species) represents as much as a third of the total insect
population. We expect that the fact that section ABC is between two firebreaks also
influences the density of insects, and further influence might be due to animal
disruption. From table 1 we can see that all trap destruction is found in section ABC
that suggests that animal interference in this section is stronger than in section DEF.
We suggest that further research is directed towards the connection between density
and diversity in similar areas, and also that a second study is done in the area of the
UWC reservation to validate the results found in this study.
Acknowledgement
A project of this size would not be possible without the generous assistance and
support of G.Raitt and the Department of Biodiversity and Conservation of Biology.
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Reference
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