Arthropod abundance and
diversity in relation to primary and early successional habitats
Participants
Gail Boyarsky, Jeff Hall, Leo Pena, Betty Carvellas, Stacey Britton, Susan Van Kleef, Tom Guyer
At La Selva Biological Station, Serapiqui, Costa
Rica, successional and primary forest plots were analyzed for arthropod
abundance and diversity through sampling of leaf litter and soil. Diversity, as measured by the Shannon-Weiner
Diversity Index, was found to be significantly higher in the leaf litter in the
successional plots. There was no
significant difference in soil diversity.
Although diversity was higher in the successional plot, the total number
of individuals was greater in the primary forest plot. Although the data indicates greater species
diversity in the successional plots, further research is needed to determine if
this a permanent factor.
Introduction
In this study, we used a simple
sampling method to collect arthropods in two habitats: tropical primary forest
and tropical early successional forest. We intended to examine relative
arthropod abundance and evaluate our predictions that diversity of arthropods
will be different in primary and in early successional tropical forest. Prior to
1990, the disruption of arthropod populations in La Selva was not fully
described (Guyer, 1988). Since 1990 there has been intensive ongoing OTS
research with the Arthropods of La Selva project (ALAS).
Some of the strongest effects of
fragmentation on ecological processes involve the invertebrate communities
(Meffe, Carroll, 1997). Arthropods are critically important in decomposition,
nutrient cycling, and other ecological processes, and are quite sensitive to
the effects of fragmentation. Surveys of arthropod diversity in understory/leaf
litter provide a biological index based on sensitivity to disturbance.
Arthropod sensitivity to disturbance makes them especially useful as indicator
species. Thus, changes in arthropod diversity can be used to gauge the effects
of fragmentation on local and potentially global biodiversity.
We compared arthropod species abundance and diversity from two successional plots
(0-1 year) and two primary plots. We selected plots ten meters from the edge, and we selected one meter square study sites within each plot. Within each study site, we examined leaf litter and collected two soil samples (6.5 X 6.5 X 10 cm.). We examined leaf litter manually and counted and identified arthropods to order and morpho type. We examined soil samples in a similar manner. At each site, we recorded the following abiotic factors: soil pH, ambient temperature, and soil temperature at 3 cm below surface. In order to quantify results, we analyzed data using the Shannon-Weiner Diversity Index.
Biotic Factors:
|
Arthropods in Leaf Litter |
Successional |
Primary |
|
Total Number of Types |
43 |
36 |
|
Total Number of Individuals |
153 |
191 |
|
1.367105 |
0.936808 |
*Shannon-Weiner Diversity
Index
Arthropods in Soil |
Successional |
Primary |
|
Total Number of Types |
8 |
6 |
|
Total Number of Individuals |
17 |
12 |
Abiotic
Factors:
A
summary of the abiotic factors sampled from each plot is found in Table 3. The temperatures of the soils in both
successional and primary plots were different by only one degree. Air temperatures between the two types of
plots were different by eight degrees.
The pH of the soils was one order of magnitude different between primary
and successional plots with the primary plot exhibiting a more acidic soil.
|
Abiotic Factor |
Successional |
Primary |
|
air temperature |
88 °F |
80 °F |
|
soil temperature |
80 °F |
79 °F |
|
soil pH |
6 |
5 |
The findings showed that the hypothesis was supported. There were significantly more arthropods in the leaf litter in the successional plots than in the leaf litter in the primary forest. There was greater taxa richness in the soil from the successional plots than from the soil in the primary forests but the sample size was so small it was difficult to draw conclusions. Fisher et al (1995) also found a greater species richness of arthropods in an area that was disturbed by humans.
The results may have been influenced
by a termite colony in one of the plots in the primary forest, which lead to a
finding of a lower relative abundance. The species richness was also lower in
the primary forest. These findings do not necessarily indicate a lower
diversity in the primary forest because most of the primary productivity is in
the canopy, which provide more food sources than on the forest floor. The
successional plots have greater primary productivity near the ground.
There are several improvements that could be made in the collection techniques for a more accurate assessment of arthropod populations. In this study, the arthropods in the leaf litter were sorted and identified on the plot. The sample should be collected, sealed, and brought back to the lab for identification. The arthropods were identified as morpho types and not as actual species. By identifying the actual species, the results might change. The soil samples were small and there were few replicates. A diversity index was used to view data, but there were no significant differences between test plots. Increasing size of sample and replication would provide a more precise representation of the populations.
To get a more accurate
representation of the effects of the creation of successional plots for study,
future research could establish if the greater abundance of arthropods is
temporary or permanent. Community structure should be examined to determine if
any species are lost or gained over time.
References
Borror, Donald, Triplehorn C., and Johnson, N. An Introduction to the Study of Insects.
Harcourt Brace College Publishers, Fort Worth. 6th ed. 1989.
Guter, C. “Seasonal Patterns of Norops humilis in Costa Rica.” Rev. Biol.Trop., 1986: 34:247-51.
Meffe, Gary K. and C. Ronald Carroll. Principles of Conservation Biology. Sinauer Associates,
Inc., Sunderland, MA. 1997:
Olson, Eric J. and Peggy Stern, ed. Tropical Biology: An Ecological Approach. Costa
Rica. Aug. 1995: 117-199.
Zar, Jerrold H. Biostatistical Analysis. Prentice Hall, New Jersey. 4th ed. 1999: 40-44.
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