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Tamsey Ellis Sue Sprenke Anne Zellinger
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Abstract
Introduction - La Selva is a biological research station located
between the Sarapiqui and Puerto Viejo rivers, northeast of
San Jose, Costa Rica. It
consists of over 1600 hectares of tropical rainforest and is owned by The
Organization for Tropical Studies (OTS). La
Selva Biological Station has been used for research for over thirty years
resulting in a substantial accumulation of information on its composition and
ecology (McDade et.al. 1994). La
Selva contains several managed habitats, one of which is a series of
five successional plots (parcelas) within the station. “The specific
objective (for these plots) was to maintain representative patches of the early
successional stages by cutting one strip (ca. 0.5 ha ) each year on a five year
cycle……This cutting is always done in February.”. (McDade et. al. 1994)
Each of these .5 hectare plots is cut periodically.
This periodic cutting enables researchers to study changes that might
occur in organisms inhabiting this area after a mass (high magnitude)
disturbance. Parcela one was clear cut one to two years ago (2000),
parcela two was cut two to three years ago (1999), parcela three was cut three
to four years ago (1998), parcela four was cut four to five years ago (1997)
parcela five was cut zero to one year ago (2001) (Vargas 2001).
Because the plots are clear cut, this can be considered a high magnitude
disturbance and the periodic nature of the cuttings result in
low to high frequency disturbance. A
question was raised about the effect of low and high frequency, high magnitude
impacts (clear cutting) on insect diversity within the plots.
We believed that the plots showing a “medium” frequency disturbance
of high magnitude (cut on the 2-3 year cycle) would show more insect richness
and abundance than the plot cut with high frequency (0-1 year cycle) and the
plot cut with low frequency (4-5 year cycle).
Materials and Method - The successional plots run parallel to the SOR trail at La Selva approximately 800m from the river station and approximately 1.5km from the lab compound. They are bounded on the north by swamp and on the west and south by primary forest. Each plot lies next to the other, separated by a path 2m wide, running the length of the plot. The plots are connected on the east by a dirt path running along the ridge above the river Puerto Viejo. Three of the five successional parcelas were chosen as the sites for this survey of arthropod diversity. Since the hypothesis is that diversity (richness) of species will be low after a massive disturbance, low after the site has recovered, and high in the middle of this succession, the following parcelas were chosen: parcela five (0-1 years; the most recently cut plot - 2001), parcela four (4-5 years; the oldest plot 1997) parcela two (2-3 years; the arithmatical mean in the time scale of cutting). The plots were ordered in an unusual sequence. Parcela 1 (1-2 year cutting) was the first plot on the north, parcela 2 (2-3 year cutting) was next and then parcela 3. The last two plots were parcela 4 (4-5 year cutting) and parcela 5 (0-1 year cutting). This put the highest and lowest frequency impact plots next to each other. Initially, sweep nets were used in all five parcelas. However the numbers of insects collected by sweep nets were so small that the data were not analyzed. The parcelas were too overgrown for effective use of this technique.
After a discussion with Dr. Thomas Langen (WWNFF), two collection methods were chosen and samples were taken in the three chosen parcelas. The techniques werebush whacking and water traps in parcelas 2,4 and 5. The bush whacking technque consisted of placing a white pillowcase under a shrub and then beating the leaves and stem of this shrub with a bamboo rod, one-meter in length, ten times. The pillowcase was immediately bunched up to prevent any organisms from escaping. The organisms were aspirated into jars and frozen. The collection was identified by Order using Borror, Triplehorn and Johnson (1992). Three water traps were placed in the parcelas of interest and collected 4 hours later.
Results - The data show that parcela 5 (0-1 year) had the greatest overall arthropod abundance, using the bushwacking technique, with decreasing abundance connected to decreased frequency of cutting. The Order richness was slightly increased in the median plot, however the increase was probably not significantly different from the other two plots. The spider subset showed the same trend in abundance (decreasing abundance with decreasing frequency of cutting). However, the richness by morpho-species of spiders was greatest in the 0-1 year plot.
The water trap data showed highest abundance in the 0-1 year plot and the 4-5 year plot, but a marked decrease in abundance in the median plot. The Order richness was the same in all three plots.
Discussion - The trend of decreasing abundance in plots with decreased frequency of cutting was surprising. We had expected that plots exposed to the shock of high frequency, high magnitude anthropogenic disturbance would show little biodiversity. However, the trend of decreasing abundance was demonstrated not only with the bushwacking sampling technique, but supported by the spider subset (graph 1, graph_2). In addition, the water trap sampling method demonstrated the higher abundance in the 0-1 year plot (graph 3). The trend of high abundance in the 0-1 year plot was supported by two different sampling techniques. These data support the validity of the collection results in that parcela. The 0-1 year plot showed a great deal of biomass close to the ground. The plants in that plot were, for the most part, fast growing, shade intolerant plants. Because this plot is clear cut every year, selection would favor plants that grew and seeded quickly. These types of plants do not produce a great deal of toxins as defense. Their energy is directed toward growth and because of their rapid growth, they can tolerate insect attack. As a result, this plot is insect friendly. The data show this clearly. The high insect abundance in the 0-1 year plot may also explain the extremely high abundance and richness of the spiders in the same plot (graph 2). You would expect to find the spiders where the insects are!
We used three different sampling techniques, each with their own bias. The sweep nets were not efficient in the plots due to the heavy vine and undergrowth. The bushwhacking technique is very efficient for picking up spiders and other non-flying arthropods living in the undergrowth. It is not efficient in collecting flying insects. The water traps, on the other hand, are used to collect flies and wasps. The data set shows this bias clearly in the Order richness (table 3). These techniques are not effective in collecting arthropods that exist in the midlevel and canopy of the trees found in the 2-3 year plot and the 4-5 year plot. Arthropods will live in the entire vertical structure and the sampling collected from only one level. This may explain the decreasing arthropod numbers. The sampling method may be best suited for the environment of the 0-1 year plot. The unexpected drop in arthropod abundance in the 2-3 year plot using the water trap at this point in time has no explanation and needs to be examined closely (graph 3).
The data show a possible problem with the layout of the succession plots. The similarity in Order richness (graph 1) and abundance in flying insects (graph 3) imply that these insects are, in fact, the same population. These plots are close together and flying insects, in particular, would have no problem crossing over the separating path. The data suggest that, if the succession plots are going to be used to show insect succession, the arrangement of the plots might be reconsidered.
The trend of decreased arthropod abundance with decreasing frequency of impact was supported by both sampling methods used. However, the biases in the techniques limited the effectiveness of the sample. Additional sampling methods should be used in order to obtain a more complete data set. Anthropogenic impacts on ecological systems are still poorly understood and yet continue to occur at faster rates. It is important to be able to understand both the short term and long term implications of these impacts.
Bibliography
Borror, D., C. Tripplehorn, and N. Johnson. 1992. An Introduction to the Study of Insects. Harcourt Brace College Publishers.
McDade, L, , K. Bawa, H. Hespenheide, and G. S. Hartshorn. 1994. La Selva, Ecology and Natural History of a Neotropical Rain Forest, The University of Chicago Press.
Vargas, R.
2001. Personal communication.
Acknowledgements - We would like to thank Dr. Thomas Langen and Senor Orlando Vargas for their invaluable help in negotiating the unfamiliar terrain of both insect collecting and La Selva Biological Station, and Christopher Davis for his help in negotiating the computer glitches. You made this project fly!
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