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Comparison of the Relative Abundance and Diversity of Terrestrial Macroinvertebrates Between Regenerated and

a Primary Tropical Rainforest at La Selva, Costa Rica

Rose Davidson, Sue Lloyd, Ken Perry, Christopher Soldat and Nancy Vankirk

Abstract

Costa Rican rain forests have been severely reduced by deforestation for agricultural purposes. At La Selva Biological Station, research is ongoing to gain an understanding of how to manage regenerating forest resources. This investigation looks at biodiversity using macroinvertebrates in leaf-litter to compare a regenerated forest to a primary forest. Results from this cursory investigation suggest that greater biodiversity exists within the leaf-litter of the regenerated forest than the primary forest.

Introduction

The forests of Costa Rica have one of the highest rates of deforestation in Central America (Leonard, 1987). The rate of deforestation has been a relatively recent event in the history of this country. By comparison, most of North America underwent massive deforestation during the 19^th century. In 1943, approximately 70%-75% of Costa Rica was forested. By 1987, only 29% of the country remained forested (McDade, 1994). At the La Selva Biological Station operated by the Organization of Tropical Studies (OTS) research is ongoing to study the regenerative process of reforestation. This research will help us understand the process in which a tropical rainforest is capable of recovering following human-impacted deforestation.

An investigation was undertaken to study leaf-litter macroinvertebrates as an indicator of biodiveristy. For the purpose of this investigation, biodiversity is measured by the number of species and their abundance. Comparative samples were taken along trails in a four to five year old regenerative plot and in a primary forest. The objective of this study is to determine if primary forests have more diversity and species abundance than a young regenerated forest.

Site Location

The regenerated tropical rainforest samples were taken from a successional research site at La Selva Biological Station. This research plot has been clearcut at regular time periods to demonstrate forest regeneration in a tropical setting. The most mature plot (4 to 5 years old) was used in the study. The understory height was approximately three meters high. Protruding tree height was approximately 10 to 12 meters high. The vegetation in the plot appeared fairly dense. The density of the vegetation within the plot made access difficult. The size of the plot was 75 by 100 meters.

The primary tropical rainforest samples were taken from a section of primary tropical rain forest that was surrounded by at least a 100 meters of similar, established forest. The understory was sparse making it possible of having easy access and visibility. The canopy was at least 30 to 40 meters high.

Samples were taken along a one hundred meter transect at twenty-five meter intervals. The reforestation area samples were taken along the Sendero Holdridge (SHO) as shown in yellow on the La Selva Biological Station map (Figure 3). Primary forest samples were taken along the Camino Circular Lejano (CCL) as shown in red on the La Selva Biological Station map.

Materials and Methods

Using the Direct Searching method (Sutherland, 1996), five leaf-litter samples were collected along a trail bordering the four to five year old regenerative plot and along the trail within the primary forest. Samples were taken in the regenerative plot approximately one meter along the perimeter of the forest from the trail (Figure 2).

Samples were taken one meter from the trail in the primary forest. Collection of samples were taken between the two hundred and three hundred meter marks to insure the sample was taken directly from a primary forest.

To standardize the sample area, a clipboard (31x 23 cm) was used to determine the area of the sample and used as a scoop to collect the sample. The clipboard was inserted into the leaf-litter until it reached the mineral level and the leaf-litter completely covered the clipboard to a depth of two centimeters. The sample was placed into a Ziplock bag with a cotton ball saturated with ethyl alcohol to act as a killing agent. The bags were stored overnight to allow time for the killing agent to affect as many of the macroinvertebrates as possible. The following morning, the samples were inspected using a dissecting microscope and macroinvertebrates were counted and sorted by taxonomic Order using Borror, et al (1989).

Data from the Geographical Information System (GIS) lab at La Selva were used to produce the map to locate the sample sites for this report. In addition, soil type and watershed data at the study sites was also available from the GIS lab. Rainfall data prior to sampling time on July 23, 2000 was also obtained from the lab.

Results

Comparison of Regenerative and Primary Tropical Rainforest Terrestrial Macoinvertebrates (by taxonomical Order)

Plot # Mass of sample (in grams)***	*R1 145	**P1 290	R 2 110	P 2 185	R 3 135	P 3 130	R 4 135	P 4 105	R 5 275	P 5 110
Arachnida	0	0	1	0	0	0	3	0	0	0
Diplododa	1	0	4	0	0	0	0	0	2	0
Isopoda	0	0	2	0	1	0	1	0	4	0
Annelida	0	0	1	0	0	0	0	0	3	0
Coleoptera	0	0	0	0	0	0	3	0	0	0
Clollembola	0	0	1	0	0	0	0	0	0	0
Diptera	1	0	0	0	0	0	0	0	0	0
Hymenoptera	0	0	0	0	0	0	0	0	2	0
Total # of macro-invertebrates	2	0	9	0	1	0	7	0	11	0

*R=Regenerated Tropical Rain Forest (4-5 years old)

**P=Primary Tropical Rain Forest

*** Average mass of the sample: 160 g - Regenerated

144 g - Primary

In comparing the data contained in the table above, there was a difference in the biodiversity and abundance of macroinvertebrates found in the regenerated forest samples compared to the primary rain forest samples. The total number of macroinvertebrates sample sites was thirty. All, which were found in the samples, collected in the regenerated forest plot. No macroinvertebrates were found in the samples taken.

According to GIS data, both sample plots were identified as being in alluvial type soils. It was also determined from GIS data that both sites were above the 100-year flood level in the same watershed system.

Discussion

Our results do not support our original hypothesis that there is greater biodiversity of macroinvertebrates in the primary tropical rainforest than a regenerated tropical rainforest. We expected to find a larger variety of species in the primary forest but instead we found none.

Our direct observations of the leaf-litter between the two samples are qualitatively

different. In the regenerative sample, the leaf-litter is smaller, partly decomposed and is

composed mainly of leaves and stems. The primary leaf-litter is less decomposed and has

more intact fibrous material. In sampling the sites we had to cover more surface to obtain

similar size leaf-litter samples. The average mass of the leaf-litter samples collected in the

regenerated forest was 160 grams and the average mass of the litter collected in the

primary forest was 144 grams. The difference in mass may be due to the difference in the

density of understory found in a primary forest as compared to young successional forest.

This lack of understory in the primary rainforest, and corresponding lack of leaf-litter for cover, may have driven insect populations elsewhere for shelter and food.

Another possible explanation for the absence of macrointertebrates may be due to current localized weather condition at La Selva. Prior to collecting the samples considerable rain fell. On the day of collection, 55 mL of rain fell in the primary forest between 10:30 a.m. to 4:30 p.m. (Enloe, et al., 2000) As a result of these weather conditions our experimental data may have been affected. During rainfall insects typically seek shelter in understory of leaf-litter (Borror, et al., 1989). The recent heavy rainfall and the composition of the leaf-litter are plausible explanations for this lack of biodiversity in the macroinvertebrates in the primary forest.

Our sampling technique could be improved by making quantitative measures of the surface area of leaf-litter collected at each site. Additional sampling of each plot would decrease the uncertainty of our results. It would also be beneficial to sample during a dry period rather than right after a recent rain.

Classroom Application

In order to convey the idea of biodiversity to students a field study similar to this can be carried out in a localized setting. School sites, nature areas, state parks and federal land offer a variety of places to learn about biodiversity.

Different sampling techniques can be utilized in this study such as sweep nets, pit fall traps, quadrant and transects. Team building and cooperative learning techniques will be reinforced during the field study experience.

Acknowledgements

Members of the Bug Off group would like to extend our thanks and appreciation to our institute leaders, David Silverberg and Evan Notman, for the guidance and expertise in developing our small group research project. We would also like to thank Matt Clark of the La Selva Biological Station GIS lab and our colleagues from Group Three for providing the rainfall data used in our study.

References Cited

Borror, D., Triplehorn, C. and Johnson, N. 1989. An Introduction to the Study of Insects. Saunders Publishing Company.

Enloe,T., Hellstern, R., Lee, C., McCandlless, B. 2000. What is the Canopy Effect on Rainfall Amount and its Chemical Composition? Woodrow Wilson National Fellowship Foundation Environmental Science Institute. Costa Rica.

McDade, Lucinda, et al. 1994. La Selva Ecology and Natural History of a Neotropical Rain Forest. University of Chicago Press.

Sutherland, William J. 1996. Ecological Census Techniques. Cambridge University Press.

La Selva

Hotter than Hades,

We’re go’ in crazy

Is it a snake we hear in the woods?

Mosquitoes we’re swatting,

Run to the potty,

Gallo pinto is what we live for,

Wasting away again at La Selva

Look’ in for our lost bottle of Deet,

Some people say there’s David to blame,

But we know, da da da da da it’s our own damn fault, da da da da da

Put on our booties,

Now we are Woodies,

Out in the forest to get our stuff done,

Doing our transects,

For tropical insects,

What we do we always have fun,

Wasting away again at La Selva

Look’ in for our lost bottle of Deet,

Some people say there’s David to blame,

But we know, da da da da da it’s our own damn fault, da da da da da

Booze in a bottle,

The worm we will swallow,

Telling tales we soon will regret,

Up at daybreak,

To look for the big snake,

See how many bugs there are in the net,

Wasting away again at La Selva,

Look’ in for our lost bottle of Deet,

Some people say there’s David to blame,

But we know, da da da da da it’s our own damn fault, da da da da da

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