A Comparative Study of Organisms Found in Leaf Litter

To the Classroom Rubric Table 1 - Arthropod Orders Table 2 - Plot Conditions+Total Morphotypes

Table 3 - t-test results Chart 1 - Total Morphotypes Chart 2 - Mean Morphotypes

Chris Barnett--Niles West High School, Skokie, IL

Chris Davis--Franklin High School, Somerset, NJ

Tracy Gleghorn--Austwell-Tivoli High School, Tivoli, TX

David Inskeep--Northwestern High School, Kokomo, IN

Introduction:

As the Earth’s human population expands, more and more encroachment into old-growth forest is thought to increase the human impact on global environmental change. Insects are widely believed to be the most diverse group of living organisms and the most diversity is found in the moist forests in the tropics. Many studies attempting to characterize the diversity of insects in the tropics have focused on the canopy and the understory. For the purpose of this study, we will attempt to characterize the diversity of insects and other arthropods in the leaf litter on the forest floor and evaluate the ability of a Costa Rican lowland forest to recover from clear cutting and other types of human invasion. The specific questions we asked were: (1) Does the total number of types of arthropods differ in a recently disturbed site and an old growth site? (2) Is there a difference in the abundance of different types of organisms in a recently disturbed site and an old growth forest?

Hypothesis:

A greater diversity of organisms living in leaf litter will be found in a primary forest as opposed to a similar, disturbed area. We expected a higher diversity of organisms in the secondary forest because there are more habitats as well as a greater diversity of plants. Further, the age of the secondary forest would allow for greater numbers of organisms to evolve or immigrate into the region. The disturbed area would have destroyed habitat thus eliminating many organisms.

Methodology: The study was conducted at La Selva Biological Station, Puerto Viejo, Heredia Province, Costa Rica, from July 16 – 19, 2001. Two sites were selected for study: the first site is a recently disturbed site that was clear cut less than one year ago located along the SHO trail between 200 and 225 meters and the second site is an old growth forest along the SHO trail between 600 and 625 meters.

At each site, two transects were laid out perpendicular to the trail at random locations. Each transect was 20 meters in length.

Samples were taken at 10 meters, 15 meters, and 20 meters. Samples were taken by removing all of the detritus and leaf litter from inside a 35 by 46-cm wooden frame and placing it inside a bag and taken to the lab. At each site, air temperature, soil temperature, and leaf litter depth measurements were also recorded. All samples were returned to the lab for analysis.

Upon return to the lab, the leaf litter samples were massed and recorded.

The litter was then placed in a sieve for sifting. Each sample was shaken in the sieve 10 seconds for each 100 grams of mass by two of the team members while the other two team members participated as timers and watched for organisms attempting to escape. Aspirators were used to collect small organisms. Collected organisms were placed in petri dishes with a cotton ball and ethanol.

Collected organisms were identified to Order and then sorted by morphological type using a stereoscope. We used analysis of variance to compare mean number of morphotypes from different sample plots. We also compared total number of morphotypes found in the two habitats.

Results: The total morphotype data collected in this study generally support the hypothesis that a greater diversity of organisms living in leaf litter will be found in a primary forest as opposed to a similar, disturbed area. Data Table 1 lists the counts arthropod orders found at each sample plot while Graph 1 depicts the relative number of morphotypes found in the primary forest plot vs the successional plot. Data Table 2 lists total morphotype counts and habitat conditions at the time of sampling. Ninety-seven total morphotypes were found in the old growth forest site compared to forty-five total morphotypes that were found in the 0 – 1-year secondary growth site. However, when means of individual plots were compared using a T-test, no statistical difference was detected (see Data Table 3 and Chart 2). The T-test results indicate that more sampling is needed to make a definitive statement about the diversity of the two sites. Due to the limited time available to complete the study, habitat data collected at each site were not correlated with diversity data.
	Click on the thumbnails below to see the full-sized graphs
Discussion: The total morphotypes collected supported our hypothesis that greater diversity would be found in the primary forest than in the secondary growth site. However, due to the great variation among the different sample plots, further sampling and analysis would be required to reach a definitive conclusion. Several factors may contribute to the increased diversity found in primary forest. The primary forest leaf litter arthropod community has persisted under similar conditions for many generations with continuous development and diversification of niches. Specialized relationships have evolved between flora and fauna, predators and prey. With more herbivores come more predators, and the diversity of organisms rapidly increases. The environment of the primary forest is also moderated and protected by the canopy. The temperature is moderated and more constant because direct sunlight does not reach the forest floor. The canopy also reduces the pounding and erosion effects of rainfall.	Graph 1 Total Morphophytes
	Graph 2 Mean Plot Morphophytes

When the forest is cleared most of the micro-arthropods are poorly equipped to survive the changed conditions. Differences in leaf litter also affect survival. In the primary forest leaf fall/litter accumulation is relatively steady. The diversity of organisms feeding on the litter allows for rapid recycling. This contrasts with the relatively thick accumulation of simultaneously decaying matter in the clearing that functions as compost generating further heat and harsh conditions. Most micro-arthropods- while capable of population explosions under ideal conditions – are not well suited to colonize new areas. They do not move over large distances and will not survive a trek outside of their sheltered environment. Thus, once a clearing is isolated from the primary forest by environment and distance it is difficult for the diversity to be reestablished. Corridors could possibly aid in the migration of these organisms. This limited study does support the idea that primary rainforest needs to be preserved intact to preserve its inherent diversity.




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