Maliau Lichen Colonisation Pattern project

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Contents

Abstract

Lichens have not been researched to anywhere near the same extent as animals and plants. Lichen colonies of many different morphotypes live on the same substrate, such as leaves. We expected there to be a pattern in differences between size distribution between leaf age categories within each morphotype. Size will serve as a proxy for age. Leaves were collected and catergorized into "young," "middle" and "old." Two lichen morphotypes were selected. The diameter of the lichens were measured with a caliper. For the analysis, the data points were categorized based upon the leaf category and size. Pearson Chi Square Test and Fisher's Exact Test were performed on the data. The results revealed that Morphotype two had significant differences in ratios of size (p-value < 2.2 e-16) but Morphotype one revealed that there was a a marginally significant difference (p-value = 0.05). As leaf age increases, the old to young ratio of both lichen morphotypes increases. However, this study was limited by the small sample size and a broader sampling of lichen morphologies and leaf ages are needed to improve the study.

Introduction

Lichens are an unexplored realm of fungal diversity. Lichens can colonize on a wide array of substrates, including the forests of Maliau Basin in Sabah, Malaysia. An abundance of lichens of various morphotypes are often found living together in habitats as small as a single leaf. These single leaf communities provide an opportunity to examine various lichen colonization patterns through comparing size abundance, which serves as a proxy for age abundance, of various morphotypes in order to provide a simplified view of Lichen abundance and age progression across variously aged leaves. In other words,this study is looking at lichen age to determine whether or not lichen colonization abundance is linked to the age of the lichens habitat.

Question

Is there a pattern in the age distribution of lichen morphotypes across habitat age?

Hypothesis: Across different leaf age categories we expect there to be a difference in size frequency for each lichen morphotype.

Methods

A single tree specimen was selected from which leaf samples were collected. The tree branch was selected by its accessibility (2 meters or shorter), morphology (the closer the leaves were to the tip, the younger the leaves-- see picture of tree), and the presence of multiple lichen morphologies on the leaves. Since there was no method of correctly identifying the exact ages of the leaves, the three selected leaves were categorized into “young,” “middle,” and “old” relative to each other. New leaves were defined as those that shinier, greener, and closer to the budding portion of the tree, while older leaves exhibit these same qualities in progressively decreased manner.
File:Lichen tree.jpg
Tree from which the leaves were picked
Two morphotypes of lichen were chosen for both their apparent abundant presence and their unique characteristics. Morphotype 1 was silvery-green with a branching morphology and Morphotype 2 was green and rounded.

The leaves were closely examined under a field microscope. The lichen diameters on the leaves were measured to 1/100 of a mm using a caliper. The lichens of morphotype 2 that were smaller than 0.2mm were classed into one category. The diameters of all other lichens of morphotype 1 and 2 were recorded and later put into categories. The data was then analyzed in the program 'R'.

Analysis

A Pearson's Chi-squared Test for Count Data (performs chi-squared contingency table tests and goodness-of-fit tests) and a Fisher's Exact Test for Count Data (performs' Fisher's exact test for testing the null of independence of rows and columns in a contingency table with fixed marginals) were performed for each morphotype.

Morphotype 2:

The data recorded had copious amounts of lichen colonies that were smaller than 0.2mm. Hence, there was an inherent category of lichen colonies smaller than 0.2mm and one category with lichen colonies that were 0.2mm or larger.
Morphotype 2 chi-squared table

Morphotype 1:

From Morphtype 2, there was reason to believe that leaf age category would also have an effect on the size distribution of Morphotype 1. The data was divided into two categories. Since these categories had no inherent 'cut-off' in the data as Morphotype 2 did, three different cut-offs were tried: 0.5mm, 0.75mm and 1.0mm. If the age of the leaves had no effect on the age distribution of the lichens, then there would be no significant difference between leaf age category no matter what the lichen size categories.
Morphotype 1 results tables in different categories

Results

Size Abundance Graphs

Statistics

The different distribution of Morphotype 2 were highly significant. The Pearson's Chi-Squared Test and Fisher's Exact Test returned a p-value of 2.2e-16.

Morphotype 1 had more varied results depending on the categories chosen:

An increase in the proportion of larger lichens to older lichens with increasing leaf age shows a pattern in the difference of age distributions across leaf age for this sample. The significance for each morphotpe is represented by the above p-values.

Discussion

Our results confirmed our hypothesis for Morphotype two, with support (p-value < 2.2 e-16) for a significant difference in size distribution between the young, middle and old leaf age groups. In addition, by dividing the Morphotype one data into two groups with a cut-off of 0.5 mm, there was a moderately significant difference in size distribution between leaf categories (p-value 0.05).

The main improvement that could be made to this experiment is a larger sample size (more leaves and more lichen morphologies). The increased amount of information is vital in our ability to make broad claims. Currently, even though we observed many interesting results and came up with new hypotheses, we are unable to make legitimate claims due to the possible extreme bias of our set of results.

Another issue with our results is the human error in our methods. For example, our measurements were based on measuring the width of the lichen colonies with a caliper, and there were three different people measuring at various times. If we were to redo this experiment, it would be helpful to take a high definition picture of the leaf and have one person measure all of the lichen colonies on the computer using an exact measurement program. There is also the possibility of different measurers having varying ideas of what the lichen morphologies were, which is an important point considering that Morph 2 was harder to distinguish from other lichen colonies that Morph one.

There is much room for further research on this topic. One possible area of study is to explore the true age distribution of various lichen colonies and see how this distribution changes in progressively older leaves. These results would examine the how lichen colonies age over time. Furthermore, based on our limited results, it appears that Morph one has a more stable age distribution pattern between the three leaf age categories while Morph two's age distributions vary more. A continued research project could look at the differences in age distributions among different these lichen morphologies. Another idea that came up in our analysis and results was whether the cut-off point in dividing the population of one morphology affected the significance of a difference in size distribution between the leaf categories. Doing preliminary tests, we observed a specific cut-off in which all the cut-offs greater than that one point created a less significant difference than those below it. With a greater sample size, it would be interesting to test whether lichen populations have a specific size group within the population that is more susceptible to variation. Finally, in our small sample size we observed that Morph one was most abundant in the middle aged leaf while Morph two was most abundant in the oldest leaf. Although we cannot make any conclusions based on these preliminary results since we did not correct for size of the leaf, the question of whether certain lichen morphology populations thrive at certain leaf ages is interesting and merits continued research.