No obvious patterns were recognised. Once the number of squares laid over lichens was counted it was found that the number of squares laid over lichens on the northern aspect of the wall exceeded that of the southern aspect. This suggests there is a difference in lichen growth and that the directional hypothesis should state that more lichen growth would occur on the northern aspect. The Man-Whitney U test was carried out to determine whether any significant difference existed.
The critical value for the Man-Whitney U test was 127. This value was more than the lower value of U or U1 so therefore the null hypothesis was accepted and statistically speaking there was no significant difference found between the growth on lichens on the northern or southern aspect of a wall.
Introduction
‘Lichens are stable, consistent and identifiable combinations between algae and.or cyanobacteria (the photobiont or phycobiont) and a fungus (the mycobiont)’. A symbiotic relationship exists between the algae and fungus of a lichen. The fungus provides structural support to the alga by surrounding it with protective strands of hyphae. Also it provides nutrients that are absorbed from the substrate and a stable microenvironment in which the alga can live. The alga, in turn photosynthesise and provide carbohydrates for the fungus. Most lichens attain their shape from the fungal component as the algal component is restricted to a single layer of cells. The fungal component forms the body of the lichen known as the thallus.
A different fungus always occurs in different lichen species, but the same algal species may occur in lots of different lichen species. Lichens are an evolutionary success. They have colonised many underwater regions to mountain tops, to desert lands and cold polar tundra. Some can even live for hundreds of years before they die. The process by which a fungus and alga join to form the lichen is called lichenization. When a lichen is separated in a laboratory into its fungal and algal components and each of these is cultured separately they grow normally as they would if originally found isolated. However it is more difficult to reform the original lichen from its constituent fungus and alga. This can occur but is rare and needs very specific conditions. The morphology, physiology and biochemistry of lichens differ greatly to that of its component fungus and alga. However many species are unable to survive environmental disturbances and hence also pollution. Therefore towards the centre of cities only pollution resistant species of lichen are to be found. From this it may be appropriate to say that many lichen species will be found growing on a wall in the countryside. Lichens have been used to make dyes and perfumes as well as traditional medicines.
Identifying lichens (urban) can be done by checking what type of stone it grows on. There are two main types: basic and acidic. Basic stone includes limestone, marble or martar. These stones fizz when they come into contact with lemon juice. Usually orange crustose lichens are found growing on basic stones. Acidic stone includes granite, slate and many silicious stones (stones containing silicon e.g. sandstone). Acid stones support foliose and crustose lichen species and are unaffected by the addition of lemon juice. Crustose lichens are found firmly attached to their stones and cannot be removed easily by hand. Foliose lichens are leafy in appearance and can be lifted from their stones by hand, they are usually found attached by root like hairs. Foliose lichens resemble bushes and are rarely found in urban settings. Lichen colour may vary dramatically depending on the species and external environmental conditions such as weather, humidity etc. Therefore the appearance of the lichens that will be crossed in the experiment may be due in part to the season.
Many lichens reproduce asexually, either by vegetative reproduction or through the dispersal of diasporas containing algal and fungal cells rather than producing fruiting bodies. Dispersion may be done by wind, rain or animals etc. Another form of diaspore involves outgrowths from the thallus that fragments and can be dispersed mechanically. This is called isidia. However many lichen fungi appear to reproduce sexually in a manner typically of fingi. The thallus of most lichens differ greatly to that of their constituent alga and fungus and may resemble simple plants in form and growth. ‘The fungus surrounds the algal cells, often enclosing them within complex fungal tissues unique to lichen associations. There is evidence to suggest that lichen symbiosis is parasitic or commensalistic, rather than mutualistic… However this now needs to be re-examined’. The photobiont can exist in nature whereas the mycobiont cannot. Furthermore, photobiont cells are routinely destroyed during nutrient exchange. This mutualistic/parasitic/commensalistic association is only able to exist because the photobiontic cells reproduce faster than they are destroyed.
The evolution of lichens and the class Ascomycota is not fully understood. There are however thirteen different Ascomycete orders and for this reason scientists generally agree that different lichens evolved independently from one another. Lichens are named based on their fungal component, making up most of the lichens mass (‘though in filamentous and gelatinous lichens this is not always the case’). Taxonomists used to place lichens in their own division – mycophycophyta but this is no longer the case as alga and fungi are acknowledged to belong to different lineages. Today lichens belong to either the Ascomycota or more rarely the Basidiomycota groups.
The growth of lichens on the wall will only differ if the conditions on either side of the wall differ as the northern and southern aspects of this wall are in such close proximity to each other. The only foreseeable difference that may occur on either side of the wall is amount of sunlight, which may result in a difference, however this is difficult to asses as there are so many variables when conduction such an experiment.
Method
A compass is first used to determine the north and south aspects of the wall selected for investigation. A ruler is then used to measure one metre into the edge of the south aspect of the wall (this is done on both sides of the wall and attempts to ensure quadrats are placed on the same spot on either side of the wall). The same ruler is then used to measure 1.5 metres vertically. The bottom left hand corner of the 30cm by 30cm quadrat is then placed in this position and the number of squares laid directly over lichens are then counted and recorded. Before removing the quadrat a photometer should then be used to measure light intensity within the quadrat. The photometer has a number of settings it was placed on the highest (x100), and placed in the as near the centre of the quadrat as possible without being in the direct rays of the sun. The value, measured in lux, was then viewed and recorded in the tabulated sheet. Care was taken to prevent the photometer from being exposed to direct sun rays as this produces inaccurate values as the lichens are largely not exposed directly to sunrays due to such things as clouds blocking the sunrays for example.
This procedure was then. Using the ruler, measuring from the edge of the quadrat and making sure that the ruler is horizontal by aligning it with bricks, a 50cm interval was then measured. Once this was done, the lower left hand side of the quadrat was then placed in this position, again at a height of 1 metre above ground level and checking with the 1 metre ruler. Again the light intensity and also number of squares laid over lichens is measured/counted and recorded on the tabular sheet. This procedure was repeated until 10 separate results were obtained. However due to obstruction by plants and bushes measurements further down the wall could not be obtained. Therefore this same procedure of attaining and recording results is repeated in the opposite direction at a height of 0.5 metres until 20 readings in total have been obtained.
This exact same procedure is then repeated for the north aspect of the wall.
Results
Average = ∑ readings / total number of readings
Therefore, average number of squares covering lichens on the southern aspect = 114 / 20 = 5.70
Therefore, average number of squares covering lichens on the northern aspect = 135 / 20 = 6.75
Analysis of Results
A one tailed test is used because the prediction of the outcome was decided i.e. there was an alternative directional hypothesis.
Step 1
Let n1 be the smaller sample and n2 be the larger sample
Step 2
The data must then be ranked in order from smallest to largest values. If there are two or more values occupying the same rank, the average of the 2 ranks should then be found and placed within this table (these are called ties). The results for the southern aspect of the wall is on the bottom of this table and the results of the northern aspect of the wall is on the top of the table. The upper middle part of this table contains the results for the rank of the northern aspect and the lower middle part of this table contains the rank of the results of the southern aspect.
Step 3
The sum of the ranks of the smaller sample is then calculated:
4 + 7.5 + 7.5 + 13 + 13 + 13 + 13 + 19 + 19 + 22.5 + 28.5 + 28.5 + 28.5 + 28.5 + 28.5 + 37 + 37 + 37 + 37+ 37 = 459
Step 4
The value of U is then calculated:
U = [n1n2 + n1(n1n2) / 2] – R
= 20 x 20 + [20(20 + 1)]/20 – 459
= 400 + 210 – 459
= 151
Step 5
n1n2 – U = 400 – 151 = 249
Step 6
The critical Value (U crit) corresponding to n1 and n2 = 127. As this value was obtained from the one tailed Man-Whitney U-Test there is only a 2.5% level of significance. However as the value 127 is more than the lower value of either U or U1, therefore the null hypothesis, which states there is no difference between growth of lichens on northern and southern aspects of the wall should be accepted.
On the southern aspect of the wall there is not any correlation between light intensity/photometer reading and the number of squares laid over lichens that can be deduced. This is because photometer readings do not increase or decrease in proportion to the number of squares laid over the lichens as seen through the first five values: the quadrat readings stay fairly constant but however there is a general decreasing pattern as follow, 8, 9, 7, 5 & 8. However the photometer readings increase at a much faster rate and decrease when the quadrat readings at the fifth interval: 5, 20, 60, 82, 20. From this alone one may postulate that there is an inversely proportional association but this is not the case as seen in further values where the photometer readings drop right down to a value of 1 and from 21 the quadrat readings increase slightly, then remain constant and increase a little more: 1, 4, 8, 8, & 9. Even when values for quadrat readings were the same the value for photometer reading would differ as seen when the first 9 quadrat squares were laid over lichens a photometer reading of 20 lux was given and later when 9 quadrat boxes were laid over lichens a photometer reading of 1 lux only was given. This is not mean to say that there is no correlation between light intensity and growth of lichens as there must be due to the fact that the algal constituent needs sunlight to photosynthesise and produce carbohydrates, however the most comprehensible explanation for this is that the photometer readings attained on this particular experiment are inaccurate or misrepresentative of their usual intensities. This may be for a number of reasons such as the weather on the day fluctuated, or the clouds constantly blocked the sun light at random times making the wall more shaded at random times during the experiment or even that the photometer was inaccurate as it was never a fixed value for more than a second or so.
On the northern aspect of the wall there is a correlation seen between photometer readings and quadrat readings for the first 7 readings. As one increases or decreases so does the other. Light intensity is however always higher than quadrat readings which would support the fact that not all sunlight energy is used up during photosynthesis as some is the wrong wavelength, or some misses the photosynthetic organism completely or even is reflected away from the photosynthetic organism. The correlation between the photometer and quadrat readings suggests that a relationship between the two does in fact exist but may just not be evident in the rest of the results and this will be due to the number of flaws within the experiment. At the eighth reading the values for light intensity shoot up, however they are still in sink with quadrat readings and as one increases or decreases so does the other. This was due to the fact that there was a shadow on one upper corner of the wall where the measurements began. As the quadrats moved on from this shaded area they received more sunlight and hence the photometer readings are much higher, and are still proportional to the quadrat readings.
The number of squares in which lichens were found differ from one aspect to the next. The northern aspect was found to have 135 squares laid over lichens whereas the southern aspect only has 114 squares laid over lichens. This however is an insignificant difference. The statistical test suggests that the aspect of the wall has no effect on the growth of lichens which supports the idea that the difference between 135 and 114 is insignificant. The Null hypothesis should be accepted as being correct in this particular experiment.
Evaluation
The purpose of the investigation was to determine whether the aspect of a wall affected the growth of lichens. The statistical data supports the null hypothesis which states there is no significant difference for growth of lichens on that particular wall chosen.
However these results are not 100% accurate for a number of reasons. To begin with values were only taken from one wall. This poses an inaccuracy because the conditions subject to that wall are not necessarily the average conditions subjected to walls around the area, for instance, there was a bush hanging over both sides of the wall, and on one side more so than the other. Also depending on the time of day that one conducts his experiment the sun will cast a bigger shadow on one side of the wall than the other producing another flaw within the experiment; and hence if there actually was a difference in growth of lichens depending on aspect of growth this may not have been possible to find on this particular wall as all almost all measurements were done in the shade of the bush.
There were other factors which also prevented a fair test from taking place such as the fact that some bricks were broken or missing providing a shaded area in which lichens could grow. This shaded area receives less sunlight than the rest of the wall and gets rid of the uniformity of the wall. The number of broken bricks on either side of the wall was not uniform and therefore the conditions on either side of the wall was not equal preventing a fair test.
Another factor that prevented a fair test from ensuing would be the time factor. At different times of days the light intensities vary. Also on different days the weather conditions vary. These affect the results and may prevent the person carrying out the experiment to attain inaccurate results. Also the time of year and hence season will affect the amount of sunlight. Had this experiment been carried out in winter the amount of lichens on the wall would have been different and the average light intensities would have generally been lower. To reduce these errors the test should be conducted as fast as possible so that the results are all attained within a narrow margin of time. Then this should be repeated for different times of the day, and also different times of the year. With this data one could then determine whether the aspect of a wall affected the growth of lichens more reliably.
Another point to make is that the photometer was unreliable as its value kept changing due to your hand shaking or the shadows of the bushes and trees around the wall. Also some parts of the wall had no shade on them and therefore the photometer values for that particular part of the wall were disproportionately larger than the rest of the readings. This could be remedied by taking many photometer values within each quadrat, eliminating any anomalous results and finding the mean value and taking a number of readings over a number of days and cross referencing to see the average values in lux for each part of the wall.
The quadrat that was being used was made of wire and the squares within the quadrat were not completely straight or exactly the same size as all the other squares. Therefore this poses another uncertainty. A quadrat made of completely straight edges should be used for more accurate results. Also the range of heights at which quadrats were placed should be increased to gain a wider perspective of the growth of lichens on the wall and also the frequency of readings taken should be increased i.e. the interval space between each quadrat should be decreased.
The experiment was however conducted in a reasonable manner. Care was taken to prevent any accidents from occurring. Care was taken to ensure that quadrat settings were not biased and that many readings were taken to find good results. The overhanging bush could not have been avoided and this problem was specific to the particular wall one had to work with rather than any technique used. The photometer was kept out of direct sunlight as far as possible and all results were recorded quickly on paper to make sure nothing was forgotten later on.
Improvements
- Increase the range and frequency of measurements.
- Repeat the experiment a number of times within the week to find an average conclusion as to the effects of aspect on lichen growth and also over the year to see how this may change with seasons.
- Use smoother, straighter quadrats and ruler.
- Take an average of many photometer readings for each quadrat placing.
- Measure the effect of aspect on lichen growth on many different walls.
- Determine and compare what species occur more prominently on each aspect of the wall and factor this into your conclusion.
Conclusion
The null hypothesis is correct as stated by the Man Whitney U test. However this cannot be fully accepted due to the number of flaws within the experiment as mentioned in the evaluation.
Bibliography
- Lichens: an illustrated guide to the British & Irish Species (by Frank S Dobson)
- A Photographic Field Guide, Mosses Lichens & Ferns of Northwest North America (by Dale H Vitt, Janet E Marsh, Robin B Bovey)
- Guide to common urban lichens 2 (on stone and soil) by Frank Dobson
- Advanced Biology Statistics by David Druce
