Ecological Niches of Caterpillars

Colour used to indicate the raw data used to calculate example mean

Colour used to indicate the raw data used to calculate example standard deviation

Observations:

* Notches were mostly at the edges of leaves but were quite deep.

* Sometimes notches didn’t go to the very end of leaves.

* Windows were very long and didn’t go completely through the leaf.

* The cabbage tree leaves taper off significantly.

* The midrib is difficult to see.

* There were some yellow spots on the leaves.

Processed Data Table showing the mean distance from the midrib of the cabbage tree leaf to the nearest point of the feeding site (mm) of 50 samples from notched and window shaped feeding sites on cabbage tree leaves and the standard deviation.

Shape of Feeding site

The mean distance from the midrib of the cabbage tree leaf to the nearest point of the feeding site (mm)

Standard Deviation of the 50 samples of the distance from the midrib of the cabbage tree leaf to the nearest point of the feeding site

Notch

Window

Note: Processed data was put to the same number of decimal places as raw data

Key

Calculated example mean result

Calculated example standard deviation result

Calculations:

Mean

From: http://easycalculation.com/statistics/standard-deviation.php

Standard deviation

From: http://easycalculation.com/statistics/standard-deviation.php

T Test

A T Test has been conducted to determine whether there is a marked difference in distance from the midrib of the cabbage tree leaf to the nearest point of the notched and window feeding site.

Null hypothesis: there is no significant statistical difference between distance from the midrib of the cabbage tree leaf to the nearest point of the notched and window feeding site.

Alternative hypothesis: there is a significant statistical difference between distance from the midrib of the cabbage tree leaf to the nearest point of the notched and window feeding site.

Calculating the degrees of freedom:

(sample size) – (the number of samples) = degrees of freedom

50+50 – 2 = 98

df= 98

The critical t value is obtained from the tables using the p value of 0.05. As the table does not include our df value of 98. We round our df value to the nearest value on the table which is 120.

Therefore critical t value is 1.98 or 2 (same number of decimal places as raw data)

Table from: http://www.ruf.rice.edu/~bioslabs/tools/stats/ttable.html

From: http://graphpad.com/quickcalcs/ttest2/

Calculated t value = 5 (same number of decimal places as raw data)

Critical t value = 2

Our calculated t value of 5 is greater than the critical t value of 2. Therefore my null hypothesis that there is no significant statistical difference between distance from the midrib of the cabbage tree leaf to the nearest point of the notched and window feeding site can be rejected and the alternative hypothesis that there is a significant statistical difference between distance from the midrib of the cabbage tree leaf to the nearest point of the notched and window feeding site can be accepted. Meaning I am 95% certain that there is a significant difference between the two means, and therefore there is a significant difference in distance from the midrib of the cabbage tree leaf to the nearest point of the feeding site from notched and window shaped feeding sites on cabbage tree leaves.

Graph showing the mean distance from the midrib of the cabbage tree leaf to the nearest point of the feeding site (mm) of 50 samples from notched and window shaped feeding sites on cabbage tree leaves and the standard deviation.

Comment on Graph:

My bar graph shows that mean distance from the midrib of the cabbage tree leaf to the nearest point of the window feeding sites was considerably lower at 6mm than the mean distance from the midrib of the cabbage tree leaf to the nearest point of the notched feeding sites at 9mm. The error bars were calculated to show one standard deviation either side of the mean, displaying where 68% of the data lay. My error bars are quite large which means there is a large spread of data, which indicates that there may have been systematic errors. The overlap of the error bars indicates that there may not be a major statistical difference between the data sets, yet as I have already shown my t test indicates that there is a statistical difference.

Conclusion and Evaluation

Conclusion

In my experiment I investigated whether the Window and Notching caterpillar share the same ecological niche.

From my results I can infer that the Window and Notching caterpillar do not share the same ecological niche. The mean distance from the midrib of the cabbage tree leaf to the nearest point of the window feeding sites was considerably lower at 6mm than the mean distance from the midrib of the cabbage tree leaf to the nearest point of the notched feeding sites at 9mm. This indicates that the caterpillars live on different parts of the leaf and therefore do not share the same ecological niche.

My data was reasonably imprecise as indicated by the large standard deviation/error bars. Because our sample size was 50 our results were reliable and it is more likely that this impreciseness is cause by the fact that organisms are random in behaviour and therefore there will be some distances which vary greatly from the average which will have affected our error bars, these outliers though naturally occurring, like the window feeding site which was 11mm from the midrib factors which cause these outliers shall explore in my evaluation. Although this does explain the inconsistency of my original samples, yet the subsequent mean calculations and t test ensures that the integrity of the data is intact. There was no difference in the size of error bars between the notched and window feeding sites, with both having a standard deviation of 3. This indicates to me that there must have been some systematic errors within my experiment otherwise I would expect the error bars to be similar, yet these systematic errors must have effect both types of caterpillar.

This result is explained through the science of ecology and various theory’s and principles within this discipline. Each species within a habitat has a unique niche, where niches of different species overlap, competition between them occurs. The Window and Notching caterpillar both have the same fundamental niche (the area which a population can occupy in ideal conditions¹), yet the competition which occurs in this fundamental niche has resulted in the caterpillar species inability to co-exist in the same habitat as eventually one caterpillar species will outcompete and eliminate the other. Thus our results can be explained by Gauses competitive exclusion principal which states: “if species are to coexist in the same habitat, their niches cannot be exactly the same².” In the case of the two caterpillars, they both have different physical adaptations have enabled them to coexist because they feed on different parts of the leaves. The notching caterpillar mouthparts which enable it to bite, so eats from the edge of the leaf. While the window caterpillar has a rasping radula³ that scrapes holes in the leaf close to the midrib. Thus the caterpillars can successfully exist on the same cabbage tree leaf while not in sharing the same niche, as their adaptations reduce competition creating different realised niches in the same fundamental niche.

However my results are reflected by these other experiments such as the one a found online which directly measured the frequency of different types of feeding sites against the distance from the midrib. Their results are shown in the graph belowâ´.

I also found another similar studyâµ which looks at the caterpillar feeding sites on flax, this also supports my findings.

The findings though not as extreme were very similar to my own, as the window feeding sites were located closer to the midrib while the notches were closer to the edges of the leaves. Therefore I can conclude that my findings and my explanation is correct as it is confirmed by outside sources.

Evaluation:

Limitation of the method

How significantly could this have impacted on your results and why

Improvement

Our sampling of the area was not truly random, due to the limited number of feeding sites.

This would have affected the reliability and validity of our results as because we were not completely random it becomes more likely that we selected feeding sites that were easily visible on the leaves. This would have made it more likely for us to select the larger notches and windows as there were immediately more visible. This would have resulted in them being closer to the midrib, therefore decrease our mean. Thus effecting both the reliability and validity of our results.

We could have the random number generator from this website http://www.random.org/integers/ to randomly select a number, we would then count that many leaves working clockwise from the top most leaf of the plant, stopping when we reached our number. We would then measure all of the feeding sites on one leaf, averaging all of the distances to the different feeding sites on one leaf, we would then repeat with another leaf and eventually we would average all of the results.

Due to the limited number of feeding sites we tended to measure clusters of feeding sites.

This would have affected the reliability of our results as because we often measured multiple feeding sites from one leaf of one plant and therefore are more likely to have a similar distance. This would have caused and increase in modes, and our mean to have been effected.

Once a feeding site had been measured we would mark the site with a red sticker bought from http://www.amazon.com/Map-Dot-Stickers-Red-Diameter/dp/B003M6WPPW, we would then only take a maximum of 2 measurements of each type of feeding site from each leaf, this would ensure we didn’t measure whole clusters from one leaf. Once we had collected all 50 samples we could return and remove all of the stickers.

In the areas we were measuring there were many other plant species such as flax.

Because there were plant species there would been other food for the caterpillars, as both the notching and the window caterpillars feed on flax. One kind of vegetation may be more favourable for one of the species of caterpillar, this would lead to an increased number of caterpillar on that type which would present more competition for the other species of caterpillar, and therefore their feeding sites of the less prevalent caterpillar according to Gauses law would be further away to avoid competition. This would have affected our experiment as we are not sure that the niches of the caterpillar overlap evenly. Because of this our validity and reliability would have been significantly affected.

If we were to conduct this experiment again I would buy a stock type 09 cabbage tree from the website below: http://www.southernwoods.co.nz/cart/prices.asp lmcommon=Cabbage+Tree&Species_Latin=Cordyline&Name_Latin=australis&Name_Common=Ti%20Kouka. I would then isolate this plant in a greenhouse this would mean there was no other vegetation around which could cause uneven distribution and hence competition of the caterpillar species.

The surrounding area where we were testing is sometimes sprayed with pesticides

If pesticides were used on the plants then they could have impacted on the caterpillars. As pesticides work in different ways one species of the caterpillar may have been better adapted to cope and therefore survive while the other died. This would have led to a larger population of the better adapted caterpillar, which would present more competition, and therefore their feeding sites of the less prevalent caterpillar according to Gauses law would be further away to avoid competition. This would have affected our experiment as we are not sure that the niches of the caterpillar overlap evenly. Because of this our validity and reliability would have been significantly affected.

If we were to conduct this experiment again I would buy a stock type 09 cabbage tree from the website below: http://www.southernwoods.co.nz/cart/prices.asp?lmcommon=Cabbage+Tree&Species_Latin=Cordyline&Name_Latin=australis&Name_Common=Ti%20Kouka .I would then isolate this plant in a greenhouse and keep it there for 12 months watering it regularly. This would ensure that the plant was not treated with any pesticides for a period of 12months prior to my testing. This would mean there wouldn’t be an uneven distribution and hence competition of the caterpillar species due to pesticides.

Cabbage trees often suffer from a fungi called Sphaeropsis cordyline which causes leaf spots this made it difficult to distinguish between window feeding sites and other damage to leaves.

Sometimes it was difficult to distinguish between feeding sites and other leaf spots which resembled window feeding sites. This meant that sometimes we may have accidentally measured damage, instead mistaking it for a feeding site. This could have introduced random errors because the distance from the midrib to the damage would have not correlate with our data for the distance from the midrib to the feeding site. Thus the spread of our data and mean for the window feeding sites and could have caused the spread and mean to either increase or decrease. This will have almost certainly affected the both the validity and reliability of our results, as we were not measuring feeding sites and then we tried to draw conclusions from incorrect findings. This would make it difficult to replicate the results obtained in this experiment also.

If we were to conduct this experiment again I would buy a stock type 09 cabbage tree from the website below: http://www.southernwoods.co.nz/cart/prices.asp?lmcommon=Cabbage+Tree&Species_Latin=Cordyline&Name_Latin=australis&Name_Common=Ti%20Kouka. I would then isolate this plant in a greenhouse and inspect it to see if it was infected with fungi if not I would wait for a period of 12months prior to my testing, checking for signs of fungus monthly. If there were signs of fungi I would purchase another plant and repeat the process. After the 12months I would then take my measurements. While conducting the measurements I would also have colour pictures of the two different feeding sites to scale which I could use to compare to the feeding sites on the cabbage tree.

We may have accidently measured some feeding sites twice.

This would have caused our mean to be inaccurate. Because we measured the same feeding sites the same distance. This would have caused and increase in modes, and our mean to have been effected either increasing or decreasing in relation to our modes. Thus significantly effecting our reliability and validity of our results.

Once we had taken measurements from a leaf we could place a small sticker brought (http://www.amazon.com/Map-Dot-Stickers-Red-Diameter/dp/B003M6WPPW) on the tip of that leaf to indicate that we had already taken measurements then we could move onto the next leaf. Once we had collected all 50 samples we could return and remove all of the stickers.

We had some difficulty in telling where the midrib was.

This would have caused our mean to be inaccurate. As our distances to the feeding sites would have altered if we had an incorrect starting point due to our inability to determine where the midrib was. This would have significantly effecting our reliability and validity of our results.

As the midrib is easier to view on the underside of the leaf. Therefore we would undertake all of our measurements using the underside of the leaf.

The leaves taper to a point so depending on where we measure the distance to the midrib from the feeding site will be less.

This would have caused our mean to be inaccurate. As our distances to the feeding sites would have altered getting smaller as the leaf tapered off. Therefore it would not be clear if we measuring a feeding site which was extremely close to the midrib or it was actually in the middle of the leaf but the leaf was narrower at that point. This would have significantly effecting the validity of our results.

If we were to do this experiment again I would not include any results from the first 15cm of the leaf or the first 15cm from the base to make the distance from the midrib to the outside more equal.

Sources

1)https://www.google.co.nz/url?sa=t&rct=j&q=&esrc=s&source=web&cd=14&ved=0CIkBEBYwDQ&url=http%3A%2F%2Fstudents.kaikourahigh.school.nz%2Fbiology%2F12resources%2Feniche.doc&ei=VWmMUu3POInyiAeUvYGwCw&usg=AFQjCNH42DsZ8PV29T6nuBoMtADGCOS4EA&sig2=fMwOpmm0LcJoP5tVn6ZsiA&bvm=bv.56643336,d.aGc&cad=rja

2) http://www.nobraintoosmall.co.nz/students/biology/NCEA_Level2/pdfs/bio2_90461_habitats.pdf

3) http://wiki.answers.com/Q/What_is_the_difference_in_the_mouthparts_of_a_notch_and_a_window_flax_caterpillar#ixzz2lAbEk3no

4)https://www.google.co.nz/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&ved=0CDsQFjAC&url=http%3A%2F%2Fcarmel.ultranet.school.nz%2FModules%2FResources%2FDownload.aspx%3FID%3D1795&ei=GGeMUr7uHqWiigfnrID4Dg&usg=AFQjCNFrkoI9ZNmdgiAOu587ghFW8Rweig&sig2=oQQROaX6cZ0934xfrqdcoA&bvm=bv.56643336,d.aGc&cad=rja

5) http://eng.keitemohiokoe.tki.org.nz/Overview-of-Biology/Harakeke-5/Activity-Friend-or-Foe-Are-harakeke-caterpillars-fighting-for-the-same-food