Literature Review 4
Study Site 24
Processing of Findings 29
1: Plagiarism Report
3: Raw Data
4: Photographic Images
5: Copy of First Draft
The following research and experiment will be conducted to determine if herbivory has an effect on the aspect ratio between leaves and spines on Acacia grandicornutas. Due to the research conducted in the literature review it is possible to create an aim and a hypothesis.
The aim of the experiment is to determine if herbivory causes the plant to have an induced response causing its aspect ratio between its leaves and spines to change.
The hypothesis is that herbivory will have an effect on the spine to leaf aspect ratio making spines longer than leaves on plants that are exposed to herbivory than on plants that are not exposed to herbivory due to the plants induced response.
Plants can produce a variety of induced responses that follow stress in order to defend themselves from herbivory. Plants can respond to herbivory in localised areas or they can respond over the whole plant. Plants induce responses rather than have consecutive responses as responses are toxic to herbivory and may also be toxic to the plant itself, as well as responses use lots of the plants energy Karban, Myers (1989). Thus trade-offs result between defense responses and growth as there is not enough resources available for plants to invest in both simultaneously Ward, Kirkman, Zinn (2007). Therefore it is possible for herbivory to cause plants to invest in growth in spines (defense) rather than leaves (growth of plant.)
This experiment is to be carried out in two of the Nkuhlu exclosures which are bush savannas with sandy soil on granite bedrock situated in the south east part of the Kruger National Park in South Africa at (25?0?0??S, 31?45?0?? E) Engdahl (2008). The first site is the Full exclosure which excludes all animals larger than hares and the second site is the Control site which excludes no animals. Therefore the experiment can be tested on Acacia grandicornutas that are exposed to herbivory and on Acacia grandicornutas that are not exposed to any herbivory. By measuring the lengths of spine-leaf pairs in the browsing range of herbivores in the different exclosures it will be possible to determine the spine-leaf aspect ratio and compare the ratios of the two sites to determine if herbivory cause induced responses.
This experiment and research will be conducted through the Skukuza Research Camp with their permission to work in the Kruger National Park. Identification of trees and the harvesting of their branches is executed under supervision of armed rangers, for safety, and people qualified in this field. This experiment is important as it shows how plants defend themselves from herbivory and how this affects the availability of food for herbivory. With recent droughts in South Africa plants have begun to experience trade-offs as there is little water available to them, causing them to focus their energy on storing water and defending themselves. This leads to smaller, less leafy trees with higher defenses which results in less food available in the environment to sustain food chains.
The following literature review will provide all the necessary background theoretical knowledge behind induced plant defense responses, how to conduct this form of experiment and data that the results can be compared to. After completing the experiment and using to data and the research in the literature review a full conclusion will be possible to draw.
The topic of my research is the effect of herbivory on Acacia, specifically the induced physical responses that act as a defense mechanism. The following sources include theoretical knowledge on induced responses, previously performed experiments and research that relate to the topic.
Titel: Induced Plant Responses to Herbivory
Author(s): Richard Karban, Judith H. Myers
Authors Credentials: Richard Karban has a B.A. in Environmental Studies from Haverford College and a Ph.D. in Biology from the University of Pennsylvania. Judith H. Myers is the Professor of Zoology at University British Columbia and is qualified in Population Ecology, Biological Control and Invasive Species.
Year of Publication: 1989
Published in: In Annual Review of Ecology and Systematics Journal.
Summary: In this source, Karban and Myers review current research of the phenomena of induced plant responses.
Induced responses in plants follow damage or stress that may be caused by herbivores and act as defenses. Induced response is referred to if stress or injury changes the quality of a plant. Induced resistance is referred to if the induced response decreases herbivory preference or performance. Induced defense is referred to if the reduced herbivory performance or preference increases the plant fitness.
Secondary metabolite, physiological and morphological changes are changes in a plant due to damage. Secondary metabolites and phytoalexins changes are changes in tannins and phenols and result in the changes in the concentration of metabolites: the toughness, bitterness and nutrient content of leaves. Physiological and Morphological changes are changes of photosynthetic and transpirational rates, these changes can cause the plant to return to juvenile growth form which results in: the increase in density and/or length of prickles, spines and hairs. All these changes make the plant less desirable to herbivory. Induced responses are highly specific and therefore when conducting tests artificial damage will not necessarily induce results similar to those induced by herbivory. For example: when leaves are picked or pruned the saliva from herbivores will induce responses that pruning with scissors will not.
Plants can respond to herbivory in localised areas e.g. a single leaf on the whole plant. Localised responses can cause herbivores to feed elsewhere on the same plant which results in the spread out of damage or may cause the herbivore to abandon the plant. It is believed that plants under stress can send chemical signals to neighboring plants to become more resistant. These changes can take place in minutes or over evolutionary time. After the damage has occured, the increase in phenolics, the decline in nutrient concentration, the regrowth of juvenile tissue and the changes in morphology, can be seen in the plant tissue that develops during the growing season after marked defoliation. Therefore long term responses of a plant following an attack will most likely not affect the individual who attacked, however it’s later generations will suffer the consequences.
Plants induce responses rather than have consecutive responses because the products produced are toxic to herbivores and may also be toxic to the plant itself. Therefore the plant only produces toxins locally when necessary. These constantly changing, unpredictable defenses prevent herbivores from building resistance.
Induced defenses are also less costly to the plant. To allocate resources to defenses costs the plants however is worth it if the loss to create defense prevents less loss due to damage by herbivory. It would also cost the plant too much to constantly upkeep defenses and is better to induce defenses when they are needed.
Karban and Myers concluded that plants change and induce responses due to herbivory, however, there is no single mechanism that is able to explain all the diverse responses.
Validity: This source is valid as it cites 111 sources. This source is valid as it is peer reviewed and published in the Journal: Annual Review of Ecology and Systematics.
Usefulness: This source is useful to my research as it provided all the background knowledge terminology and information on my research topic.
Reliability: This source is reliable as the research was reviewed by authors who have doctorates in population ecology.
Limitations: This review was published in 1989 making this an old source, however, it is valuable as it forms the basis of the theoretical knowledge in this field.
Reference in Harvard Format: Karban, R., Myers, J.H., Induced Plant Responses to Herbivory. Annual Review of Ecology and Systematics, online Available at: Accessed 31 March 2018.
Titel: Herbivory on woody plants and induced responses in two similar species of Acacia in the Kruger National Park, South Africa.
Author: Fredrik Engdahl.
Authors Credentials: Report of research conducted by Engdahl at Sweden’s Lantbruks University. Supervisors of the report include: Joakim Hjältén and Tuulikki Rooke of the Sida/Swedish Research Councils research links program.
Year of Publication: 2008
Published in: Swedish University of Agricultural Sciences – University in Uppsala, Sweden.
Summary: Engdahl’s research looks at how large herbivores, mainly elephants and giraffe, impact vegetation and how plants, in particular: Acacia exuvialis and Acacia grandicornuta, have adapted to protect themselves against herbivores by using inducible defenses.
The aim of his research is to test how the absence/presence of giraffes and elephants affect browsing severity and if this severity is dependent on distance to water as well as the affect large herbivores have on the lengths of shoots and leaves and the abundance and length of spines.
Engdahl conducted his experiment in the Nkuhlu exclosures which are situated on the northern bank of the Sabie river, in the south east part of the Kruger National Park. The treatments consist of the Full Exclosure (exclude all mammals larger then hares), the Partial Exclosure (excludes only elephants and giraffes) and the Control Area (accessed by all animals.) Engdahl used transect sampling to conduct his experiment. He randomly placed thirty 2×40m transects in each of the three treatments, using a fixed grid with squares of roughly 30×30m,the starting points were located using GPS. These starting points for each transect were at the center of each square. To avoid biased results a 30m boarder was set between the two vegetation types in the crest and foot slope in each treatment. Acacias measured were roughly the same size and were not chosen if they were taller than 150cm of if the accumulated browsing exceeded 50%.
The results for woody plant abundance showed that there was greater numbers found on the crest than on the foot slope. As well as the full exclosure had more individuals than the other treatments on the foot slope vegetation but fewest individuals on the crest. The results of browsing severity show that the most browsed out of the control and the partial exclosure was the Control and the least browsed was the Partial Exclosure The results for induced responses show that spines on the A. exuvialis were significantly shorter in the Full Exclosure (±3.5mm) than in the Control (±6.2mm), but there was no significant difference in the A. grandicornuta. The spine abundance were higher in the Partial Exclosure than in the Full exclosure and the Control Area. Leaf lengths and shoot lengths were longer in the Partial Exclosure than in the Full exclosure and Control area.
Engdahl concluded that the increasing populations of elephants and giraffes could cause heavy damage to parts of the ecosystem and plant responses in the form of longer spines and shorter leaves making them less palatable. However he notes that even if the exclusion of elephant’s decreases browsing damage, this is only one affecting factor. When elephants are excluded other factors may increase such as competition between plants.
Validity: This source is valid as it is an experiment that was conducted by a scholar at Lantbruks University. This experiment has been supervised by: Joakim Hjältén and Tuulikki Rooke. While conducting this experiment in the Kruger National Park he was assisted by the Scientific Services at Skukuza. The research was reviewed by a fellow researcher of Engdahl, Elin Gunve.
Usefulness: This source is useful due to the fact I am also investigating spine to leaf ratios. This will allow me to follow similar methods of measuring the spine and leaf lengths. This experiment was conducted in the exclosures in the KNP which is where I am conducting my experiment therefor this source is useful because I can expect similar results.
Reliability: This source is reliable as the experiment was carried out the same in all of the treatments. The tests were repeated on multiple trees to gather a large amount of data (767 leaves were measured) to form an accurate average and therefor making the results reliable. Due to the repeated method in collection of multiple data to find accurate averages and the authors good credentials we can say that this source is trustworthy and reliable.
Limitations: The limitations to this source would be that there are only one of the resource topics that relates to my research and therefore many of the pages in this document do not apply to my work. In this research Engdahl has limited his measurements to new shoots only whereas in my research I am measuring from developed branches and removing the new growth. Therefore I cannot compare my length measurements to Engdahl’s due to the chemical variables in new shoots however I can compare the spine to leaf ratios. Another limitation would be that the source was published in 2008 which is 10 years ago and the growth of the savanna and browser population is likely to have changed.
Reference in Harvard Format: Fredrik Engdahl, 2008. Herbivory on woody plants and induced responses in two similar species of Acacia in the Kruger National Park, South Africa. Lantbruks Universitet, online Avaliable at: Accessed 9 February 2018.
Titel: Inducible defenses in Acacia sieberiana in response to giraffe browsing
Authors: David Ward, Kevin P Kirkman, AD Zinn.
Authors Credentials: David Ward is the appointed Art ; Margaret Herrick Endowed Professor of Plant Biology at Kent State University. Kevin P Kirkman is a Professor of Grassland Science at the university of Kwa-Zulu-Natal. AD Zinn has contributed scientifically many times, is affiliated with University of KwaZulu-Natal and other places.
Year of Publication: 2007
Published in: African Journal of Range and Forage Science
Summary: Ward’s, Kirkman’s and Zinn’s research looks at and investigates the inducible chemical and physical defenses in Acacia sieberiana trees as a response to different intensities of giraffe browsing.
The food selection of herbivory, such as giraffes, is based on plant palatability (this is related to the plants nutrient content as well as the plant’s defense mechanisms.) Production of defenses is costly however the defenses have better benefits then the costs. Therefore even though it is costly plants produce inducible responses but only in response to herbivory. The Resource Availability hypothesis that suggests that trade-offs exist between plant defense and plant growth because plants are not able to invest resources into both simultaneously due to the availability of resources in the environment.
Due to the resource availability hypothesis Ward, Kirkman and Zinn decided to an observational study to examine the defense trade-offs and induced defenses in Acacia sieberiana under the different levels of giraffe herbivory.
The study and sampling was conducted from March to June 2005 on the 3400ha iHlanzi Game Ranch in the Otto’s Bluff district of Kwa-Zulu-Natal in South Africa (29?27? S, 30?19? E). The landscape’s vegetation consisted of mosaic grassland, valley thicket and Acacia woodland.
Bases off of fixed-point photographs three sampling sites were identifies: Low-intensity, Medium-intensity and High-intensity giraffe browsing sites. 20 A. sieberiana trees were randomly selected in each of the 3 sites. Measurements were taken from 3 branches per at a height of 2.5m from the outer 0.5m of each branch. The number of leaves and thorns were counted on each branch as well as the measurement of 10 spines and 10 leaves selected at random were taken per branch. Leaves were also collected for chemical analysis.
The results showed that leaf length and width were significantly greater in low browsing areas compared to high browsing areas however density of leaves showed no significant difference. Significant differences were found in spine length and density. The spine length in the high intensity site was larger than in the medium intensity sight and even larger compared to the low intensity being on average 1.1cm larger. Density between high and low did not vary however medium intensity had a much lower density spine count. There was no significant difference found among all three sites concerning tannin concentration, nitrogen percentage in leaf material. However there was a significant difference in hydrogen cyanide – levels increased per site as browsing intensity increased.
Ward, Kirkman and Zinn concluded that the increasing levels of hydrogen cyanide is a means of conserving nitrogen and herbivores do not prefer to eat cyanogenic plants. They also concluded than spine length increases and leaf length decreases with increased herbivory which could be a defense trade-off as well as the results showing no increase in majority of chemicals which could act as defenses shows trade-offs as the plant focused its energy into spine length increase or it could also show that chemicals such as tannins could be a fixed defense.
Validity: The authors of this source are well studied in this field and have good credentials. Advice was obtained by Craig Morris. Laboratory assistance was obtained from Stephne? Stuart and Vanessa Stuart. Assistance in the field was performed by Chippie du Toit, Dieter Rodewald, Andrea Wellmann and Penny Pistorious. Tuulikki Rooke and an anonymous reviewer reviewed the source and made considerable improvements. A National Research Foundation believed in their project and funded their research. This source is valid as it contains 42 references and uses citations.
Usefulness: This source is useful to my research as I am testing for similar results – spine and leaf lengths and ratios based on browsing. This research measurements are taken from grown Acacia branches and not shoots which is what I am measuring from. I am conducting my experiment in the same country where this experiment was conducted – South Africa – and the climates in Mpumalanga (where I am conducting my research) and Kwa-Zulu-Natal (where their research was conducted) is very similar and therefor my factors will be similar and I can expect similar results.
Reliability: This research was conducted by Professor of Plant Biology, a Professor of Grassland Science a well-educated individual who contributes scientifically and is affiliated with the university of Kwa-Zulu-Natal. When conducting field work they followed a scientific method that was carried out in each of the sites and leaf sampling tests to insure fair collection of data and accurate results. Data was collected over 4 months March to June and a vast number of data was collected to insure reliable and accurate averages. All measurements of a same type were taken at the same time in the different sites to avoid problems with sampling plants that for example could lead to confounding errors regarding secondary compound levels. Leaves and thorns were selected at random to measure to avoid selective sampling. Due to the vast amount of data collected to find accurate averages, the scientific method followed in collection of data and sampling of data and the authors’ good credentials we can conclude that this source is reliable.
Limitations: A limitation to this source is that it is just over ten years old which makes it a fairly old source. Due to the measurements of different Acacia species results cannot be compared.
Reference in Harvard Format: Ward, D., Kirkman, K.P., Zinn, A.D., 2007. Inducible defenses in Acacia sieberiana in response to giraffe browsing. African Journal of Range and Forage Science, online Available at: Accessed 29 March 2018.
Titel: Spines of Acacia tortilis: What Do They Defend and How?
Author: Juan H. Gowda
Authors Credentials: Currently a research associate at the National University of Comahue. Gowda was previously a PhD student at Swedish University of Agricultural Sciences who worked under the super vision of Tomas R. Palo on plant defenses and nutritional ecology.
Year of Publication: 1996
Published in: Oikos
Summary: Gowda’s research tests the effect of spine densities on Acacia tortilis on feeding rates and methods of goat browsing. Goats have two different methods of browsing: Picking (only leaves are removed) and Pruning (both twig and leaves are removed.) In prior research Gowda found that previous studies focus only on comparing the effects of herbivory on presence and absence of spines rather than testing for quantitative variation in spinescence due to the browsers preference of branch and method of browsing. Gowda also found that previous studies showed that spines caused as an induced response was localized to only act as a defense to branches exposed to herbivory.
Due to this research Gowda decided to test as experiment 1: if biomass removal is affected by the presence of spines and whether spine density affects feeding rates. Gowda also tested as experiment 2: if spineless shoots were more affected by browsing when positioned next to spiny shoot.
Gowda conducted his experiments at the Livestock Production and Research Institute in Mpwapwa, Central Tanzania from February to March 1995. For experiment 1: in an open paddock (no trees or shrubs) 12 sticks were presented at 1m intervals. At a height of 1m two shoots (±42cm), either spiny or spineless, where attached horizontally (with apices pointing in opposite directions) to each stick.
One animal at a time was allowed to feed until it left the experimental area. Immediately after the animal left the area, shoots were removed and analyzed. This experiment was repeated with 5 different goats for sticks with spineless shoots and with 6 different goats for sticks with spiny shoots. A similar procedure was followed for the second experiment with the difference being that spiny shoots were paired next to spineless shoots on the same stick. Feeding rate was calculated as biomass removed from the pair divided by time spent at the stick.
The results from Gowda’ experiment show that only 1/116 spineless shoots was not pruned. 10%-15%of spiny shoots were not pruned. A significantly greater amount of biomass was lost by spineless shoots compared to spiny shoots and feeding rates were higher on spineless shoots. Goats removed significantly more biomass through pruning than picking. More focus was on pruning when browsing on spineless shoots and an equal amount of focus was placed on pruning and picking when browsing spiny shoots. However when browsing spineless shoots associated with spiny shoots both methods of browsing were important as like above when browsing on just spiny shoots.
By putting more focus on picking, less biomass was removed, however, a large amount of leaves were still removed. Gowda concluded that spine density leads to lower feeding rates and lower biomass removal. Spines may act as a defense to the apices of its twigs rather than to the leaves. This therefore makes the defense more important against large browsers. From his research Gowda could also conclude that increasing spine density leads to associative protection of neighboring shoots, however whole plants are more complex than two shoots and more research should be conducted before making further assumptions.
Validity: This report was peer reviewed by R.T Palo and P. Grubb to improve it. This work contains 19 references and makes use of citations. When conducting the experiment Gowda insured only one animal was in the experimental setup at a time to insure there was no competition in feeding. All results were presented in dry mass values to account for water loss. All experiments were performed after goats had been grazing for at least two hours to insure hunger drive did not play a role.
Usefulness: This source is useful as it gives me reverse insight into the results I am testing for. This source allows me to cross check my theoretical research.
Reliability: This source was conducted by a PhD student of Agricultural Sciences and reviewed by Tomas R. Palo a professor on plant defenses and nutritional ecology. Multiple tests were conducted to obtain a good amount of data from which accurate results could be drawn. Ten sticks were removed before analysis due to time-recording problems and no visit by the animals to insure reliable results. The same scientific method was followed throughout the experiments to insure reliable data.
Limitations: The author indicates that there where time recording problems when conducting the experiment, these problems were accounted for however there is the possibility that some could have gone un noticed. This source was published in 1996 making it an old source however there is limited sources regarding this take on this topic and it is necessary to use this source.
Reference in Harvard Format: Gowda, J.H., 1996. Spines of Acacia tortilis: What Do They Defend and How?. Oikos, online Available at: Accessed 29 March 2018.
Titel: Lecture on Acacia Tree Defense Mechanisms
Author: Melissa Holbrook Schmitt
Authors Credentials: Dr Schmitt is a postdoctoral researcher. She is affiliated with the South African Environmental Observation Network and the University of California Santa Barbara. Schmitt’s research focuses on herbivore-plant interactions – particularly plant responses to herbivory in the form of chemical defenses and what reaction of mammalian herbivores is provoked in response to these chemical defenses.
Year of Publication: 2018
Source Type: Interview
interview: Dr Schmitt advised that Acacia grandicornutas would be the best specimen to sample from in the exclosures as they grow in abundance, have prominent spines and are preferred by many herbivores especially impalas, and thus would be the best specimen for my research.
Dr Schmitt explained the advanced defense mechanisms of the tree and it’s well defined induced responses which are chemical and physical. Dr Schmitt described how Acacia grandicornutas releases chemicals and tannins when browsed by herbivores. These chemicals repel herbivory as they are released into the leaves, are bitter and can be toxic when consumed in large amounts. The trees also release an airborne hormone in response to herbivory that alerts surrounding trees of the danger and causes them to induce defense responses. Dr Schmitt explained that testing for these chemical responses is extremely difficult and takes more time and equipment than what was available and therefore she recommended rather testing the physical responses as they are easily visible as well as measurable.
Dr Schmitt listed the variety of induced physical responses that could be tested for, of which included: branch shape, tree shape, spine length, leaf length, bark thickness and leaf and spine density.She further advised that removing the first 20cm of each branch tested would make the experiment more reliable as new buds that are not fully developed would not be measured. Dr Schmitt also advised that comparing the aspect ratio of spine-leaf pairs of the different trees would be more reliable and accurate for this experiment. It would be unreliable to just measure the spine lengths and compare those as the leaves could grow larger as the spines grow larger and therefore would make no difference to attainability of leaves for the herbivory.
Dr Schmitt advised to use younger trees as they are still in the process of developing and thus it is important for them to invest in defenses to protect themselves. She also explained that measuring the grey spines would make the experiment more reliable as they have finished growing.
Dr Schmitt explained that if it can be proven that with observations that the tree has to defend itself then the spine-leaf aspect ratio is a result of an induce response to herbivory as a defense mechanism and not as a result of environmental factors.
Validity: The information from this source is valid as it is based off of current and past research. Dr Schmitt specialises in this research and still currently works in this field, thus as the interview was conducted this year, the information is valid.
Usefulness: This source is useful as Dr Schmitt specialises in herbivore- plant interactions and plant responses to herbivory. In this interview Dr Schmitt gave useful advice on what the best experiment to conduct for my investigation would be as well as useful advice on how to go about conducting the experiment and making it more accurate and reliable.
Reliability: The information from this source is reliable as the interviewee has good credentials and is highly qualified in this field. Dr Schmitt is affiliated with the South African Environmental Observation Network and the University of California Santa Barbara. Schmitt’s research focuses on herbivore-plant interactions – particularly plant responses to herbivory in the form of chemical defenses and what reaction of mammalian herbivores is provoked in response to these chemical defenses.
Limitations: As this interview was not recorded, not all the information was taken down, however all the useful and important information was noted.
Reference in Harvard Format: Schmitt, MH. 2018. Lecture on Acacia Tree Defense Mechanisms. Interviewed by Rutherford, S. Potgieter, M. Written Notes Skukuza Research Facility 13 January 2018.
The sources in this literature review provide theoretical knowledge behind the topic as in Source 1, by explaining how plants such as the Acacia grandicornutas have induced response defense mechanisms that are triggered by damage or stress caused by the browsing of herbivory Karban and Myers (1989) or by airborne hormones Schmitt (2018). These defenses can be physical or chemical and result in making it difficult and unpleasant for herbivory to browse as in Source 4. These sources also provide knowledge on how to go about testing for my research as they give examples of different methods used and advice on how to make the experiment more reliable as in Source 3 and Source 2.
Source 2 and Source 5 give good background knowledge to the site of where my experiment is to take place as a similar experiment was conducted in the same location in Source 2 – thus also providing knowledge on how to go about conducting the experiment. Source 5 is an interview with a postdoctoral researcher currently researching at the site where the experiment is to take place and is also therefore helpful in knowledge of the site and how to conduct the experiment as she specialises in this particular field.
However Source 2 and Source 5 have a contradiction, in Source 2 Engdahl measures the shoots of leaves of the acacias, does not use aspect ratio and does not remove the first 20cm of the branch Engdahl (2008). Whereas in Source 5 Dr Schmitt advises not to use shoots as they are not fully developed and will give inaccurate results, to uses aspect ratio to make a better comparison and to remove the first 20cm of the branch to remove new shoots and the part of the branch that is likely to already be browsed thus giving accurate results Schmitt (2018).
The literature review gives a full overview of the process: causes, responses and effects.
• Harvesting of branches from the Acacia grandicornuta trees are executed under supervision to ensure that the habitat will not be disturbed and the tree will not be damaged.
• As this experiment took place in one of South Africa’s National Parks (Kruger National Park) it was essential to obtain permission from the Skukuza Research Facility to conduct our field work.
• During field work we will be accompanied by experts on the Acacia grandicornutas, the wildlife and ecosystems of the Kruger National Park. This will help with the correct identification of the Acacia grandicornutas and insight on their habitat.
• All information gathered on the topic of thorn leaf ratio and Acacia grandicornutas is acknowledged and referenced ensuring no plagiarism occurs.
• Safety is insured during the field work in the Kruger national Park by being accompanied at all times by to two park rangers armed with firearms.
• Due to harsh weather conditions it is necessary to ensure one has a sustainable amount of water to avoid dehydration and sunscreen and hats to avoid severe sunburn.
• Permission to conduct the field research was granted by the Skukuza Research Facility.
3. Selection of Study Site:
This experiment will be conducted on the 13th of January 2018. The fieldwork will be carried out in two of the Nkuhlu exclosures. It will be carried out for the duration of 2.5 hours in each exclosure. The Nkuhlu exclosures are bush savannas with sandy soil on granite bedrock situated in the south east part of the Kruger National Park in South Africa at (25?0?0??S, 31?45?0?? E) Engdahl (2008).
The site consists of three areas of 25-30 ha each: The full exclosure excludes all mammals larger than hares. The Partial exclosure excludes giraffes and elephants. The Control site excludes no animals. This experiment takes place in the Full exclosure and the Control site. Refer to the map of the sites in Image 1.
Image 1: Map image of Study Site
Nkuhlu Exclosures 24°59’12.0″S, 31°46’25.6″E.Google Earth (2018).
• Hedge shear
• 2m metal ruler
5.1 Measuring leaf length and thorn length in the Full exclosure:
5.1.1 Identify 5 Acacia grandicornuta trees of relatively the same age (5-7 years.)
5.1.2 Measure out the browsing range of a browser on each tree with a metal ruler – between 0.5m and 1.5m above the ground.
5.1.3 In this range randomly select 3 branches.
5.1.4 Cut off the first 20cm off the end of the branch with pruners to eliminate new growth which is momentarily not at its full grown length.
5.1.5 Cut the branch off the tree with the pruners where it branches off the trunk.
5.1.6 Measure (and record in a table) the lengths of each thorn and leaf of the first 10 thorn-leaf pairs with a caliper, starting at the end where the first 20cm of the branch was removed.
5.1.7 Repeat steps 5.1.4 to 5.1.6 on each of the three branches.
5.1.8 Repeat steps 5.1.2 to 5.1.7 for each of the 5 trees.
5.2 Measuring leaf length and thorn length In the control:
5.2.1 Identify 5 Acacia grandicornuta trees that are the same age (5-7years) as identified in step 5.1.1 to ensure they have had the same amount of time to develop.
5.2.2 Repeat steps 5.1.2 to 5.1.8.
5.2.3 Analyse results by:
188.8.131.52 Calculating averages for lengths of spines and leaves for each tree and the average length of spines and leaves in each site, and record results.
184.108.40.206 Using average spine length and average leaf length for each site create the average spine-leaf ratio for each site and record data.
220.127.116.11 Compare the average spine-leaf ratios of the two sites.
While conducting the field work to obtain results a few interesting observations were made that relates to this research topic. The vegetation in the Full Exclosure was very dense compared to the open area of the Control Site. When measuring the leaves and thorns in the Control site it was seen that a few of the leaf stems growing out from the branch where growing parallel to the branch rather than perpendicular. This can be seen in Image 2 and Image 3 in the appendix.
An observation of the branches in the Control site also stood out. The ends of the branches showed no growth of leaves what so ever, however this cannot be seen in the results due to in the method the first 20cm of the end of the branches were cut off to prevent measuring new growth. This can be seen in Image 4 in the appendix. Due to conducting our research on trees out in the open savanna it was expected to see insects crawling on the trees. However there where a noticeably increase in the amount of ants crawling on the subject trees in the Control Site compared to the amount on the trees in the Full Exclosure. A photo of the ants observed can be seen in Image 5 in the appendix. The trees in the Full exclosure where considerably larger in size and their branches where more spread apart than those in the Control site.
Table 1: Comparison of mean Spine and Leaf Lengths in the Full Exclosure and Control Site.
Location: Mean Spine Length (mm) Mean Leaf Length (mm)
Full Exclosure 18,41 48,44
Control Site 42,42 33,00
Table 2: Comparison of Ratio of mean Spine and Leaf Lengths in the Full Exclosure and Control Site.
Location: Spine to Leaf Ratio
Full Exclosure 1:2
Control Site 1:0.8
Processing of Findings
Herbivory has effects on Acacias, specifically they cause Acacias to have induced physical responses that act as a defense mechanisms. In my research I studied the effects of herbivory on the spine to leaf ratio of Acacia grandicornutas Engdahl (2008). The research was conducted in the Nkuhlu Exclosures in the Kruger National Park on Acacia grandicornuta trees. The same methods for the experiment where used in each exclosure which lead to reliable interesting results.
The experiment was conducted in the Full Exclosure and the Control site of the Nkuhlu Exclosures in the Kruger National Park – bush savannas with sandy soil and granite bedrock Engdahl (2008). The Full Exclosure excluded all animals larger than hares and thus the trees were not exposed to any herbivory whereas the Control site was an open area that did not exclude any animals and thus the trees were exposed to herbivory.
The test subjects of the experiment were the spine and leaf pairs on Acacia grandicornuta trees. These trees were advised by Dr Schmitt to uses as subjects due to the abundance of them found in the exclosures, their prominent spines and their preference by herbivory for browsing Schmitt (2018). It is known that many plants such as Acacia grandiconutas have induced responses that act as defense mechanisms following stress or damage to the plant. These responses can be costly to the plant as they cause trade-offs however they are more beneficial as they protect the plant Karban, Myers (1998). This is why Acacia grandicornutas where a perfect test subject for this experiment.
The experiment was conducted first in the Full Exclosure and then in the Control Site, under the supervision of qualified researchers in this field and with the protection of armed rangers. The method was kept the same in both sites and repeated on multiple branches of multiple trees to find accurate averages to insure reliable results. Five Acacia grandicornuta trees were identified in each of the sites. Three branches where randomly selected within browsing range on each tree. The first 20cm was cut off of each branch to remove unreliable new growth Schmitt (2018). Then the length of the first 10 spine-leaf pairs where measured on each branch and recorded. Measurements were taken using calipers and the browsing range was determined using a 2m metal ruler – to insure accuracy of results. The method used had validity as it was created based off of Engdahl’s research as part of his research included the measuring of spines and leaves of Acacia grandicornutas in the same sites Engdahl (2008). However this method was altered due to Dr Schmitt’s more recent incite on measuring spines and leaves – she advised to remove the first 20cm of the branch to remove unreliable new growth whereas Engdahl’s measurements included the measuring of shoots. Thus this method used is based off of previous experiments that have been improved by a postdoctoral researcher in this field Dr Schmitt. Therefor this method has validity and is reliable.
600 measurements were taken in total, 300 spine-leaf measurements consisting of 150 spines and their 150 leaf partners were measured and recorded in each of the sites. The average spine length for the trees in the Full Exclosure was 18.41mm. The average spine length for the trees in the Control Site was 42.42mm. The average leaf length for the trees in the Full Exclosure was 48.44mm. The average leaf length for the trees in the Control site was 33mm. The average spine to leaf ratio for the Full Exclosure was 1:2.3. The average sine to leaf ratio for the Control Site was 1:0.78.
These results clearly show that the spines in the Full exclosure (18.41mm) where smaller than the spines in the Control Site (42.42mm). The leaf measurements show that the leaves are longer in the Full Exclosure (48.44mm) than in the Control Site (33mm). These are the types of trends that where expected, as the site with no herbivory as expected in the hypothesis had shorter spines and longer leaves than the site subjected to herbivory. However this data does not mean much without the ratio of the corresponding spines and leaves Schmitt (2018) because to know if the spines are acting as a defense mechanism they would have to be longer than their partner leaves, aspect ratio is necessary to compare in order to determine if lengths are a result of induced response.
After analyzing the data collected from the spines and leaves the average spine to leaf aspect ratios were determined and the results showed that the aspect ratio for the Full Exclosure was 1:2.3 and the average aspect ratio in the Control Site was 1:0.78. This clearly shows that when the Acacia grandicornuta trees are not subjected to herbivory their leaves are 2.3 times larger than their spines thus causing their leaves to be unprotected as the leaves project further out than the spines making them more vulnerable to browsing. In the Control Site however, the leaves were only 0.78 of the spines length. This shows that when the Acacia grandicornuta is exposed to herbivory its spines project further out than its leaves. This can be seen as a defense mechanism because Gowda’s research shows that when spines protrude further than leaves they protect the leaves from browsing as the spines that project further out than the leaves and poke, prick and make it harder for herbivory to brows thus lowering herbivory’s performances and protecting the plant Gowda (1996).
Karban and Myers research shows that herbivory saliva that comes in contact with the plant after the herbivore has caused damage to the plant can induce responses on plant and cause them to return to juvenile growth which results in increasing density/length of prickles, spines and hairs Karban, Myers (1989). Increased spine density and length according to Gowda acts as a defense mechanism for plants Gowda (1996). Therefore due to this experiment showing that the aspect ratio of thorns to leaves is smaller in the Full Exclosure than in the Control Site it shows that the presence of herbivory play a role in how the plant grows and uses trade-offs.
Is research shows that in the Full exclosure (experimental control) the trees were not subjected to herbivory and that resulted in them putting more energy into growth (leaf surface for photosynthesis) and less into defense (spines) as their leaves were significantly larger than their spines. However when the Acacia grandicornuta trees were subjected to herbivory, in the Control Site, the collected data showed a difference to that collected in the Full Exclosure. The data collected in the Control Site – where the trees are subjected to herbivory – showed that the spines were longer than the leaves. This shows that the subjection to herbivory caused the trees to adjust their trade-offs and rather focus on defense (spine growth) rather than their growth (leaves) unlike in the Full Exclosure. This data collected corresponds with the hypothesis made at the beginning of this experiment.
According to Dr Schmitt observations of other physical defense differences between the two areas are necessary to note as they prove that the spine to leaf aspect ratio is due to an induced response that acts as a defense against herbivory and not as a response against environmental factors Schmitt (2018). The following observations where made while conducting the experiment and back up the theory that the increase in spine length and decrease in leaf length in the Control Site is due to herbivory:
The Acacia grandicornuta trees where considerably larger in the full Exclosure than in the Control Site and their branches where more spread apart. According to Gowda when branches are closer together as seen in the Control Site their spines act not only as protection for the branch they grow on but also for the surrounding branches and is a defense response to herbivory Gowda (1996). Due to the trees being larger in the full exclosure it shows that the tree’s trade-offs in the Full Exclosure allocate majority of its energy to its growth as it needs no defense as it is not subjected to herbivory browsing whereas the trees in the Control site allocate more energy to defense and they are smaller in size but have spine protection – this is a sign of a induced defense mechanism in the Control Site due to herbivory Schmitt (2018).
Another observation made was that the leaves were not only shorter in the Control Site than in the Full Exclosure, they had also grow in parallel to the branch (seen in image 2 and 3 in appendix 4) rather than perpendicular to the branch line in the Full Exclosure. Thus positioning the leaves in the Control Site closer to the branch making them harder for herbivores to reach as the thorns project out even further than the leaves. The way the leaves had grown in the Control Site is another example of a defense mechanism of the tree against herbivory. The final observation that was made that could be a defense mechanism had to be with the leaf growth of the branches. In the Full Exclosure the leaves grew right until the ends of the branches, however in the Control Site the ends of the branches showed no growth of leaves whatsoever (seen in image 4 in appendix 5), however this was not recorded in the results due to: in the method the first 20cm of the end of the branches were cut off to prevent measuring new growth. This shows that the leaves grew closer to the stem of the tree thus making them further out of reach for herbivory and thus protecting the plant.
Due to there being many observations of differences between the trees in the different sites. Due to the differences in the trees in the Control Site to the trees in the Full Exclosure act as defenses against herbivory, it can be said that the difference in aspect ratio of spine to leaf length is due to an induced response acting as a defense mechanism against herbivory and not due to environmental factors. The increase in spine length and decrease in leaf length in the Control Site compared to the Full Exclosure show that the presence of herbivory have an effect on the Acacia grandicornuta trees and when herbivory are present it causes the trees to induce a defense response against herbivory that increases spine length making the spines longer than the leaves. These results back the hypothesis made at the start of the experiment.
This experiment tested the effect of herbivory on the Acacia grandicornuta tree’s thorn to leaf aspect ratio. The aim was to test if the presence of herbivory caused the tree to induce a physical defense response that affects it’s spine to leaf aspect ratio. It was hypothesized that the presence of herbivory would have cause an induced response in the trees and cause a change in the spine to leaf aspect ratio: an increase in spine length and a decrease in leaf length, the spines will be longer than the leaves when exposed to herbivory and the leaves will be longer than the spines when the trees are not exposed to herbivory.
Once the data was analyzed it showed results that were predicted in the hypothesis. The average spine length in the Full Exclosure (18.41mm) was smaller than the average spine length in the Control Site (42.42mm), the average leaf length in the Full exclosure (48.44mm) was longer than the average leaf length in the Control site (33mm). When combining the data the results showed that the average spine to leaf ratio in the Full exclosure (1:2.3) was smaller than the average spine to leaf aspect ratio of the Control Site (1:0.78).
To conclude, based on the results, herbivory do cause Acacia grandicornutas to induce a response in their spine-leaf length ratio which causes spines to be longer than leaves when subjected to herbivory and spines to be shorter than leaves when not subjected to herbivory, thus the hypothesis was proven correct. This data shows that when the trees are not subjected to herbivory they put more of their energy in to their leaves to provide them with more energy to grow rather than into growing the length of their spines because there is no need for protection as they are not subjected to herbivory which cause harm to the tree. The data also shows that when the trees are subjected to herbivory they induce a defense response that causes them to have trade-offs – Ward, Kirkman, Zinn (2007) – which causes them to put more energy into increasing their spine length to protect the leaves from the herbivory and this results in shorter leaf lengths as plants are not able to invest resources into both spines and leaves simultaneously Ward, Kirkman, Zinn (2007).
The following observations of other defense mechanisms on the trees in the Control Site recorded prove that the change in spine to leaf aspect ratio from the Full Exclosure to the Control Site is a result of herbivory and not environmental factors Schmitt (2018): significant increase in the amount of ants on the trees in control site, the trees in the Full Exclosure were larger with more spread out branches than those in the Control Site, the leaf stems in the Control sight grew parallel to the branches unlike those in the Full Exclosure that grew perpendicular to the branches and there was no leaf growth at the ends of the branches in the Control Site unlike the Branches in the Full Exclosure which had leaves that grew right to the tip of the branches.
The method was kept the same in both sites and many measurements were taken with suitable equipment to ensure accurate and reliable results. However there is one limitation to this experiment: according to Schmitt (2018) trees release airborne hormones as a result of stress caused by herbivory that cause surrounding trees to induce defense responses, this means that the trees in the Full exclosure that where next-door to the Control Site could have been triggered to induce defense responses even though they themselves where not subjected to herbivory.
This means the measurements taken in the Full Exclosure could possibly not be the measurements of a tree that has not induced any defense responses. The herbivory while not in contact with the trees could still have had an impact on them. In the future it would be more reliable to measure the spines and leaves of trees that are isolated from any exposure to airborne hormones that could have an effect on their growth. Engdahl’s research and results of spine lengths support the results of this experiment however the leaf length measurements cannot be compared to Engdahl’s results as he included the measurements of new shoots whereas in this experiment the measurements of shoots where left were not taken as they would be unreliable as they are not fully grown. The trees chosen to measure where randomly selected however it would be more accurate to in the future to use transect sampling like in Engdahl’s experiment Engdahl (2008).