Cecropia trees and Azteca ants: FINAL

This discussion topic submitted by Roopa Kamesh ( roopakamesh@hotmail.com) at 12:44 pm on 7/13/01. Additions were last made on Sunday, October 20, 2002.


Abstract
The Cecropia tree and Azteca ant relationship is one of the most conspicuous associations among all the ant-plant associations in Costa Rica. The hollow tree trunk and the production of food bodies rich in glycogen, attract the Azteca ants onto the Cecropia trees. The ants in return for this food and shelter are said to protect the trees from herbivores and prevent growing of vines. A study was conducted on the herbivory of Cecropia trees in Monte Verde , a cloud forest and the herbivory of Cecropia trees in Corcovado, a lowland forest to determine if the ants did infact play a role in protecting the trees. The Monte Verde region is believed to not have ants present due to the high altitude. Therefore it was assumed that herbivory rates will be lower in Corcovado. The results of the study showed that the trees in Corcovado, compared to Monte Verde showed both higher herbivory rates and lesser overall damage. The results of this study may be considered to be imprecise to a certain degree. This is because no Azteca ant colonies were present on any of the trees being studied. These results should instead be used as baseline data, which could assist in future studies on the relationship between Cecropia trees and Azteca ants.

Introduction
Plants that provide food and/or housing for ants, are conspicuous members of the tropical plant communities ( Agrawal et. al 1999). These ants then become constitutive and inducible mechanisms of resistance against herbivores that may feed on the trees. In fact studies involving the removal of ants from the trees indicate that ants do protect the herbivory of the trees and also protect it from competing plants. This is especially believed to be true, in the case of the Neotropical ant-plant association between Cecropia trees and Azteca ants.
Cecropia trees are referred to as “pioneer” species that quickly invade and colonize forest clearings, riverbanks, abandoned farmland, and any area that has light gaps (Janzen 1983). They are known to grow rapidly, sometimes reaching a height of 2.8 feet in one year. The hollow trunk is thin and spindly with bamboo like rings on the outer surface. The leaves are large lobed with six-eight leaves per bunch. The underside of these leaves are hairy that may help ants or other insects to climb. The trees form a “tropical buffet” not only for insects, but for birds, iguanas, and also sloths (Kricher 1997).
Azteca ant communities are found only in the New World tropics, and are extremely aggressive as a species, leading to mosaic colony distributions. They are famous for their arboreal nesting habits, preferring hollow nodes or stems in which they can tend large populations of mealy bugs which provide the ants with sugar, vitamins, and amino acids (Janzen 1983). Azteca ants in particular are especially attracted to the tree, as the tree produces special structures in velvety-brown glandular patches under the leaf stems that provide nourishment for the ants (Kricher 1997). They live in the domatia within themodififed hollow pith of the stem and feed on the nectarines and bead bodies. The ants are extremely aggressive and protective of the tree. They attack any living organism that may cause a threat to the tree.
The association between the trees and the ants is believed to be mutualistic. The Cecropia provides food (between 19-47%) (Sagers et al., 2000) , and shelter to the ants, while the ants protect the trees and also receive nitrogen from the ant carcasses (Agrawal et. al 1999). The level to which ants provide protection is still ambigous. Not many studies have shown how effective the ants are in protecting the tree from damage. To understand the realtionship better, a study was conducted in the Monte Verde cloud forest (where the high altitudes are not conducive for the presence of ants), and in Corcovado a lowland forest (where Cecropia trees and Azteca ants are said to co-exist), both in Costa Rica. The null hypothesis was that there will be no dofference in the herbivory in both the study sites, and the alternative hypothesis was that owing to the presence of ants, the trees in Corcovado will show less herbivory than teh trees in Monte Verde.

Methods
The data required to analyze the mutualistic relationship was obtained from two types of rainforests in Costa Rica. These were the Monte Verde natural preserve and Corcovado National Park. The first being a cloud forest, and the second a lowland forest. This comparison was important to highlight any changes that may occur in the relationship between the Cecropia trees and the Azteca ants based on varying altitudes and weather conditions.
A total of twenty trees were studied in both Monte Verde and at Corcovado. These trees were randomly selected as they were encountered along the trails. The canopy of each chosen tree was divided into four quadrants, and a sample of five leaf groupings were chosen from each quadrant. This was done to ensure an adequate study of the total coverage of the herbivory in each tree. The herbivory of each leaf stalk sampled was initially divided into damaged and undamaged. The damaged herbivory was further ranked into light damage, moderate damage, and heavy damage based on the extent to which the leaves had been damaged. This method was followed in noting damaged and undamaged levels of herbivory for the twenty trees in both Monte Verde and Corcovado. Presence or absence of vines or ants in the trees was also noted. Presence of ants was noted by tapping or shaking the trunks of the selected trees. The results were analyzed using Minitab and StatView software packages. Several Chi-Square tests were performed on the results noted in total, light, moderate, and heavy damage to identify any difference in herbivory between the two study sites. Pictures were taken to help serve as indicator tools to better understand the sclae of light, moderat, and heavy damage.

Results
Certain trends were identified based on the results. An average of the degree of damaged and indamaged leaves was taken in both Monte Verde and Corcovado. On comparison, this showed that in Monte Verde, 68% of its leaves would be damaged, and 32% undamaged. While in Corcovado, 85% of the leaves would be damaged, and 15% undamaged. This says that the rates of herbivory in Corcovado are higher than that of the trees in Monte Verde.
For further study, a similar study was made on the type of damage in the leaves in both the study sites was also conducted. As a result, the leaves in Monte Verde showed 48% light damage, 26% moderate damage, and 26% heavy damage. In comparison, the leaves in Corcovado showed 60% light damage, 29% moderate damage, and 11% heavy damage. Based on this we can observe that the leaves in Corcovado are prone to higher levels of light damage, and the leaves in Monte Verde are prone to higher levels of heavy damage.
A Chi-square test (95% confidence interval) was performed for each of these comparisons, and the results are as follows:
-- total damage of an average tree in Monteverde versus the total damage of an average
tree in Corcovado; p=.023
-- light damage of an average tree in Monteverde versus an average tree in
Corcovado; p= .110
-- heavy damage for an average tree in Monteverde versus an average tree in
Corcovado; p= .0016
-- moderate damage for an average tree in Monteverde versus an average tree in
Corcovado; p= .546
A statistical difference can be observed for total damage, light damage, and heavy damage. Whereas, in the case of moderate damage, there is no apparent statistical difference. These results reiterate the earlier findings that state that, the trees in Corcovado showed a higher percentage of light damage, compared to the trees in Monte Verde; and also in Monte Verde, heavy damage was more prominent in the leaves compared to Corcovado.

Conclusion
The findings of the study show that the degree of herbivory is higher in Corcovado, compared to the herbivory rates in Monte Verde. This disproves our initial hypothesis, which states that the herbivory in Corcovado will be lower than that in Monte Verde. This could be attributed to various limitations that were faced while conducting the study.
The foremost difficulty that was faced, was the lack of ants on any of the trees that were being studied. The aim of the study was to compare an area where there were no ants on trees to an area where ants were present on Cecropia trees, in order to observe any noticeable difference between the two sites, and to determine how effective the Azteca ants are in protecting the Cecropia trees. Though the results did display a difference in herbivory rates, it went against our assumption, as Corcovado, which is believed to have ants, displayed higher herbivory levels. Hence, a question arises, if there would have been a difference, if the trees that were studied had ants on them. Also, this does not help us deduce any specific conclusion on the level to which ants help in protecting the tree.
The absence of ants could be attributed to other limiting factors in the study. The trees that were studied were chosen based on convenience and location near the trail; there was no restriction on age of trees, any tree irrespective of potential age was chosen; also the trees were only tapped or slightly shaken to check for presence of ants, more complicated techniques like slitting the bark of the tree was avoided. Also there was a certain degree of subjectivity involved during visual collection of data on the different levels of damage on the trees. These factors could have affected the observation of ants on the trees.
The results generated not only prove the contrary of what was expected, but also leads to another question, ie., despite lack of ants on the trees, why were the herbivroy rates different. This could be due to various reasons that were out of bounds in the study conducted. The altitude in the study areas, and the kind and number of organisms found in the study sites could prove to be important factors that could be studied to derive a better understanding of varying herbivory rates.
These limitations prove that further, more complex studies must be conducted, to arrive at a better understanding of the kind of relationship that exists between the Cecropia trees and the Azteca ants. This study has only shown that there is a difference between levels of herbivory damage, but the reasons are still ambiguous. The limiting factors that are visible in this study could serve as a basis for future studies that could be conducted. The data that has been generated could also serve as baseline information for further analysis.

References
1. Kricher, John. A Neotropical Companion, Princeton University Press,
Princeton, New Jersey, 1997.

2. Janzen, Daniel H Costa Rican Natural History, The University of Chicago Press, Chicago and London, 1983.

3. Agrawal, Anurag A. and Benjamin J. Dubin-Thaler. "Induced Responses to herbivory in the Neotropical ant-plant association between Azteca ants and Cecropia trees: responses of ants to potential inducing clues", Journal of Behavior, Ecologoy and Sociobiology, Vol. 45, 1999.

4. Janzen, Daniel H. "Dissolution of Mutualism between Cercropia and its Azteca ants", Biotropica, Vol. 5 Issue 1, 1973.

5. Longino, John T. "Geographic Variation and Community Structure in Ant-Plant Mutualism: Azteca and Cecropia", Biotropica, Vol. 21 Iss. 2, 1989.

6. Folgarait, P.J., H.L. Johnson, and D.W. Davidson. "Responses of Cecropia to Experimental removal of Mullerian Bodies", Functional Ecology, Vol. 8 Iss. 1, 1994.

7. Janzen, Daniel H. "Alleopathy by Myremecophytes: The ant Azteca as an Allelopathic Agent of Cercropia", Ecology, Vol. 50 Iss. 1, 1969.



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