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ABSTRACT
The neotropical Cecropia tree has an interesting mutualism with an ant in the Azteca genus. The ants are allowed to live in the hollow stem of the tree and eat of the nectar producing bodies that the tree produces. In return, the Azteca ants provide the tree with defense though biting and stinging herbivores and trimming encroaching vines. Cecropia trees were sampled from three locations in Costa Rica: Monteverde, Corcovado National Park, and Gandoca. The trees were placed into three size categories and then the amount of herbivory was measured using an herbivory index. From the results it appears that when the Azteca ants are present on a Cecropia tree the amount of herbivory is reduced. The Azteca ants and Cecropia trees have tightly evolved into this mutualistic relationship with not much room for conflict.
INTRODUCTION
Cecropia trees are found in the Neotropics mainly in areas of large light gaps and secondary growth. The seeds from Cecropia remain viable in the soil for up to two years and the trees are adapted to grow rapidly when the necessary light becomes available. Cecropia trees are easily located along roadsides and along streams, as well as in abandoned fields. These trees are easy to identify with a spindly gray trunk and leaves that are very large and deeply lobed. It has been said that the leaves resemble a parasol. The undersides of the leaves are whitish and often damaged by herbivory. The truck of the Cecropia is hollow, which may be an adaptation for rapid growth to reach a height that enables the Cecropia to out compete other plants for light. The trade off in this situation is more growth for less production of wood (Kricher 1997).
Cecropia are utilized by approximately 48 animal species. The species range from mammals, such as bats and monkeys that eat the fruits, sloths, which make a meal out of the leaves, birds that may come in search of insects, insects like leaf-cutter ants, and iguanas (Kricher 1997). Not all animal species utilize Cecropia solely for their own bnefit. There is one species of ant that has formed a unique relationship with the Cecropia tree.
Inside the hollow stems of the Cecropia tree Azteca ants can be found. It has been noted that this ant-plant relationship is a case of evolutionary mutualism. The tree will provide these ants with a house in the hollow stem as well as nectar producing bodies that supply some of the ants' diet. In return the ants protect the tree from insects and encroaching vines, as well as providing a source of nutrients from frass, insect carcasses, and other insect debris that they leave behind in the stem (Sager et al. 2000).
Many previous studies have led to hypotheses that the ants may be on the same level as other chemical defenses that are employed by plants for protection. It does not appear that using ants as a plant defense is as simple as the presence or absence of the ants. Like chemical defenses, the concentration of the ants may be dependent upon many environmental conditions. The level of defense seems to be related to the dynamic behavior of the ants. The ant behaviors may be directly related to their effectiveness of the protection they provide to the Cecropia trees.
Another similarity to chemical defenses is the ant defense seems to be induced by herbivory. Studies have documented in temperate plants that phytochemical and physical responses are induced following herbivory. Other studies have shown that some tropical plants are able to respond to herbivory also. For example the African Acacia drepanolobium has been recorded to increase the length of newly formed thorns as a result of giraffe herbivory.
It was noticed that disturbance to ant bearing plants resulted in the ants swarming to the disturbance ready and willing to bite or sting anything that it came across. The following aspects of ant biology make them suitable to be an inducible plant defense: 1) acute sensory mechanisms to detect disturbance and chemical cues, 2) often aggressive and well developed defense mechanisms, 3) have recruiting mechanisms making them able to rapidly deploy to the site of disturbance, 4) individuals within the ant colony are willing to die in the defense of the colony or it resources (Agrawal 1998).
Mutualisms consist of interspecific interactions in which all organisms involved benefit in some way. In some sense, mutualism can be viewed as "reciprocal slavery…each species has been selected to exploit its obligate partner without being able to avoid being exploited" (Anstett et al 2002). In recent studies it has been noted that conflict underlies all cases of mutualism in the sense that one species may be cheating in the mutualistic relationship by taking the benefits without paying the cost (Sager et al. 2000). Conflicts are seen as limiting evolution of the mutualism by inhibiting the evolution towards higher benefits for each partner. In other words, the conflicts that are within a mutualism tend to destabilize the interaction (Anstett et al 2002) In this particular ant-plant mutualism one potential conflict is ants consuming nutrients from the nectar producing bodies at the cost of the tree while not providing benefits to the Cecropia tree.
Another potential conflict is the plant could be exploiting the ant colony for the nitrogen source from the debris left behind by the ants, but this would not be an extra cost to the ants (Sager et al. 2000). To determine the amount of conflict within this system Sager et al. (2000) looked at carbon and nitrogen isotopes trace nutrient exchange between Cecropias and Azteca ants. They found that the ants were receiving the majority of their diet from external sources such as insects and therefore not exploiting the food provided from the tree thereby reducing the amount of conflict that this system contains.
The mutualism described above that exists between the Cecropia trees and Azteca ants seem to benefit both sides of the system: the tree and the ants have formed what seems to be a perfect relationship. The hypothesis that was formulated was: if the different species of Cecropia trees are inhabited by any ants in the Azteca genus then the effects of herbivory on the trees will be greatly diminished.
METHODS
The study was located in three areas of Costa Rica in May of 2002. One site was located in a cloud forest in Monteverde, Costa Rica. Monteverde was at the highest altitude, had the coolest temperature, the highest humidity, and the heaviest rainfall of the three sites. The second site was located in a rain forest within Corcovado National Park. This area was at sea-level altitude, has higher temperatures than Monteverde, and had less humidity and standing water in comparison with Monteverde. The third site was located in Gandoca, Costa Rica. This site was also at sea-level altitude, was very humid, and had high temperatures.
The following size ranges classified Cecropia trees. A tree was placed in the small classification when between 2 and 5 meters tall. A medium tree was considered 5 to 8 meters tall. Large trees were 8 to 15 meters tall. Each size classification was used at Monteverde. At Corcovado National Park small trees were not observed and Gandoca did not have small or medium sized Cecropia trees.
An herbivory index was created to allow for herbivory measurement. The herbivory index is as follows: 0, no observable herbivory; 1, very little herbivory; 2, little herbivory; 3, moderate herbivory; 4, high herbivory; 5, very high herbivory. Trees were observed from the ground level and several were observed using the zoom function on a video camera. Before the herbivory index was used an observation was made to record if Azteca ants were present or not present on a given tree. To test for ants the tree was knocked on by using a fist or by a rock to imitate a disturbance of an intruder onto the tree.
RESULTS
Table 1: The average herbivory on Cecropia trees located in Monteverde Cloud Forest, Costa Rica with corresponding presence or absence of Azteca ants.
Size Avg Herb Ants
Small 0 no
Medium 1.5 no
Large 2.5 no
AVERAGE 1.3 no
The average overall herbivory on Cecropia trees in Monteverde was 1.3 on the herbivory index indicating that there was very little herbivory taking place within the cloud forest (Table 1). It also appears that the large Cecropia trees (8-15 meters in height) were at 2.5 on the herbivory index indicating that they were experiencing little herbivory (Table 1). It is interesting to note that at this particular study site no ants were present on any of the trees that were observed (Table 1).
Table 2: The average herbivory on Cecropia trees located in Corcovado National Park, Costa Rica with corresponding presence or absence of Azteca ants.
Size Avg Herb Ants
Small . .
Medium 1 yes
Large 0 no
AVERAGE 1 yes
The average overall herbivory on Cecropia trees in Corcovado National Park was 1 indicating that there was very little herbivory taking place in this part of the country (Table 2). At this particular study site the medium Cecropia trees were also experiencing little herbivory (herbivory index 1), but in contrast to Monteverde the large Cecropia trees that were included in this study at Corcovado National Park were experiencing no observable herbivory (Table 2). At Corcovado National Park the medium trees that were observed showed the presence of ants, while the large trees showed the absence of ants (Table 2).
Table 3: The average herbivory on Cecropia trees in Gandoca, Costa Rica with corresponding presence or absence of Azteca ants.
Size Avg Herb Ants
Small . .
Medium . .
Large 1 yes
AVERAGE 1 yes
The average overall herbivory on Cecropia trees in Gandoca was similar to Corcovado National Park with an herbivory index of 1 indicating very little herbivory (Table 3). At this particular study site small and medium Cecropia trees were not located and therefore could not be included in the average of Gandoca.
DISCUSSION
The results of this experiment support the following hypothesis: 'If the different species of Cecropia trees are inhabited by any ants in the Azteca genus, then the effects of herbivory on the trees will be greatly diminished'. It appears that the average herbivory for each location was lower when Azteca ants were present. This is shown through the comparison of Monteverde average herbivory with Corcovado National Park and Gandoca herbivory index averages.
It was observed that the ants were absent in a marsh-like, water-heavy environments, such as Monteverde. At Monteverde ants were absent from the Cecropia trees and as in our hypothesis the herbivory was slightly higher compared to when the ants were present in Corcovado National Park and Gandoca. Some reasoning behind this finding may be the high amount of tannins and lignin in younger leaves, which may be a replacement for the ants.
Another observation that was noted was that the ants, when present on the tree, are not usually found on the main leaf, but instead on the trunks and nodes of the leaves. This provides an opportunity for small insects and beetles to go unnoticed by the ants as they eat parts of the leaf. So, even though the ants are present there are still chances available for other organisms to create the herbivory that was seen on the Cecropia trees. Some studies have been completed to determine what types of cues are used to induce the defensive mechanisms of the ants. Some have hypothesized that the plants may give off volatile compounds that that the ants can sense. Other hypotheses include disturbance, visual signs, pattern of the herbivory, and possibly saliva from the herbivore for the inducible ant defense (Agrawal 1998).
The conclusions from this study are that Cecropias do benefit from the presence of ants by deterring the majority of organisms from eating the tree. Also, the placement of energy serves as a portion of defense showing that ants do not function as the only defense mechanism. The Cecropia trees have adapted to rapid growth when the environment is right. More energy can be placed into growing, while using the Azteca ants as a defense mechanism instead of taking energy from growth and using it for other defense mechanisms, such as chemical defense. The benefit of having Azteca ants on the Cecropia trees must be higher than the cost of the Cecropia tree allowing the ants to inhabit the tree and use parts of the tree for food.
Some of the errors that were encountered during this study were the small sample size of each Cecropia tree size rage that was taken from the field, ground observance of the leaf herbivory, lack of observance of herbivory presently taking place, rapid construction of the herbivory index, and the length of the study was done in a small time frame.
For future research it would be beneficial to complete an actual canopy investigation to more precisely measure the amount of herbivory that has taken place on a given tree. It would also be beneficial to complete a comparative study of different tree species that also have a mutualistic relationship with ant species to determine if other mechanisms of defense are present. It would be interesting to determine the actual cues that the plant uses to notify the ants of an herbivory and research if similar cues are used in other ant-plant mutualisms.
REFERENCES
Agrawal, Anurag A. and Benjamin J. Dubin-Thaler. 1999. Induced responses to herbivory in the Neotropical ant-plant association between Azteca ants and Cecropia trees: response of ants to potential inducing cues. Behav Ecol Sociobiol. 45: 47-54.
Agrawal, Anurag A. September 1998. Leaf damage and associated cues induce aggressive ant recruitment in a neotropical ant-plant. Ecology.
Anstett, Marie Charlotte, Martine Hossaert-McKey, and Finn Kjellberg. 2002. Figs and fig pollinators: evolutionary conflicts in a coevolved mutualism. Tropical Ecosystems of Costa Rica. Put together by Dr. Hays Cummins and Dr. Chris Myers of Miami University.
Kricher, John. 1997. A Neotropical Companion: an introduction to the animals, plants, and ecosystems of the new world tropics. Princeton University Press. P.451.
Sager C. L., S. M. Ginger, and R. D. Evens. March 2000. Carbon and nitrogen isotopes trace nutrient exchange in an ant-plant mutualism. Oecologia. 123: 582-586.
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