Ant-Plant Mutualisms

By Claire Jackan      

          For our final project of the semester, my partner and I investigated the extent of the relationship between ants and the plant Pentaclethra macroloba in varying degrees of light intensity. Here at La Selva, we participated in independent, as opposed to faculty-led, projects. This means that my partner and I came up with the idea for our experiment and designed the methods ourselves.  We were curious to learn more about how environmental factors may affect ant-plant associations.  Some ant-plant relationships are strict mutualisms; in other words, the plant produces nutritious food bodies to attract a specific species of ant, which in turn protects the plant from herbivory by attacking insects and removing larvae from the leaves.  However, the relationship between ants and P. macroloba is known as a diffuse mutualism because while both the plant and the ants benefit from the interaction, it is an indirect benefit and not required for their survival.  The plant still produces food (within extrafloral nectaries) to attract the ants, but it is a smaller reward in terms of nutritious value. The ants that come are typically generalists (not necessarily a specific species) and they still provide some protection against herbivory by patrolling the leaves and occasionally removing a potential herbivore.

            We thought that plants with access to more sunlight would have more resources and energy available to allocate towards producing higher quality extrafloral nectaries that would in turn attract more ants. We tested this hypothesis by placing small pieces of tuna on the leaves of P. macroloba plants and measuring how long it took for the ants to appear on the leaf and how long it took for them to actually make contact with the tuna. Other variables we recorded included the height of the plant, the number of extrafloral nectaries, percent herbivory, and canopy cover.  Counting the extrafloral nectaries proved much more difficult than I anticipated, as they are so small that I had to use a magnifying glass and a light in order to be able to see them at all. The weather was also an obstacle at times; we couldn’t set up the tuna baits when it was raining because the ants are not active during rain and it would have just washed off the leaves anyway.

            At the end of the project, we were able to share our results not only with the class but also at a poster session in which we presented in Spanish to the local community.  Though at times it was challenging to communicate exactly what I wanted to say, I’m really grateful to have had the opportunity to practice my Spanish, especially since I had to utilize a more scientific vocabulary than I am accustomed to. I think the combination of the independent project and poster presentation was the perfect way to culminate the semester by bringing together everything I have learned and sharing it both with my peers and with the local community.

My Student Research Project

By Geoffrey Nathan

With two weeks of our Costa Rica trip left to go, it was time to start our final research project. My partner and I chose to work with extrafloral nectaries and their relationship with ants. Extrafloral nectaries are glands on certain plants that provide ants with a source for sugar. When these ants occupy the plant, they protect the plant from herbivores. We wondered whether extrafloral nectary production in seedlings influenced herbivory and whether extrafloral nectary production in seedlings was influenced by light intensity.

            In order to answer these questions, we set out into the La Selva forest and located seedlings of the plant Pentaclethra macroloba, which are known to produce extrafloral nectaries. I struggled to identify the plant at first, often mistaking it for a fern. However after four days of data collection and 96 seedlings used, identification became second nature.

 We attracted ants to the plants by placing tuna on the leaves. We then checked for any ant presence in the plant for five hours. Ant presence was used as a proxy to determine if ants regularly occupied that specific seedling.

We also had to count the number of extrafloral nectaries on each seedling. Because the seedlings were small to begin with, identifying extrafloral nectary glands seemed a daunting task. However we were then introduced to the jewelry magnifying glass, a device that when worn looks like it’s from the future, but is simply a hands-free magnifying glass that is worn on the head. This made identifying the glands much easier and more efficient.

We used a densiometer in order to measure light exposure in each plant. Densiometers are convex mirrors that are used to estimate canopy cover overhead. Estimating canopy cover seemed like it would be a simple task, but I had to hold still with both arms extended in order to keep the densiometer level and the mosquitoes had a field day taking advantage of this. One day after data collecting, I counted nine bites on the back of my hand alone.

Although data collection was time consuming and took a lot of work, the fact that it took place in the unique La Selva forest made it an incredible experience. One day I heard thumping sounds of things falling next to me. After a couple of minutes of this it occurred to me that Capuchin monkeys overhead were throwing shells of a fruit they were eating at me. I amusedly watched this happen for a little while until one of the shells hit me on my shoulder. I decided to hurry up with the plant I was working on so as to move on. The following day I was lucky enough to spot endangered spider monkeys swinging around in trees above me. This was only the second time I had seen spider monkeys on the trip and by far the best view I had of them. I was amazed at how long their tails were and how much taller they were then the other monkeys of Costa Rica.

After hours and hours of time spent in the forest, our results did not produce any significant data, but I built both a project and experience I will never forget.