Title
Preferential foraging on invasive plants by Atta texana at Brackenridge Field Lab
Abstract
Texas leaf-cutting ants, Atta texana, forage on plants so that, in the mound, they may produce a fungus that it will serve as their food. Atta texana use specific plant material so that they may optimize fungal growth. The objective of this research is to understand if Texas leaf-cutting ants preferentially forage on invasive plants rather than native plants. I hypothesize that they preferentially forage on invasive plants when harvesting plant material at the Brackenridge Field Laboratory. To test this hypothesis, I will create transect lines radiating from Atta texana mounds at BFL and observe the plants on which the ants are foraging. This will allow me to compare the frequency at which the ants forage on invasive and native plants. Proposed findings will potentially allow for further investigations regarding the importance of Atta texana as invasive plant regulators.
Background
Atta texana are very specific in their selection of plants to forage and often travel extensive distances to collect specific plant materials (Texas A&M, 2006). Within each colony, there are twelve morphologies that are group into three categories: small specialist workers, foragers, and soldiers (Boyd and Martin, 1975). Foragers prefer new leaves, flowers, and plant stems rather than mature ones and prefer material with higher water content (Texas A&M, 2006). Atta texana is known to forage for new leaves of live oak (Quercus fusiformis) (Waller, 1989), a species native to the Brackenridge Field Lab. Larger soldier ants protect the colony and guard medium-sized ants as they forage (Moser, 2006). Plant material is used by smaller specialist ants that chew and manipulate the leaf material into a pulpy substrate; then, specialists take the pulp and contribute a special fungus onto it (Texas A&M, 2006). These workers never leave the nest and do not forage or protect the colony (Grossman, 2007). The fungus is then available as the primary source of food, making process of locating and harvesting specific plants vital to the survival and success of the colony (Texas A&M, 2006). The amount of resources accessible from the nest’s location dictates the size of the colony as well as the area that a colony covers (Drees and Merchant, 2006). Leaf-cutting ants will travel up to 200 meters in order to access preferred plant species and will even make tunnels under creeks and streams in order to attain the desired material (Moser, 2006). Interestingly, Atta texana prefers nonnative plants over native plants (Texas A&M, 2006).
The Brackenridge Field Lab is home three invasive plants, Ligustrum sinense, Ligustrum japonicum, and Nandina domestica; however, since 2012, these species have been subject to management and mechanical removal with the goal to restore native plant communities at the field laboratory (Brackenridge Field Laboratory, 20XX).
Specific Aim and Hypothesis
The aim of this research is to understand if Atta texana preferentially forage on invasive plant species relative to native plant species. My hypothesis is that Atta texana preferentially forage on invasive plants when collecting plant material at the Brackenridge Field Laboratory.
Methods
Having already located three mounds to study using information provided by Dr. Gilbert of the Brackenridge Field Laboratory, I will collect data through developing transects lines radiating from each mound. I will mark eight transect lines from the center of each mound. I will do this from a distance, however, as to not disturb the mount itself. Each transect will be 45 degrees a part in order for them to be equally spaced. Using a compass, the four initial transect lines will each follow a cardinal direction. Additionally, using a measuring tape, I will extend each transect line 20 meters from the center of the mound. Thus, the transect lines will encompass the mound in an area of approximately 1250 m2.
Once transect lines are in place, data collection can begin. I will walk each transect line and observe the plants in each one’s path, with a scope of 1 meter on each side of the line. For each plant I observe, I will record three things. I will find out if the plant is one of the three invasive plants, Ligustrum sinense, Ligustrum japonicum, and Nandina domestica, or if it is native. Secondly, I will observe and record if there is damage or defoliation or not. If a plant is completely defoliated, I will look at the stem and bark to identify it. Lastly, I will record the plant’s distance from the mound. I will repeat this process for each of the eight transect lines. After performing a preliminary analysis, I will be able to see how prevalent damage is at further distances. With that information, I may extend the range in which I collect data. With this data, I will divide the total number of damaged plants by the total number of plants observed, and I will do this for each of the three invasive plants as well as the native plant. This will allow me to compare the frequencies of damage between invasive and native plants as well as between the three invasive plants.
Limitations
A major limitation is the amount of area surveyed. As I stated in the closing sentence of the “Methods” section, I may extend the range in which I collect data depending on the results of a preliminary analysis. It may be the case that ants are going much further than 20 meters from the mound to collect plant material. I will assess this by looking at how prevalent damage is at different distances from the mound. Once I see the range in which leaf-cutting ants are traveling to collect plant material, I will adjust my procedures accordingly. As I stated in the “Background”, Atta texana have been known to travel up to 200 meters to access preferred plant material. With a relatively small area, the Brackenridge Field Lab may limit the possibility of collecting data at further distances. This is because I will not be able to differentiate between two colonies of Atta texana if there is damage in an area of overlap. However, depending on how far defoliation extends from each mound, I may or may not have to deal with this issue.
Another confounding factor in this research is the variation of invasive plant species at Brackenridge Field Laboratory. The goal of this investigation is to see whether or not Atta texana at BFL prefer invasive plant material over native plant material. However, there are only three primary invasive plant species at BFL: Ligustrum sinense, Ligustrum japonicum, and Nandina domestica. All three of these plant species are evergreen; thus, it is possible that any findings regarding preferential foraging and these plants could be a result of the plants being evergreen. In addition, as there are only three invasive plants to consider, it is possible that leaf-cutting ants particularly prefer one, two, or all three of these species but do not prefer other invasive plant species. That is not something that I will be able to see in this study, but it is a limitation to consider when drawing conclusions from the data collected. Similarly, it is possible that leaf-cutting ants at BFL use these invasive plants because they are prevalent and easily accessible in this area. This limiting factor, however, could be avoided in future studies by researching in an area with more diversity with regards to invasive plants.
Significance
My proposed findings will potentially build off of prior research regarding the forage preferences of Atta texana. Not only could this investigation inform us of whether or not BFL leaf-cutting ants prefer invasive plant material, but also, the information that this study could provide can be used to study why leaf-cutting ants prefer invasive to native plant material. In addition, findings could provide data for how Atta texana shape the environment by controlling invasive plants.
Understanding the relationship between plants and Atta texana is important for maintaining biodiversity, especially in a system such as the Brackenridge Field Laboratory. As I have discussed, invasive plants are being removed from BFL. It may be dangerous to leaf-cutting ants as well as the biodiversity at BFL to continue this removal without knowing the extent to which other organisms rely on invasive plants. On a larger scale, herbicides and other mechanisms of invasive plant species removal could harm more than just the invasive plants. The proposed findings could offer insights that suggest being more careful about removing invasive plant species.