Lack of Effect of Predator Presence on Optimal Foraging in Ants

Introduction

In theory, optimal foraging is the process in which a living organism obtains life-sustaining nutrients under favorable environmental conditions. The utilization of foraging to acquire food correlates with Darwin’s theory of natural selection. Individuals that are consistently successful at developing strategies to hunt their prey are inherently selected for. This is important because future generations use these specialized skills to survive, feed, and reproduce. It is over these continuing generations that the fitness of the species evolves and becomes more adaptive to the changing conditions of the habitat. In every ecosystem, organisms must learn to adjust to the hostile conditions that work against their favor. Certain external components, such as predators, can impact an organism’s ability to collect resources. To study the theory of optimal foraging, the scavenging behavior of ants was observed. Because they work together in complex mechanisms for feeding, ants are an insightful tool into the scientific study of foraging strategies.

In efforts to observe ant behavior specifically when foraging for food, the experimental design focused on predation in feeding environments. The risk of predation is a consistent threat that many organisms face in their social domain. The key to sustaining life is balancing the trade-offs that come with spacial competition between predator and prey. Maintaining the costs and benefits of survival in individual species is crucial to the diversity of the ecosystem and how it thrives. The initial hypothesis for this experimental study was to observe decreased foraging in ants, measured by the number of individuals when a predator was present. Also expected was a greater decrease in foraging as the number of predators increased.

Methods

Study site: This experimental study took place at the Louisiana Tech University Arboretum located on the South Campus in Ruston, LA. All six replicates were located on a path that branched off of the main trail (32°30’28’’ N and 92°38’54” W). The first three replicates were in sunny locations with very dense, tall grass and surrounding brush and trees. There were two ant nests nearby. The fourth replicate was located in a more shaded location boarding the denser area. There was less grass but just as many trees nearby. The final two replicates were directly on the path mentioned. These were also in a more shaded area with little grass. There were trees and bushes nearby these locations as well. For all of the locations, there was little to no wind or humidity. The temperature was around 66° though it had dropped below freezing the night before the experiment was conducted.

Study design: To investigate the ecology of ants, an experimental study was developed from the hypothesis that the presence of a predator would influence the amount of ant foraging. This experimental design included six replicates of two treatment groups and one control group. The independent variable was the number of spiders and the dependent variable was the foraging and number of ants. Spiders were chosen as predators because they naturally prey on ants. Even though it would have been more accurate to use real spiders, fake spiders were used for safety measures. The plastic spiders were black and small in appearance. Eighteen plastic spiders were needed, three for each replicate. In order to prepare for this experiment, eighteen cotton balls (three for each group) were soaked in a 20% sugar water solution in the lab since the ant nutrients were not being manipulated. The cotton balls were transferred to a ziplock bag to contain the moisture until the experiment was ready to begin. Eighteen note cards were labeled by the group. For example, the three-note cards used in replicate group one were labeled 1A, 1B, and 1C. The number in front of the letter was determined by which replicate group the note cards belonged to.“A” stood for the control group, “B” stood for Treatment 1, and “C” stood for Treatment 2. After obtaining the rest of the supplies from the lab, the experiment moved to the Louisiana Tech Arboretum. Six spots at least fifteen feet apart were marked for the setup. At each spot, three-note cards were placed in a triangle shape spacing about six inches apart from one another. An individual cotton ball was transferred from the ziplock bag onto each notecard. The 1A note card only contained a single cotton ball since it was the control group. The 1B note card consisted of one cotton ball and one plastic spider leaning against it. Finally, the 1C note card had one cotton ball and two spiders propped up on opposite sides of the ball. As stated before there were six of these replicates scattered throughout the designated area in the Arboretum. Since the weather had been cold that morning, the goal was to set up the replicates in an area of the sun rather than shade. As soon as each replicate was positioned, the start time was recorded in a data table. The replicates were examined every fifteen minutes for four intervals, which added up to an hour of observations.

Statistical design: After the collection of our data, we performed an ANOVA test through SPSS to determine if the presence of predators significantly influenced the foraging behavior of ants. The null hypothesis, stating that there is no influence of predators on the foraging behavior of ants, was tested. The number of predators was the dependent variable and the number of ants (amount of foraging) was the independent variable.

Results

Throughout the experiment, very few ants were seen at any of the replicates, which made determining trends difficult. Generally, most individuals were seen on the sixth replicate. Some ants were seen alone on other replicates. There was not a consistent demonstration of a trend for the amount of foraging seen over time. As seen in Table 1, the number of individuals present at the control fluctuated, as well as for treatment 2. There was an increase in the number of individuals seen at treatment 1, though it was a sudden increase at 60 minutes rather than a consistent increase. For all replicates, there were only one species of ant present. It is seen in Figure 1 that the control group containing the sugar-soaked cotton ball without any spiders had over twice as many ants as the two treatment groups.

Considering our raw data, the ants had a preference toward the control group over the treatment groups, although statistical analysis did not support this assumption. An ANOVA test was performed to determine the statistical significance of our data. It was found that the data was not statistically significant (p=0.701) because it is greater than 0.05. This means that the null hypothesis could not be rejected.

Discussion

Finding food is essential for an organism’s survival, yet there are many factors that influence its foraging behaviors. Ants use different strategies to forage for resources based on their species and the amount of food available to them. When searching a large area, it can be extremely difficult for an ant to find nutrients necessary for survival. This obstacle is overcome by using teamwork to actively search large areas, which in turn promotes efficiency. Group foraging tends to be most effective for ants when food abundance is low in harsh conditions, such as a long winter. These foraging behaviors can be influenced by predators in a way that causes individuals of different species to completely avoid an area due to the presence of a predator.

Based on this knowledge that predation decreases foraging behavior, our hypothesis projected that the experiment would have similar results to the ecological norm. The raw data led us to believe that the statistical analysis may support our hypothesis because most ants were seen on the control group notecard. Because the ANOVA test calculated p=0.701, the null hypothesis could not be rejected. We were unable to confirm that the presence of predators influences the foraging behavior of ants based on these findings, and therefore our hypothesis was not supported.

Our findings are not supported by other peer-reviewed scientific sources. However, the results most likely were affected by the climate of the study site. Due to the recent cold weather, it is possible that there was a reduced the amount of foraging outside of the nest. Had more ants been present during the experiment, the results could have potentially had a different outcome that would have supported our hypothesis that predation directly affects foraging.

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