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|An Investigation of the Moisture Content of Plant Resources Selected by Atta.cephalotes in Different Habitats|
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Purpose: To determine whether there is a relationship between the type of habitat in which an A. cephalotes colony is located and the moisture content of resources carried into the colony.Hypothesis: We hypothesize that the removal of natural vegetation from primary forest would decrease canopy cover and humidity and increase air and soil temperature. Such alterations in the abiotic factors surrounding an A. cephalotes colony would raise the temperature of the colony, causing an increase in evaporation of moisture. As these ants farm fungi by planting it on a vegetative substrate produced from resources carried into the colony, the increase in evaporation will cause the ants to seek resources high in moisture to support the growth of the fungi. We predicted that the least favorable abiotic factors for maintining colony moisture would be found in the developed area, with the abandoned agroforest being more favorable, and the primary forest the most. Therefore, we expected to find that the level of moisture in resources carried to the colony to be highest in the developed area, moderate in the agroforest, and lowest in the primary forest. Materials: Tweezers Ziplock bags Colored Markers Stop watch Counter Compass Balance
Thermometer Sling psychrometer & conversion chart for humidity Desicator Flag Markers
Clip board & paper Canopy cover transparency Map of La SelvaMethodology:
Old Growth Habitat
Abandoned Agroforestry Habitat
Results & Discussion:
Abiotic Factors:Our data shows that there does not appear to be any significant difference in atmospheric conditions or canopy cover across habitat types on the sampling day (See Figure 1). Variation in the data from site to site is more likely a reflection of time of day and imprecision of measuring instruments than any other factor. A longer-term study could illustrate significant differences in abiotic factors between different habitats and how they may affect A. cephalotes. Worker Frequency: Looking at the data, there does not appear to be a correlation between worker frequency and habitat type or time of day. Trails with a high and low frequencies of workers were found in all habitat types and throughout the day with no apparent overall pattern (See Figures 2 & 3). There may be daily patterns of varying activity level in each colony. However, it would require sampling each colony at several different times of day over a period of time in order to discover this. Worker frequency is probably dependent most on the total population of the colony, the availability of desired resources, daily weather conditions, and disturbances such as trail blockages. We observed during our sampling that even minor trial blockages, such as fallen twigs across the path of the ants, caused the outgoing workers to stop and remove the obstacle or reroute the trail before resuming harvesting efforts.
For example, when observing the ant trail during the four minute intervals in which we did not collect samples, we sometimes noticed that the frequency of workers varied. This raises the question of whether the periods of time in which data were acquired are really representative of the average flow of the trail. In addition, collection of data became difficult when frequency of workers was high, so numbers may actually be too low for Trails 4 & 6.Hitchhiker Frequency: There is no relationship between the frequency of hitchhikers and the type of habitat. The frequency of hitchhikers directly correlates with the frequency of workers (See Figure 4), and in some ways may depend on the type of resource being cut and carried. Hitchhikers were observed on both leaves and flowers, but not on any other type of resource. Qualitative observations demonstrate a relationship between the size of the resource and the number of hitchhikers, with small globs of fruit or pollen being both unable to support hitchhikers and unnecessary to require them. Whereas, larger leaves and flowers tended to have hitchhikers, with the highest number of hitchhikers on the largest pieces. Apparently, there have been competing hypotheses as to the role of these hitchhikers. Some researchers claim that these smaller ants clean the resource as it is being carried. Others claim that these ants are martyrs, willing to sacrifice their lives in order to protect the workers below. However, we observed several times that when workers carrying large cuttings encountered difficult terrain, they were helped by the hitchhikers. Hitchhikers crawled off the large cutting and assisted in lifting it over twigs or around obstacles. In fact, the highest hitchhiker frequencies were all along trails in which the primary resource collected was green leaves cut into relatively large pieces. Resource Type: It was observed that A. cephalotes cut and carried several types of plant material used to farm their fungus. Although green leaves were the overwhelming majority of the material, small purple flowers, parts of bright pink flowers, brown and yellow leaves, seeds, seed husks, stems, stamens, pistils, globs of fruit, and even pollen were harvested. Colonies usually have many trails leading to several different entrances, each of which may be bringing in slightly different mixture of resources. We also noted, in a few cases, that multiple ant trails lead to different types of plants. These trails converged into one larger trail and the final mixture was brought into the colony. Since we tended to take samples less than 5 m from the colony entrance, our collected samples represent this final mixture of materials coming into the colony. Resource Moisture: Moisture of resources was consistent across all three habitat types (See Figure 5). Disregarding the outlying data point for Trail 4, the average resource moisture was 66% regardless of habitat type or resource type mixture. The value for Trail 4 was 86%, which is interesting to note, because the resources carried on this trail were over 90% composed of flowers and flower parts. Although fresh leaves must be available to this colony, due to their proximity to two other colonies in the same habitat, this colony seemed to strongly prefer harvesting flowers. One idea resulting from this is that colonies may choose flowers over leaves due to the high moisture content, if given a preference. However, much further study needs to be done to explore this idea. For instance, do the two nearby colonies also have access to the flower resource? If so, why do they prefer leaves? If not, is it because the colony of Trail 4 is dominating this resource and preventing other colonies from accessing it? We do not know how available this resource is in terms of resource plant population density in the area or frequency of blooms. When discussing flower versus leaf preference with a local researcher, another idea was raised. As flowers are ephemeral in nature, trees may not invest much energy in creating toxic anti-herbivorous compounds in them. Therefore, not only do the flowers have high moisture content, but also they may be desired by A. cephalotes for farming fungus because of the lack of certain chemical compounds. Conclusions: Our hypothesis was not supported. There were no significant differences in abiotic factors between the three habitat types, despite differences in their degree of deforestation. Likewise, there were no significant differences in the moisture content of resources collected by ants between the three habitat types. However, our data raises the question of whether, regardless of colony habitat, A. cephalotes will show a preference for resources high in moisture content when given a choice.