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Activity patterns of the Atta cephalotes (leaf cutter ant) under different variations of light on native tropical plantation trees

Text Box: By Tom Bogard, Michelle Breinholt, Megan Hylok, Lisa Lockwood

ABSTRACT

The purpose of this investigation was to compare the activity patterns of the Atta cephalotes (leaf cutter ant) on native tropical plantation trees under different variations of light. In order to make this comparison a previously planted test plot area was used at the La Selva Biological Reserve in Costa Rica. A visual survey was taken of 20 random plots and the number of trees with leaf cutter ant trails leading up to them was recorded. Further investigation included recording the types of species harvested by the Atta, and the type of planting method used in each plot – monocultures or polycultures and how each influenced the behavior of the ants. Our results suggest that the leaf cutter ants prefer harvesting the leaves of tropical plantation trees from sunny areas. Our data also suggests that light conditions might have greater effect on the harvesting habits of the Atta, than tree species preference.

INTRODUCTION

Atta cephalotes are found in lowland tropical forests below 2000m. These dominant herbivores consume a variety of plants, but do show a preference for particular species. It has been noticed that the leaf cutter ants do not forage for vegetation immediately around the nest. They scout mainly from the ends of their trails rather than from the nests. This spreads the grazing pressure over the whole foraging area and minimizes overexploitation.[1] Pieces of plant material are carried back to large nests, often containing up to 5 million workers, and are placed in special chamber where they are used to grow fungus on which the ants feed.[2]

Leaf cutter ants are one of the most important recyclers of nutrients in the tropical rainforest. Each year they remove approximately 17% of the total annual leaf production and are the major determinants of the success or failure of plantation crops.[3] As more tropical rainforests are destroyed for hard wood logging, the success of native tree plantations is becoming increasingly important. The particular conditions of the plantation, (e.g., whether trees are established in open sun or under shade) may influence the susceptibility of such a plantation to Atta attack.[4]

The purpose of this study was to investigate the activity patterns of leaf cutter ants in monocultures and polycultures of these native plantation trees, in light and shady conditions, in order to determine successful planting techniques. We believed that we would find greater leaf cutter ant activity in areas with greater shade because we understood that leaf cutter ants were more active at night.

METHODS AND MATERIALS

In order to perform this investigation the following materials were used: map of the La Selva Biological Reserve, map of the experimental plots, measuring tape, field guide to woody plants of Central America, digital camera, zip lock bags.

A census of ant trails was done in experimental plantation plots at La Selva Biological Reserve. The test plots were set up in 15m x 25m areas. There were a total of 120 plots of which 20 were sampled for this investigation. These plots were originally planted 10-11 years ago, for the purpose of studying food preference of leaf cutter ants under different light conditions. They contain a variety of combinations of planting methods, including monocultures and polycultures of native plantation tree species. The tree species in our study include: Virola koschyni (Vk), Hyeronima alchorneoide (Ha), Stryphnodendrum microstachyum (Sm), Pentaclethra macroloba (Pm), and unidentified canopy trees species. While a portion of the original test species have not survived, we selected out plots based on current light conditions, i.e., whether the plots were in direct sun or shade.

The following procedure was used to collect data:

Located test plots at intersection of trails STR and SAT at STR 2700m.
Randomly selected 10 shady plots and 10 sunny plots.
Record edoriginal plot number according to map.
Identified tree species and planting method (mono or polyculture).
Each of 4 group members lined up evenly spaced across one end of width of plot (10m).
Walked together for the length of the plot, located trails and recorded the number leading up to trees in that plot.
Identified types of trees within plot to which those trails lead.

RESULTS

Figure 1.

Figure 2.

Plots	P1	P2	P3	P4	P5	P6	P7	P8	P9	P10
sunny	Mono 4 Vk	Mono 7 Sm	Poly 7	Mono 6 Vk	Mono 2 Vk	Mono 1 Sm	Mono 3 Vk	Mono 0 ?	Mono 2 ?	Mono 3 Vk
shady	Mono 0 Pm	Mono 0 Pm	Poly 1	Poly 1	Mono 0 Sm	Poly 1	Mono 0 Pm	Mono 0 Sm	Mono 0 Vk	Poly 0

Figure 3.

Figure 4.

DISCUSSION

Overall, our data showed that more leaf cutter ant trails led to trees in the sun than to trees in the shade. An average of 3.5 trails per plot led to trees in the sunny areas. An average of .3 trails per plot led to trees in the shady areas. (see figure 1) Further analysis of our data showed that 9 of 10 plots in the sunny areas were a monoculture planting. Of these monocultures, trails were found in 8 of 9 plots. Our data also showed that 4 of 10 plots in the shady areas were polycultures, and 6 of 10 were monocultures. (see figures 2 and 3) The only trails recorded in the shady plots were in polycultures. No trails were found in shady monocultures. These findings do not support our original hypothesis, which proposed that there would be more leaf cutter ant trails in shady plots than in sunny plots. Our data indicates that leaf cutter ants prefer to cut their leaves from trees in sunny areas. Further investigation of this theory would require more extensive investigations of the entire test plot region to gather more data. These findings lead to further questions about the effects of monocultures and polycultures on the harvesting habits of the leaf cutter ants.

This investigation also led to the question of tree species preference of the ants. Our data shows that a total of 14 Stryphnodendrum microstracium had trails leading to them in sunny plots, while this species attracted no ant activity in shady plots. (see figure 4) This indicates that light conditions might have a greater effect than tree species preference on the harvesting habits of the leaf cutter ants. Further investigation of this possibility would require further data collection from monoculture plots of Stryphnodendrum microstracium in shady and sunny conditions.

APPLICATION TO THE CLASSROOM

The following are topics to which this study could be applied.

Whole Organism Biology: Insect Behavior

Forest Preservation: Use of Native Tree Plantations, Comparative Studies to Local North American Species

Monocultures and the Natural Environment Effects on that Particular Species

Comparative Study of Monocultures and Polycultures Outside Your Classroom (growing your own)

AKNOWLEDGEMENTS

We would like to thank the following people for helping us with our project : Evan for helping us with our idea and locating the plots, Lee Dyer for giving us the plot maps and Xander for his GIS information.

REFERENCES

Covert, 1998. http://jrscience.wep.muohio.edu/fieldcostaricaarticles/leafcutterantsdraft2.html., 7/24/00

Dyer, Lee. Plot map.

Folgarait et al, 1996. Leaf-cutting ant preferences for five native tropical plantation tree species growing

under different light conditions. Entomologia Experimentalis et Applicata. 80:521-530.

Gentry, 1993. Field Guide to Woody Plants. University of Chicago Press.

Mavrides, Xander. La Selva map.

Stevens, 1983. Atta cephalotes. Costa Rican Natural History. Jansen, Daniel H, ed. University of Chicago Press.

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