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Comparison Study of Leaf Cutter Ants (Atta cephalotes) in Canopy Versus Gapped Areas at La Selva, Costa Rica

Kathy Dewain Tamsey Ellis Michael Graham Beverly Mowrer

Abstract - Behavior of leaf cutter ants (Atta cephalotes) was observed in both canopy and gap areas over a period of one day at La Selva Biological Research Station, Costa Rica.  Ants transporting plant matter in a gap area appeared to cover a specific distance in a shorterperiod of time than ants in a canopy area.  Ant productivity increased in full canopy areas. 

Introduction  - Leaf cutter ants (Atta cephalotes) are found in forests below 2000m elevation throughout Costa Rica.  The nests occur in large bare spots and may have exits up to 50m apart (Jansen 1983).  These nests may have up to 5 million ants ranging in size from 2mm to 20mm.  Atta sizes vary widely based on their job specializations.  The minima ants are 2mm in size.  They take care of the internal functions of the nest by having the roles of nurse ants and tenders of the fungi gardens.  The media ants are workers that cut and transport plant material to the nest.  Soldier ants, that can be 20mm in size, walk along and protect the workers.  The smallest ants, the guard ants, ride along on the leaves and protect the workers from parasitic wasps (Jansen 1983). 

Atta grow a spongy, bread-like fungus on the leaf material they collect.  Minimas clean and scrape the surface of the leaf removing foreign fungi and bacteria.  They chew the plant matter adding saliva and fecdal matter to the thick fungal mass.  From time to time they continue to add fecal matter.  When a dense mat is formed, ants collect the swollen tips of hyphae as food (Jansen 1983).

Because of the highly specialized nature of ant societies and their importance in the tropical forest ecosystem, it is important to determine and monitor ants’ responses to changing abiotic environmental conditions.  We suggest that Atta cephalotes will be more productive under full canopy primary forest than in a gap forested area.

  Methods - Two ant nests were chosen for this study.  One of these nests was in a gap area near the Rio Puerto Viejo to the left of the Stone suspension bridge at La Selva Biological Station.  This area is mowed periodically to keep the grass short.  There were two avocado trees near the nest, which were the source of the leaf material being cut.  The other nest was in the primary forest (canopy) near the 100m marker of the SUR trail.  Two paths were selected at each nest resulting in four sites that were observed.  Sites 1 and 2 are the gap nest area.  Site 1 was a clear path, 17cm wide and in full sun.  Site 2 was 10cm wide; covered in leaf litter and in shade.  Site 3 was on a concrete pathway in the forest or canopy area and appeared to be approximately 17cm wide.  Site 4 was also in the canopy area and in a soil area with little leaf litter and that was 17cm wide.  Measurements of soil and air temperature were also taken at all sites and showed no appreciable difference. 

  Two types of observations were made at each site.  The first observation method was to count the number of ants carrying plant matter that crossed a specific point over a five-minute interval.  Six measurements were taken and averaged for each site.  The second observation was to record and then average the amount of time it took for ants carrying plant matter to travel 1.53m.

Data

The 100 ants observed at site 1 (gap, clear path, bright sun) had an average speed of .017 m/sec.  The same number of ants at site two (gap, shade, leaf litter) had an average speed of .017 m/sec.  The average speed of ants was the same at both sites in the gap area.  Ants at site 3 (canopy, walkway) had an average speed of .015 m/sec.  The ants at site 4 (canopy, natural trail) also had an average speed of .015 m/sec. 

The average number of ants crossing a specific point over a 5-minute period at site 1 was 60 and at site 2 was 63.  In the canopy area, the average number of ants crossing a point in the concrete path was 112 over a period of 5 minutes and 105 ants along the natural trail during the same length of time (graph 1).

 Discussion - Ants transporting plant matter in the gap area appear to be slightly faster than ants in the canopy area (.017 m/sec as opposed to .015 m/sec).  Several other observations were also made during this trial period.  Larger worker ants carried larger leaf fragments than the smaller worker ants.  Almost all ants transporting leaf matter were also carrying smaller ants.  These smaller ants are thought to offer protection against predators (David Silverberg 2001).  It was noted that these smaller media ants also played a role in keeping the leaf matter load balanced.  If the leaf started to tip, they moved to the opposite side of the leaf, restoring balance.  If a leaf tipped completely over, other small ants rushed to the spot and helped get the leaf upright.  One of the media ants became completely confused and started walking in the opposite direction with the leaf matter.  Several smaller ants swarmed over the larger media ant and pushed and pulled on the confused worker until it was again going in the correct direction.

Ant productivity was calculated by dividing the average number of ants carrying leaves in the canopy by the average number of ants carrying leaves in the gap area.  There was a 58.7% increase in productivity in the full canopy area versus gap area.  Although there was no appreciable air and soil temperature difference in either site, as measured by a thermometer, the guard ants may be more sensitive to microtemperature differences.  It was noted that in the gap area, the worker ants carried the leaves a shorter distance and then stockpiled them.  This stockpile was then picked up by another group of ants.  This behavior could be the result of greater light sensitivity by the ants or again, sensitivity to microtemperature differences.  With increasing global warming, the rate at which ants harvest leaves could be seriously affected.  Less nutritional leaves may require higher levels of productivity to sustain the same amounts of fungi.  This can put additional stress on ant colonies.  Ant colonies are numerous and may last up to 20 years, playing a major role in forest decomposition and carbon storing.  The new projects at La Selva involving studying carbon retention in the soil must include parallel studies in ant activity.

Bibliography

 Jansen, D. 1983. Costa Rican Natural History. University of Chicago Press

 Silberberg, D.  Personal communication. July 28, 2001.  

Acknowledgements