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A COMPARISON
OF ATTA CEPHALOTES ACTIVITY ON THE EDGE
AND INTERIOR OF THE LA SELVA RAINFOREST
By Paul Melkonian, Giselle
Nakhid, John Neering,
Kenneth Salim and Linda Strauss
Woodrow Wilson National Fellowship Foundation
Organization for Tropical Studies, La Selva Biological Station
ABSTRACT
We compared the collection of biomass (vegetative
matter) of the leaf cutting ant, Atta
cephalotes, at the edge of the
secondary forest with the interior of the forest. The ants at the edge accumulated more biomass than those at the
interior. Increased leaf cutting ant
activity at forest edges may have serious implications on land use management
policy. There were also changes in
diurnal and nocturnal vegetation collection activity at both sites. Possible explanations for these changes will
be discussed in this paper.
INTRODUCTION
This study was designed as a
comparison between the leaf-cutting ant colonies in the forest interior and on
the forest edge at La Selva Biological Station in Costa Rica. Leaf-cutting
ants, found mostly in the tropics and subtropics of the New World, represent an
integral and essential part of the forest ecosystems, stimulating new plant
growth, recycling organic matter, and enriching the soil (Lopez, 1995).
Leaf-cutting ants use leaves and
plant cuttings as a substrate to cultivate fungus deep within the nest. The
ants eat the fungus which is an important source of many of their nutrients,
while in return they provide the fungus for cultivation and dispersal from nest
to nest. Ant nests can live to be
fifteen years old and soil mounds can be connected by numerous trails
connecting many species of trees. Large
nests could have populations of 10 million individuals, several miles of
underground chambers, and a depth of 4 to 5 meters.
The leaf-cutter ant we have chosen
to study is one of the three species native to La Selva Biological
Station. Atta cephalotes belongs to the order-Hymenoptera and family-Formicidae. All members of the species are red, have
smooth gasters, and carry either fresh leaves from treetops or vegetative
fragments from along their trail.
It has been shown that laden ants are more active at
night than during the day (Wetterer, 1990). The experimental design of our
project was to measure the amount of biomass carried by ants in the forest as
compared to the amount of biomass carried by ants on the forest edge. We also
determined individual ant speed and average number of the ants on the trail at
the two sites. All of the preceding measurements were made for both sites
during day and night sessions.
We hypothesized that leaf-cutting
ant activity might be greater in edge areas when compared to the forest
interior. If this proved true, this
would have important implications for global environmental change and rainforest
preservation which will be discussed later.
MATERIALS AND METHODS
We conducted this study with two large colonies of Atta cephalotes at La Selva Biological
Station in Puerto Viejo, Costa Rica, during a two day period in July 2000. One colony
was in an ecological reserve interior forest (hereafter referred to as Forest Site #1). The second colony was
on a forest edge area (hereafter referred to as Edge Site #2).
In order to observe day and night activity, we
collected data during a daylight and a night session. The data for the daylight
session was collected between 9AM and 11AM. The night session was conducted
between 8PM and 10PM.
All of the following measurements were made for both
the Forest Site and the Edge Site colonies and during the daylight and night
sessions. Using twine, we measured the length of two distinct trails that led
from the main colony to the base of each foraged tree. Lengths were averaged to
calculate an average trail length for each site (see Results).
We also measured the number of Atta cephalotes that passed a set point within two minutes for both
colonies. The speed of ten individual ants over a predetermined distance was
calculated and averaged to determine an average individual ant speed for each
colony.
In order to compare the amount of biomass carried by
Atta cephalotes at the Forest Site
and the Edge Site, we took a random sampling of leaf matter carried by laden
ants. Ants at the Edge Site harvested leaves from two different genera of
trees, Ficus and Eugenia. Ants at the Forest Site harvested leaves from an
unidentified genus. Forceps were used to collect a total of 50 pieces of leaf
matter at each site. The biomass collected was measured without drying on an
electronic balance as described by Wetterer, 1993.
RESULTS
Figure 1: The greatest number of Atta cephalotes counted in a two minute
span were found at night at the Edge Site.
Figure 2. The greatest amount of plant biomass carried by Atta cephalotes was measured during the
night at the Edge Site.
Figure 3. The highest rate of plant
biomass carried by laden Atta cephalotes
was measured during the night at the Edge Site.
DISCUSSION
The results shown in our data strongly support our
original hypothesis which stated that there would be greater leaf cutter
activity at the forest edge than the forest interior. The results demonstrated
in Figure 1 point to a marked difference in the number of Atta cephalotes at the forest edge compared to the number of Atta cephalotes in the forest interior.
During the course of a 24 hour period there were more than four times the
number of ants recorded at the edge site as compared to the number of ants recorded
in the forest interior.
Another finding in our results is
the data found in Figure 2. This figure shows the amount of biomass carried by
laden ants in both the forest edge and forest interior. These findings in
Figure 2 strongly support our hypothesis. There is a greater amount of biomass
moved by the laden ants at the forest edge as compared to laden ants in the
forest interior.
By measuring the amount of plant biomass carried per
minute (Figure 3), we were able to determine
that the edge site colony was more efficient in moving leaf matter at night
(2.67 g/min) than during the day (0.24g/min). Other research on Atta cephalotes shows that the amount of
biomass transported by laden ants is greater at night as compared to the day. (
Wetterer 1990 )
The same efficiency trend was not seen in the
interior forest site which may have been due to the reduced number of ants
travelling at night than during the day. We suggest that this may be due to
various factors including a greater use of unobserved trails. It is also
possible that increased moisture at the forest site resulted in reduced ant
activity. This moisture may have had an effect on the pheromone concentration
present on the ant trails. The significance of these findings suggests that not
enough trials were performed nor trails studied to account for the difference.
Our results clearly show that the average number of
laden ants and the amount of biomass transported by Atta cephalotus at the edge is greater than the average number of
laden ant and the amount of biomass transported by Atta cephalotus in the forest interior. We believe the reason for the higher numbers of laden ants and
quantity of biomass at the forest
edge may be attributed to easy access to food sources and the decrease in natural
enemies. Atta cephalotus is specialized to live in forest gaps and hence has
the ability to devestate subsistence farms and plantations cut into the
rainforest. (Cherrett and Peregine,
1976) This, therefore, has serious
ramifications for local banana plantations on the edges of La Selva Biological
Reserve.
This is significant in terms of global environmental
change in neotropical
rainforests
and poses an important question: Is it economically viable to clear cut
sections of the rainforest for agricultural purposes? Leaf cutting ants such as
Atta cephalotus live in huge deep
nests and reproduce rapidly. It is quite difficult to eradicate such a
"pest" from clear cut section of rainforest. Our own deforestation -policy may not only
lead to aiding these ants but may contribute to the additional environmental
problem of pesticide use.
In the Costa Rican rainforest, Atta cephalotus is a key contributor to its health and vitality,
but in a human-generated agricultural landscape carved from the rainforest, Atta cephalotus can be a devastating
pest requiring great quantities of money and effort to control. Due to the complex interactions inherent in
tropical rainforests, humans must first begin to understand the implications of
fragmentation before they continue to adversely impact terrestrial ecosystems.
REFERENCES
Aasen,
Kjetil. Personal Interview: La Selva
Biological Reserve,
Costa Rica, July 17, 2000.
Clark,
Matthew. Personal Interview: GIS Laboratory at La Selva Biological
Reserve,
Costa Rica, July 17, 2000.
Gentry,
Alwyn H. Woody Plants of Northwest
South America,
Washington, D.C: Conservation International, 1993.
Holldobler,
Bert and Edward O. Wilson, The Ants, Massachusetts:
Harvard University Press, 1990.
Janzen,
Daniel H. Costa Rican Natural
History, Chicago: University
of Chicago Press, 1983.
Lopez,
Francisco, "Arboretum Informational Plaque," La Selva Biological
Station,
September 1995.
Paniagua,
Maylin. Personal Interview: ALAS Project at La Selva Biological Reserve,
Costa Rica, July 17, 2000.
Wetterer,
James K., "Diel changes in forager size, activity, and load
selectivity in a tropical leaf-cutting ant, Atta
cephalotes,"
Ecological Entomology (1990)15, 97-104.
Wetterer,
James K., "Foraging and Nesting Ecology of a Costa Rican Leaf-Cutting Ant,
Acromyrmex volcanus," Psyche (1993)100, 65-76.
APPENDIX A
Raw Data Tables for Figures
|
TABLE FOR FIGURE 1 # Atta cephalotes counted
in 2 minutes |
|||
|
FOREST |
EDGE |
||
|
A.M. |
P.M. |
A.M. |
P.M. |
|
39 |
4 |
20 |
172 |
|
29 |
9 |
30 |
155 |
|
38 |
6 |
24 |
167 |
|
Avg - 35.3 |
Avg - 6.3 |
Avg - 24.7 |
Avg - 164.7 |
|
TABLE FOR FIGURE 2 Plant Biomass Carried (g) |
|||
|
FOREST |
EDGE |
||
|
A.M. |
P.M. |
A.M. |
P.M. |
|
0.68 |
0.95 |
0.97 |
1.62 |
APPENDIX B
Ant Speed
|
TABLE 4 Ant Times to Travel Distance |
|||
|
FOREST (2.5 m) |
EDGE (1.2 m) |
||
|
A.M. |
P.M. |
A.M. |
P.M. |
|
100 sec |
144 sec |
55 sec |
53 sec |
|
192 sec |
279 sec |
63 sec |
61 sec |
|
257 sec |
108 sec |
45 sec |
49 sec |
|
240 sec |
124 sec |
29 sec |
40 sec |
|
202 sec |
63 sec |
21 sec |
53 sec |
|
93 sec |
135 sec |
45 sec |
57 sec |
|
101 sec |
196 sec |
53 sec |
56 sec |
|
185 sec |
129 sec |
35 sec |
48 sec |
|
139 sec |
75 sec |
46 sec |
66 sec |
|
110 sec |
125 sec |
40 sec |
61 sec |
|
|
|
|
|
|
|
|
|
|
|
Avg - 161.9 |
Avg - 137.8 |
Avg - 43.2 |
Avg - 54.4 |
|
|
|
|
|
|
Ant Speed (cm/sec) S = D/T |
|||
|
FOREST (2.5 m) |
EDGE (1.2 m) |
||
|
A.M. |
P.M. |
A.M. |
P.M. |
|
2500cm/161.9sec |
2500cm/137.8sec |
1200cm/43.2sec |
1200cm/54.4sec |
|
15.4cm/sec |
18.1cm/sec |
27.8cm/sec |
22.1cm/sec |
