Please
Click here to return to main page
A comparison of herbivory
and ant species in Acacia collensii found in edge and light gap of secondary forest.
Participants
Stacey Britton, Pearl River Central H.S., Carriere, MS
Betty Carvellas, Essex H.S., Essex
Junction, VT
Leo Pena, Pelham Memorial H.S.,
Pelham, NY
Kenny Salim, Brighton H.S., Boston,
MA
Susan Van Kleef, Watkinson School,
Hartford, CT
Abstract
At Palo Verde National Park,
Guanacoste, Costa Rica, Acacia collensii
were studied in a secondary forest gap and edge habitat to determine the
herbivory levels found at trees colonized by (Pseudomyrmex ferruginea and P. nigrocincta , red ants), and P. belti (black ants). The study also analyzed the presence of bird
nests located within A. collensii
inhabited by red ants and black ants.
Overall, herbivory levels were found to be lower in A. collensii inhabited by red ants(P. ferruginea, P. nigrocincta).
Data showed that herbivory levels were higher in A. collensii found in the edge habitat. Although further research is needed, secondary studies indicated
a higher abundance of bird nests (rufous-naped wren, Campylorhynchus rufinucha and Northern oriole, Icterus g. galbula) in A.
collensii found in the edge habitat.
Introduction
Habitat destruction in the tropical dry forest has created many edges.
The community structure in these edges is quite different from the community
structure in the secondary forest. This study in Palo Verde National Park,
Guanacaste, Costa Rica, compares percent herbivory on Acacia collensii by three species of ant. The study was conducted
in a light gap in a secondary forest and at a secondary forest edge by a road.
The study also determined the presence of bird nests in A. collensii. It was predicted that there would be a greater number
nests in trees colonized by the red ants.
A.
collensii flourish at edges where there is more light (Hensel, 2000). It is
possible that healthy trees can produce more food for the ants that protect it,
therefore experiencing less herbivory. It is also possible that edges may
better accommodate herbivores who feed on A.
collensii leaves. It was predicted that there would be a difference between
the amount of herbivory in an edge and in a light gap.
This study also investigated whether
red ants (Pseudomyrmex ferruginea and
Pseudomyrmex nigrocincta) protected
against herbivores more aggressively than black ants (Pseudomyrmex belti). P.
ferruginea and P. nigrocincta protect
A. collensii trees with more vigor
than P. belti (Jantzen, 1983). This
is illustrated by the bare areas underneath the trees where P. ferruginea and P. nigrocincta crop all of the competing vegetation. It was
predicted that there would be less herbivory in A. collensii trees that are inhabited by red ants.
Materials and Methods
We compared herbivory and ant species in Acacia collensii found in edge and light gap areas of secondary
forest. We measured 100 meters along the edge and randomly selected five trees
populated by red ants (P. ferruginea
and P. nigrocincta) and five trees
populated by black ants (P. belti). Within the secondary forest, we selected
three light gap areas and observed five trees populated by red ants (P. ferruginea and P. nigrocincta) and five populated by black ants (P. belti). Each team member estimated
the height and percent herbivory of each tree, averaging the results. We recorded the presence or absence of nests
in each tree. Due to the lack of nests
in our original plot locations, we walked a one-kilometer section of edge and
recorded the presence of nests of the rufous-naped wren (Campylorhynchus rufinucha) and the Northern oriole (Icterus g. galbula).
Results
A total of twenty A. collensii trees were observed in this study. Ten trees were
studied on the secondary forest edge and ten trees were studied in three light
gap areas in the secondary forest. Results showed that A. collensii inhabited by the same ant species were clustered
together in an area regardless of study location. Two species of red ants were
observed: P. ferruginea exclusively
inhabited trees in the secondary forest gap, while P. nigrocincta inhabited trees on the secondary forest edge.
Trees on the secondary forest edge were subject to
greater herbivory (See Figure 1). These measurements were determined by
estimating the percent of total leaf cover removed. Trees inhabited by black
ants (P. belti) were subject to
greater herbivory than red ants (P.
ferruginea and P. nigrocincta)
(See Figure 2). There was also greater clearing of ground vegetation under A. collensii inhabited by red ants than
trees inhabited by black ants.
Ten A.
collensii were surveyed along a 1 kilometer section of road; bird nests
were found in seven A. collensii
trees protected by red ants and only three protected by black ants. There were
no apparent trends in A. collensii height
at the different locations or between trees inhabited by different species of
ants.
A variation in the structure between edge and gap habitats was observed; those trees found in the gap were more compact with less branching.
Figure 1. There was 12.4% more leaf
cover removed on A. collensii trees
at the secondary forest edge as compared to the light gap in the secondary
forest edge.
Figure 2. There was 9.6% less leaf
cover removed on A. collensii trees
protected by red ants (P. ferruginea
and P. nigrocincta) than on trees
protected by black ants (P. belti).
The
presence of herbivory on A. collensii K.N. Salim
Discussion
The results of this study support the hypothesis that A. collensii are subject to greater
herbivory on the secondary forest edge than in a secondary forest light gap
(Figure 1). It is possible that the habitat of the secondary forest edge is
more hospitable to herbivores who might move more freely in an open area.
This study also demonstrated that A. collensii inhabited by red ants (P. ferruginea and P. nigrocincta) experienced less herbivory than trees inhabited by black ants (P. belti) (Figure 2). The study results indicated less vegetation around the trunk base. All acacia-ants will kill any vegetation that touches the acacia tree and will attack any climbing vertebrates (Jantzen, 1983). However, red ants are more successful in protecting A. collensii from herbivores by aggressively preventing the establishment of competing plant species beneath the tree.
Apple and Morehead (1995) have previously found that ants of the same species occupied A. collensii in the same area. This study also found a difference in the colonizing behavior of two red ant species. P. ferruginea only inhabited trees in the secondary forest gap and P. nigrocincta only inhabited trees on the secondary forest edge. It appears that this tendency for single ant species to colonize a given area is strong because no colonies of P. ferruginea were found on the secondary forest edge, despite previous research that cites that P. ferruginea is spread over all habitats (Jantzen, 1983).
Though bird nests were not observed in any of the
subject trees, an informal survey of a one-kilometer stretch of road showed
that there were more bird nests in A.
collensii inhabited by red ants than trees inhabited by black ants. While
ants might attack a developing nest, they soon grow accustomed to an
established nest and attack neither the eggs nor the occupying bird (Jantzen,
1983). Since the red ants exhibit greater aggressive behavior and therefore
protect A. collensii more
effectively, birds might demonstrate a tendency to establish nests in these
trees occupied by red ants.
This study also found that A. collensii on the secondary forest edge exhibited a broader
branching pattern extending to a greater circumference. A. collensii in the secondary forest light gap occupied a narrower
area with less extensive branching. This may be due to the greater exposure to
light on the secondary forest edge, allowing A. collensii to grow more robustly and occupy a greater area.
While A.
collensii is highly successful in forest edge habitats artificially created
by infrastructure through secondary forest areas, it is unknown what subsequent
effects the proliferation of A. collensii
might have on other plant species. A change in the plant community caused by
fragmentation might also have adverse effects on the population of herbivores
that live in these areas. In the secondary forest gap, herbivores have an
established food source that might not be available due to dominance of A. collensii on the forest edge.
Further investigation is needed to determine if proliferation of A. collensii along the secondary forest edge on the survival of other plant species and the herbivores that rely on them. A study of the population density of A. collensii on the secondary forest edge compared with the forest light gap would assess the extent of the effects of A. collensii proliferation.
Relating the Acacia collensii Study to the Classroom
The Acacia study can be used in the classroom to teach many science
concepts. Classroom activities could include looking at how human disturbance
through the creation of an edge can affect plant productivity and community
balance. This study indicated a difference in herbivory at the edge of a
roadside compared with a natural forest gap. This could be replicated with
other plant species to determine the effect of human influence on plant
survival, as it relates to edge created by fragmentation.
The Acacia tree
is also a good organism to study because it demonstrates physical adaptations
that discourage herbivory. The unique adaptation of the Acacia tree is the symbiotic relationship it maintains with the Pseudomyrmex spp. (a.k.a., the
“acacia-ant”). The Acacia tree has
thorns that not only provide protection from large herbivores, but also provide
a home for the Pseudomyrmex spp. and their larva. The Acacia also supplies Beltian bodies and nectaries that provide food for the
ant. In return, the ants provide
protection against herbivores. This is a key mutualistic relationship
between A. collensii and Pseudomyrmex spp. that can be studied in
an ecology unit.
The process of developing this project is also an
important model for supporting scientific inquiry in the classroom. By
brainstorming questions based on observations made in the natural environment,
students are given the opportunity to have ownership for their investigations.
Beltian bodies and
nectaries that provide food for the ant
L.A. Pena
References
Apple, J. and S. A. Morehead. “Patterns in two obligate acacia-ant species distributions on acacias at Palo Verde.” Tropical Biology: An Ecological Approach, January – March 1995. Organization for Tropical Studies 95-1: 129-133.
Carson, C.D. et al. “Effect
of Pseudomyrmex spinicola vs. P. flavicornis on Acacia collensii Condition.” Dartmouth
Studies in Tropical Ecology. Department of Biological Sciences, Dartmouth College, Hanover, N.H., Oct.
1995: 1 – 2.
Fischer, E. et al. “Outcome
of Competition Among Obligate Acacia Ants.” OTS Tropical Biology, July-Aug.
Course Book 73-2. 102 – 103.
Ginsburg, M.A. et al. “The Effects of Pseudomyrmex flavicornis
and P. spinicola mulls on the
aggressive behavior of P. flavicornis.” Dartmouth Studies in Tropical Ecology. Department of Biological Sciences,
Dartmouth College, Hanover, N.H., Oct. 1995: 3 – 4.
Hensel, Phillipe. Personal
Communication. July 24, 2000, Palo Verde National Park, Costa Rica.
Jansen, D. H., ed. Costa Rican Natural History. University of Chicago Press, Chicago,
IL. 1983: 762-763.
Wray, C.D. et al. “Differential clearing of Acacia collensii by Pseudomyrmex spinicola and P.
Flavicornis.” Dartmouth Studies in Tropical Ecology. Department
of Biological Sciences, Dartmouth College, Hanover, N.H., Oct. 1995: 5 – 6.
Discussing
the finer points of herbivory Betty
Carvellas
Please
Click here to return to main page