Cecropia –Ant Mutualsim Effects on Herbivory
Instructed by Tomás Carlo
Jinhyoung Kim, Skylar Andrews, Tess Bloom, Max Kruse, Brady Boyer
January 2, 2014
Abstract
Cecropia is a well documented Neotropical plant which commonly displays a mutualistic
relationship with Azteca ants. The purpose of this study was to determine how this mutualism
influences herbivory in the Cecropia species C. pelata and C. obtusifolia. Cercopia inhabited by
ants were expected to display a lesser degree of foliage damage due to herbivory when compared
to individuals not inhabited by ants. Although this mutualism with Azteca ants is well
documented, it was unknown whether or not a larger, less aggressive ant species which also
inhabit Cecropia, provide similar benefits to the plant. This interaction between Cecropia and
larger ant species was also explored in this research. Additional factors that were considered to
contribute to herbivory rate were ant species, plant height, and Cecropia species. Possible
interactions between these variables that could affect the herbivory rate were also determined.
It was concluded that the Cecropia containing ants exhibited reduced herbivory rates,
thus indicating that the Cecropia-ant mutualism is effective. However, it was also found that
several other variables also significantly influence herbivory. Ant type (whether plants
contained no ants, large, or small ants) was found to have the strongest effect on herbivory rate;
small Azteca ants reduced herbivory rates when compared to the plants without ants, while plants
containing large ants actually increased herbivory rates. Plant height was found to have a
significant positive correlation with herbivory rate. Cecropia species type did not significantly
affect herbivory rates, however, it was determined that a significant interaction between
Cecropia species type and ant type did exist. Ultimately, this study was effective in proving that
the mutualistic relationship between Cecropia and ants reduces herbivory rates, and was also
effective in determining how other variables contribute to herbivory in Cecropia.
Introduction
Encompassed under the family Urticaceae is a group of plants of the genus Cecropia
(Kricher, Tropical Ecology, 2011). This genus contains over 61 species which inhabit well lit
openings in the Neotropical forests as well as areas of secondary growth. These trees are
characterized by bamboo-like rings surrounding a hollow greenish-grey trunk and relatively
large lobed palmate leaves (Kricher, 1997). Two species of Cecropia commonly found on La
Reserva Biologica Alberto Manuel Brenes in San Ramon Costa Rica are Cecropia peltata and
Cecropia obtusifolia. C. peltata is characterized by broader leaves and narrower stems than that
of C. obtusifolia; in contrast, C. obtusifolia have narrow, teardrop-shaped leaves and thick trunks
(Carlos, 2013). Both of these species, however, produce carbohydrate rich Mullerian bodies at
the nodes of each branch point. These Mullerian bodies provide sustenance for Azteca ants in a
well documented mutualism (Kricher, 1997).
Mutualism is a relationship in which both species benefit and achieve a higher fitness
through interacting with each other. Within the previously identified mutualistic relationship
between Cecropia and Azteca ants, the ants gain a home in the hollow trunk of the plant as well
as a food source, whereas Cecropia plants gain protection from herbivores, vines, and epiphytes
(Kricher, 2011). Throughout La Reserva Biologica, two species of ant are commonly found
inhabiting Cecropia, including the well documented Azteca ants and a larger species of ant.
Azteca ants tend to live in larger colonies and are far more aggressive towards any intruders of
their host plants, while the larger ant species tend to live in smaller colonies and are quite passive
(Kricher, 2011). Due to the fact that larger ants do not easily exhibit a conspicuous response to
intruders, they are less readily recognized on the surface of the plants. However, these ants are
easily identified through observing large holes in the trunk of the tree. One characteristic that
both ant species display is feeding from the Mullerian bodies that contain carbohydrates
(Kricher,1997). Depending on the Cecropia-ant interactions, the degrees of herbivory on the
leaves of the plant can vary (Carlos, 2013).
To determine if the relationship between Cecropia and ants is truly mutualistic, research
was completed to determine how ant inhabitants influenced herbivory rates on the Cecropia
leaves. Overall, the degrees of herbivory may be predicted due to the interactions between
Cecropia and ants. In this study it was expected that Cecropia inhabited by ants would display a
lesser degree of foliage damage due to herbivory when compared to individuals not inhabited by
ants. However, it is noted that this ant mutualism is likely not the only determinant of herbivory
rates. Other variables such as tree height, leaf age and position, tree species, ant species type,
and interactions between these variables are also expected to influence the degree of herbivory in
Cercopia, and thus were considered in this experiment. Due to the fact that tree height is
indicative of age, larger plants have been exposed to herbivores for a longer period of time, so it
was expected that there would be a positive correlation between height and herbivory rates. In
addition, it was expected that smaller ants would reduce herbivory rate whereas larger ants and
plants with no ants have no effect on herbivory rate. In addition, it was expected that smaller
ants would reduce herbivory rates to a greater degree than large ants when compared to plants
without ant mutualisms. Prior to data collection, it was observed that C. obtusifolia appeared to
exhibit mutualisms with Azteca ants more frequently than C. peltata; therefore, it was
hypothesized that C. obtusifolia would exhibit lower rates of herbivory than C. peltata.
Materials & Methods
To test the hypothesis, Cecropia leaves were collected from three different sites which
included a dirt road proximal to the station at La Reserva Biologica, the paved road which was
farther from the station, and along the river that runs through the Reserve. Data was collected
from 40 Cecropia which were in the area and accessible based on height. The Cecropia species
were characterized primarily by the leaf shape and trunk thickness. For each plant sampled, it
was noted whether the plant contained no ants, small ants, or large ants prior to collecting leaves
from the plant. In order to declare that the Cecropia plant contained small Azteca ant
inhabitants, the ants had to be present in colonies and had to exhibit swarming when the plant
was handled. A plant was recorded to contain large ants based on the identification of a
relatively large (about >0.5cm diameter) nest entrance; this was the most appropriate indicator
due to the fact that the larger ants were often not present on the exterior of their host plant, and
do not exhibit swarming. The height of each tree was estimated through comparison against a
long walking stick of known length (2.10 m).
It was unknown whether or not herbivores prefer to feed on specific leave positions. To
account for this variable, and to standardize leaf collection, the first and third leaves were
collected from each plant. A tree pruner was used to remove leaves that were out of reach. In
addition, due to the flexibility of the trunks of Cecropia, the trees were bent in order to reach
leaves on the taller plants. After leaves were collected, herbivory percentages were estimated
with the use of a wire grid containing 25 (1 cm X 1cm) squares. Data was collected using the
wire grid in four specific locations. These grids were placed in the middle of the leaf, on the two
edges of the left and right leaflets, and lastly on the edge of the top leaflet. If a hole or foliage
damage was present in over half an individual 1 cm X 1 cm square, this square was counted as a
positive case for herbivory. Ultimately, the herbivory was calculated out of 100 holes (4 sites per
leaf X 25 squares per grid), so each case represented 1% of the herbivory rate for that leaf. The
herbivory rates for both leaves collected were then averaged to calculate the mean combined
herbivory rate for each plant. Data analysis was done using the program JUMP.
Results
Generalized Linear Model
(Binomial Error Dist.)
X2 = 87.9, DF = 1, P<0.0001*
Figure 1: The effect of ants on herbivory in Cecropia
Figure 1 displays the differences in mean combined herbivory rate between plants sampled with
ant inhabitants and without ant inhabitants. Mean combined herbivory rate was calculated using
the herbivory rates observed in both the first and third leaves sampled from each plant.
As seen in Figure 1, there was a difference in the herbivory rates between Cecropia sampled
with and without ant inhabitants. A generalized linear model with binomial error distribution
indicates that plants containing ant inhabitants exhibited significantly lower herbivory rates.
Figure 2: A generalized linear model factor analysis: the degree to which different
variables influence herbivory rates in Cecropia
In this analysis the effects of Cecropia species type, ant type (whether Cecropia contained no
ants, large ants, or small ants), and plant height were tested for their significance in affecting
herbivory rate. This analysis also considered two interactions: 1) the interaction between
Cecropia species and ant type, and 2) the interaction between ant type and plant height.
This analysis indicates that Cecropia species type did not significantly influence herbivory rate.
Ant type, defined as whether the plants contained no ants, small ants, or large ants had the most
significant effect. Plant height and the interaction between ant type and plant height were found
to significantly affect herbivory rate. The interaction between Cecropia species and plant height
was not found to be significant; however, a P value of 0.0629 for this test indicates that this
interaction may produce a marginal trend in herbivory rate.
Generalized Linear Model
(Binomial Error Dist.)
X2 = 138.26
DF = 1
P < 0.0001*
No ants
Large Ants
Ant Type
Figure 3: A comparison of mean combined herbivory rates between Cecropia plants with
large ants and plants containing no ants
Figure 3 indicates that Cecropia plants containing large ants exhibited significantly higher mean
Mean Combined Herbivory Rate
combined herbivory rates than plants containing no ants.
Ant Type
Figure 4: The effect of ant type on mean combined herbivory rate on Cecropia
Figure 4 is a graphical representation of the data describing the effect of ant type on herbivory
rate in Figure 2 (DF=1, X2=13.6, P<0.0011*). The data indicate that there were significant
differences in herbivory rate between plants that contained no ants, large ants, or small ants.
Figure 4 indicates that plants containing large ants had the highest mean combined herbivory
rate, followed by plants with no ants; plants containing small ants exhibited the lowest mean
herbivory rate. The data displayed in Figure 2 indicate that these differences were significant.
Figure 5: Effect of plant height on mean combined herbivory rate.
Figure 5 shows that there’s a significant positive correlation between the plant heights and mean
combined herbivory rate. However, the R2 value indicates that the height explains only 14.9% of
mean combined herbivory rate.
Figure 6: The interaction between ant type and plant height.
This figure shows how different ant types (large, none, small) respond to the variation in height
of Cecropia. (DF = 2, X2 = 7.13, P < 0.03)
Figure 6 is a graphical representation of the interaction between ant type and plant height that
was analyzed in figure 2. These data indicate that there is a positive correlation between large
ants and plant height, while there is no relationship between plant height and the other two ant
types.
Discussion
The goal of this experiment was to assess the effectiveness of the mutualistic relationship
between Cercopia and ants in the La Reserva Biologica Alberto Manuel Brenes in San Ramon
Costa Rica. The overarching hypothesis of this experiment was that Cecropia inhabited by ants
were expected to display a lesser degree of foliage damage due to herbivory when compared to
individuals not inhabited by ants. The samples collected suggest that the frequency of this
mutualism is about 57.5%; 17 trees sampled did not contain ant inhabitants, while 23 did. At the
most basic level, this hypothesis was proven to be true with the data displayed in Figure 1; these
data clearly show that the plants which contained ant inhabitants exhibited lower rates of
herbivory. This suggests that the Cecropia-ant mutualism is effective in protecting the plants
from foliage damage due to herbivores, and that the presence of ant colonies in Cecropia can be
used to predict herbivory rates. The comprehensiveness of the data displayed in Figure 1 was
limited however, due to the fact that it did not consider the potential effects of other factors on
herbivory rate.
A generalized linear model factor analysis was utilized to gain a better understanding of
how additional variables including Cecropia species, ant type, and plant height collectively
affected herbivory rates. This factor analysis was also utilized to analyze the interaction between
Cecropia species and ant type, as well as the interaction between plant height and ant type. The
data in Figure 2 indicate that herbivory rates in Cecropia are affected by multiple different
factors, and are not determined solely by the presence or absence of ant inhabitants. It was
determined that plant height and ant type both significantly affected herbivory rates, while the
type of Cecropia species did not appear to affect herbivory. Although Cecropia species type (C.
peltata or C. obtusifolia) did not significantly influence herbivory rates, the data in Figure 2 also
shows that the interaction between Cecropia species and ant type was marginally significant
(P<0.0629). This suggests that there may be a tendency for ants to inhabit one Cecropia species
over the other. X2 and P values for Cecropia species type (X2 = 0.062, P< 0.8034, figure 2) and
those of the interaction between Cecropia species and ant type (X2 = 7.13, P < 0.03, figure 2),
suggest that there’s no feeding preference on Cecropia species by herbivores, however ants do
have preference for mutualism with one Cecropia over the other; collectively, these two analyses
indicate that one species should tend to exhibited lower rates of herbivory. Future studies should
be completed to further explore if a certain species is, in fact, more likely to be protected by
mutualism with ants over the other or if this observed trend was the result of a bias sample
population.
It was observed that the larger ant species commonly found in Cecropia lived in much
smaller colonies, and were also far less aggressive than the small Azteca ants. Therefore it was
expected that these larger ants may be less effective in protecting Cecropia from herbivores.
Surprisingly, however, a comparison between mean combined herbivory rates in plants
containing large ants with plants containing no ants (Fig. 3) showed that plants containing large
ants exhibited significantly higher rates of herbivory. This indicates that the interaction between
the large ants and Cecropia is likely not mutualistic. The data in Figures 2 and 4 further prove
that the larger ants were ineffective in protecting Cecropia from herbivores. The GLM factor
analysis indicated that ant type had the most significant effect on mean rate of herbivory (DF =1,
X2=13.6, P<0.0011*). Figure 4 also illustrates that the large ants increased herbivory rates when
compared to plants containing no ants, while small ants significantly reduced them. Thus, these
data provide strong evidence that small Azteca ants exhibit effective mutualisms with the
Cecropia, while the larger ants do not.
The test that was run to determine the correlation between plant height and ant type
showed that there was weak positive linear relationship where height explains 14.9% of mean
combined herbivory rate. Although the relationship was weak (R2 =0.149), it was significant
(P<0.0138) enough to be analyzed further. Thus, the responses of each ant type to height were
separately examined using a GLM factor analysis to determine if there were significant
interactions between ant type and plant height. Figure 6 showed that there was an interaction
between the response that large ants had on the height variation of Cecropia and the response
that the plants containing no ants had on the height variation of Cecropia. This interaction was
found to be significant with P < 0.03. The significant interaction between plant height and ant
species can be explained by the fact that large ants were only found in tall plants, and tall plants
exhibited higher rates of herbivory. However, this interaction could be misleading due to an
outlier; the tallest plant sampled also had the highest rate of herbivory, and happened to contain
large ants as well. This outlier, however, should not be ignored due to the fact that it may
contain valuable information about the taller Cecropia population we were unable to sample. To
validate this interaction, more trees of this height must be sampled to determine if large ants do
in fact inhabit taller Cecropia more frequently.
Due to the time constraints of this experiment, the method of collection and sample size
were limited. Thus, there are many future experiments that can be performed to expand upon
and enhance the validity of the conclusions drawn from this research. For example, only three
trees containing large ants were sampled. This sample size must be increased to more accurately
elucidate the effects large ants have on herbivory rates in Cecropia. Although it appears that
large ants inhabit taller trees more frequently, and taller trees have higher herbivory rates, further
studies are necessary to determine the cause of this increased herbivory. Large ants may actually
be parasitic, or they may inhibit the mutualistic protection offered by Azteca ants, thereby
increasing herbivory. Another possibility is that herbivores simply prefer to feed on more
mature trees, and the relationship between the large ants and Cecropia is actually a
commensalism. Cecropias are dioecious which means that there are different independent sexes
which bare different energy costs associated with reproduction. These energy costs may have an
effect on the plants relationship with ants. Future studies should consider sex differences as a
factor that may influence herbivory rate. Additionally, only the trees that could be reached were
sampled in this experiment, which means that the data is biased towards younger trees. It would
be beneficial to find a way to also sample the tall, mature trees for this experiment.
In conclusion, the Cecropia-Ant mutualism was found to be effective in reducing
herbivory in plants containing Azteca ants. The data indicates, however, that the larger ant
species are not mutualists, and may even be parasitic. Future studies must be completed to
elucidate the specifics pertaining to the interactions between the large ant species and taller
Cecropia. This information is useful in that it suggests introducing Azteca ant colonies to a
deforested area could aid in recruitment of various Cecropia species. Cecropia are a pioneer
species, so understanding this mutualism could be useful in utilizing them to promote
reforestation.
References
Carlos, T. (2013, December 30). Cecropia. (T. Bloom, Interviewer)
Kricher, J. (1997). A Neotropical Companion. Princeton: Princeton University Press.
Kricher, J. (2011). Tropical Ecology. Princeton: Princeton University Press.