Download Ant Species Identity has a Greater Effect than Fire on the Outcome of

BIOTROPICA 47(4): 459–467 2015
10.1111/btp.12227
Ant Species Identity has a Greater Effect than Fire on the Outcome of an Ant
Protection System in Brazilian Cerrado
Kleber Del-Claro1,3 and Robert J. Marquis2
1
^ndia (UFU), CP 593, Uberla
^ ndia, MG, CEP 38400-902, Brazil
Instituto de Biologia, Universidade Federal de Uberla
2
Department of Biology and Whitney R. Harris World Ecology Center, University of Missouri-St. Louis, One University Boulevard, St. Louis,
MO, 63121, U.S.A.
ABSTRACT
Although fire- and ant–plant interactions influence the community structure and dynamics of Neotropical savannas, no previous studies
have considered their simultaneous effects on target host plants. We monitored the effect of ant exclusion for 3 years on leaf area loss
to leaf chewing insects, thrips abundance, and reproductive output of the extrafloral nectary-bearing shrub, Peixotoa tomentosa (Malpighiaceae). We predicted that the impact of ants on herbivores and plants would depend on the ant species, and that fire would reduce the
effect of ants. We deliberately chose control plants that differed in their occupant ant species. Fire occurred in the second year of the
study, allowing us to determine its effect on the benefit afforded by ants. Ants reduced leaf area loss and thrips abundance, and
increased fruit and seed production in all 3 years. Some ant species were more effective than others, while plants with multiple ant species suffered higher leaf area loss than plants with a single ant species. In the year following the fire, leaf damage was greater than in
the other years, regardless of the ant species, and the proportional effect of ants in reducing damage was less. Interactions affecting
thrips abundance did not change following fire, nor was the benefit to the plant proportionally reduced. Overall, the identity of the ant
species had a greater effect than did the occurrence of fire on the ant–herbivore–plant interaction: the identity of the ant species influenced leaf area loss, thrips numbers, and bud and seed production, while fire only modified the impact of ants on the amount of leaf
area consumed by insect herbivores.
Key words: abiotic factors; ant–plant interactions; Brazil; Cerrado; extrafloral nectaries; fire; thrips.
EXTRAFLORAL
NECTARIES
(EFNS),
WHICH ATTRACT ANTS AND OTHER
(Byk & Del-Claro
2011), have been present in Angiosperms since the Oligocene
(Pemberton 1992). Ants visiting EFN-containing plants often
protect those plants by eating, hindering, or driving away herbivores (Bentley 1977, Horvitz & Schemske 1984, Koptur 1984,
Vilela et al. 2014). In turn, plant growth and seed production are
often higher (Del-Claro et al. 1996, Oliveira 1997, De la Fuente
& Marquis 1999, Nascimento & Del-Claro 2010).
Not all studies, however, suggest that EFNs are beneficial
for plants (O’Dowd & Catchpole 1983, Rashbrook et al. 1992,
Bronstein 1998, Mody & Linsenmair 2004, Chamberlin and Holland 2009). Differences in behavior among the ant species
attracted to plants containing EFNs influence the effect of the
EFNs (Del-Claro & Oliveira 2000, Mody & Linsenmair 2004,
Palmer et al. 2008, Nahas et al. 2012, Del-Claro et al. 2013). The
abiotic environment may also contribute, although this is much
less studied (Bronstein 1998, Vilela et al. 2014, Yamawo et al.
2014). Clarifying which factors influence the effect of EFNs is an
important starting point for understanding how these ant–herbivore–plant interactions evolved and were maintained, and for
identifying their contribution to the structure and functioning of
INVERTEBRATES THAT FEED ON THEIR SECRETIONS
Received 9 June 2014; revision accepted 26 February 2015.
3
Corresponding author; e-mail: [email protected]
ª 2015 The Association for Tropical Biology and Conservation
food webs. This is particularly true for tropical systems where
plant species with EFNs are most abundant (Rico-Gray & Oliveira 2007, Bronstein 2009, Dattilo et al. 2014).
Plants with EFNs are common in the savannas of South
America, Africa, and Asia (Rico-Gray & Oliveira 2007). Because
fire is such an important environmental factor in savanna vegetation (Alves-Silva & Del-Claro 2014), it is likely that fire, in
addition to ant species identity, influences the nature of interactions between ants, herbivores, and plants with EFNs. In Brazilian savanna (the Cerrado), EFNs occur on more than 25
percent of the tree species and up to 30 percent of individual
trees (Rico-Gray & Oliveira 2007). Fire is considered to be a
principal determinant of Cerrado vegetation structure and plant
diversity (Miranda et al. 2002). One obvious mechanism by
which fire might influence the interactions between plants and
ants visiting EFNs is through its impact on insect herbivores
and ants. Cerrado arthropod density and ant species richness
are known to decline in fire-disturbed areas as a result of direct
mortality, diminished resources, and less favorable microclimates
(Vasconcelos et al. 2009). Although both ant–plant interactions
(Del-Claro & Torezan-Silingardi 2009) and fire (Vasconcelos
et al. 2009) play an important role in the community structure
and dynamics of Cerrado (Alves-Silva & Del-Claro 2014), their
simultaneous effects on plants in any tropical savanna system
are unknown.
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Del-Claro and Marquis
The overall goal of this 3-year study was to evaluate the
interactive effects of ants and fire on herbivory and subsequent
reproduction of the Cerrado shrub, Peixotoa tomentosa (Malpighiaceae). We took advantage of the occurrence of an unplanned fire
as a natural experiment (sensu Diamond 1986) to determine the
changing impact of fire over time, along with the impact of different ant species associated with P. tomentosa. Fire is a major factor at our study site (Alves-Silva & Del-Claro 2014), and in other
tropical savannas where ant protection mutualisms occur (Miranda et al. 2002, Bond et al. 2005).
We tested three hypotheses. Our first hypothesis was that
ants visiting P. tomentosa would have a positive effect on plant
reproduction. This beneficial effect has been demonstrated many
times previously in other systems (Rosumek et al. 2009, Trager
et al. 2010). Our second hypothesis was that the outcome for
P. tomentosa (leaf area loss or fruit production) would be ant species dependent (Moreira & Del-Claro 2005). Although different
ant species commonly confer different degrees of plant protection
(Rico-Gray & Oliveira 2007), we wanted to establish a priori
which ant species provide more benefit. Factors such as large ant
size and aggressiveness toward humans do not necessarily result
in increased protection to plants (e.g., Letourneau 1983). Our
third hypothesis was that fire would reduce the protective benefits of ants. Specifically, we predicted that the protective effects of
ants would be reduced immediately following fire (year 2000)
compared to the year prior to the fire (1999) and 2 years after
the fire (year 2001). We predicted this decrease in protective
effect because Cerrado fires were previously shown to reduce ant
populations (Vasconcelos et al. 2009). Fires also result in a
predictable increase in vulnerable leaf tissue to herbivory due to
resprouting following the fire (Vieira et al. 1996, Otterstrom &
Schwartz 2006, Alves-Silva & Del-Claro 2014).
METHODS
STUDY SITE AND STUDY SYSTEM.—The study was conducted at the
tropical savanna reserve of Clube de Cacßa e Pesca Itoror
o de
Uberl^andia (CCPIU) – Minas Gerais, Brazil (18°590 S, 48°180 W).
The mean annual temperature is 22°C, the elevation is 863 m,
and mean annual rainfall is 1500 mm, with the majority falling
between October and April (Bachtold et al. 2012). The vegetation
of the reserve is dominated by Cerrado sensu strictu (Oliveira &
Marquis 2002), consisting of trees 2–8 m in height, with an
understory dominated by grasses and scattered perennial herbs
(Oliveira-Filho & Ratter 2002).
Peixotoa tomentosa A. Juss. (Malpighiaceae) is a common
decumbent shrub (0.5–2.5 m) at the study site, possessing a pair
of extrafloral nectaries (EFNs) at the base of each leaf. These
EFNs attract ants (Fig. 1A and B). In general, individual plants
are dominated during the day by a specific ant colony, but are
visited at night by other species. New leaves are produced at the
end of September, with the first rains of the rainy season, and
are retained until the beginning of the next rainy season (the following September) (Del-claro et al. 1997). The flowering period
of P. tomentosa at the study site peaks between June and July, but
individuals and groups of plants bear flowers during most
months of the year, and the plant survives for more than 6 years
(Vilela et al. 2014). The leaves and petals of P. tomentosa are
A
B
C
D
FIGURE 1. Extrafloral nectar-bearing plant and ants: (A) Detail of the extrafloral nectary (EFN) of Peixotoa tomentosa (Malpighiaceae), the arrow points to a droplet of nectar; (B) Ectatomma ant collecting nectar in an EFN of P. tomentosa; (C) Thrips (Heterothrips peixotoa) sucking at the base of reproductive structures of a
P. tomentosa flower; (D) Field site immediately after the fire, in early November 1999.
Fire- and Ant–Plant Interactions
attacked by a large diversity of insect species, with most species
being of low individual abundance (Vilela et al. 2014). However,
Thysanoptera (mainly Heterothrips peixotoa, Del-claro et al. 1997,
Fig. 1C), which feed on the bases of stamens, and endophytic
beetle larvae (Curculionidae), which feed on the interior of floral
buds and flowers, destroying the reproductive organs (Vilela et al.
2014), are abundant.
ANT EXCLUSION EXPERIMENTS.—To evaluate the outcomes of these
ant–plant–herbivore interactions over time, we conducted the
study over three consecutive years: 1999 (May–July); 2000 (February–June); and 2001 (March–June). At the beginning of the
experiment, we tagged 157 individuals of P. tomentosa, all approximately the same size (1.5 m) along one transect of 5 m 9 3 km.
We used a different subset of plants each year, chosen at random
from the original 157 marked plants, with the qualification that
they must have been flowering and have active EFNs (both antexcluded and ant treatment plants) and they must have been visited by a particular species of ant or mix of ants (ant treatment
plants). No plant was ever used for more than a year.
Each year, we randomly selected (by the flip of a coin) a
subset of 20 of these reproductive and EFN-active plants to be
included in the no-ant treatment (named ‘ant excluded’). We
applied a strip of adhesive tape covered by Tanglefoot (The
Tanglefoot Co., Grand Rapids, Michigan, U.S.A.) to the base of
the main stem, 15 cm above ground level, to prevent ants from
climbing onto the plant. We manually removed any additional
structures that could be used by ants to access the plant.
The identity and number of control plants (named ‘ant treatment’), and the identity of their diurnal ant occupants varied by
year. In 1999, we selected 40 flowering and EFN-active plants as
part of the ant treatment based on their diurnal ant occupants,
with 10 plants per dominant ant species: (a) Ectatomma tuberculatum; (b) Crematogaster sp.; (c) Camponotus crassus; and d) the last
group composed by other ants (named ‘mix of ant species’),
mainly Cephalotes pusillus, one undetermined species of Dolichoderinae, and one Myrmicinae. At night, all plants were visited by
the same dominant ant species, C. renggeri, which tolerates the visitation of C. atriceps in the same plants. It is also important to
note that C. crassus tolerates the presence of other Camponotus ants
on the same plant, particularly C. blandus.
In the 3 years preceding our study, there were no records of
significant fire in the area; however in October 1999, a humancaused fire burned 90 percent of the reserve, including our study
transect (Fig. 1D). The majority of aboveground parts of grasses,
herbs, shrubs, and trees up to 5-m tall were destroyed. Subsequently, P. tomentosa resprouted, and these new branches flowered
between January and February 2000. In the second year of the
experiment (2000), we were able to tag 25 flowering and EFNactive plants to be included in the ant treatment: 10 were used by
E. tuberculatum and 15 mainly by C. crassus during the day, and all
were used by C. renggeri and C. atriceps during the night. In the
third year of the experiment (2001), plants flowered irregularly,
and we were able to tag 10 flowering and EFN-active individuals
used by E. tuberculatum and 10 by C. crassus.
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In each year, on the first day of the experiment (1999: May 3;
2000: February 1; 2001: March 5), we measured area loss to chewing insects on all leaves of the experimental plants. Leaf herbivory
was sampled again on the last day of the experiment of each year
(1999: July 26; 2000: June 26; 2001: June 25). Leaf area loss was
measured using a clear plastic grid, with a grid size of 1 mm. An
index of herbivory was estimated for each leaf, calculated as the
proportion of points in the grid falling within damaged areas of the
leaf blade, divided by the sum of points falling within damaged and
undamaged areas of the leaf blade. The average percent area missing per leaf, calculated across all leaves of each plant, was used as
the response variable (Dirzo & Domınguez 1995). We checked
plants weekly to refresh the Tanglefoot barrier when necessary in
the ant exclusion treatment and to confirm the identity of the ant
species in the ant treatments. For all plants, we also counted the
total number of buds and flowers produced weekly (those produced above a fine, colored thread placed on the stem the previous
week), as well as the number of thrips infesting flowers, recorded
directly in the field. At the end of each year, the number of fruits
and seeds were recorded for each experimental plant. Counting
seeds in P. tomentosa is easy, as the fruit is a samarium, generally
composed of three samaras, each with a single seed inside. To estimate seed production, we counted the number of samariums, and
multiplied this number by three.
STATISTICAL ANALYSES
All analyses were run in SAS. To clarify the effects of the treatments, we distinguished between protection provided by the ants,
as quantified by leaf area missing and number of thrips, versus
the benefit of ant protection, measured as bud, fruit, and seed
production (De la Fuente & Marquis 1999). For all analyses, initial leaf missing (leaf area loss) was tested as a covariate. Significance was based on Type III sums of squares (based on
unweighted means) to account for differences in sample sizes
among treatments. Leaf damage (percent leaf missing in each
plant), a continuous variable, was logit transformed (Warton &
Hui 2011), and treatment effects on leaf damage were tested with
ANOVA (PROC GLM). Residual fit was tested with a Kolmogorov–Smirnov D goodness of fit and homogeneity of variances
among treatments with a Levene’s test. Number of thrips, and
bud, fruit, and seed production were considered to be discrete
variables. Therefore, treatment effects on these variables were
tested against a Poisson distribution with a log link or a negative
binomial and a log link with a Pearson scaling to account for
overdispersion of the data when needed (PROC GENMOD).
Variables were square root or log transformed as needed for
good model fit, which was tested with a goodness of fit chisquared test based on the scaled deviance divided by the degrees
of freedom.
In the first set of analyses, using 1999 data only, we asked
whether the identity of ants occupying a plant during the day
influenced protection (leaf damage and number of thrips) and
benefit (production of buds, fruits, and seeds). We compared the
effects of ant exclusion on these variables against the effects of a
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Del-Claro and Marquis
A
B
C
D
E
F
FIGURE 2. Final leaf damage (% of herbivory) and number of thrips on shrubs of the extrafloral nectar-bearing plant Peixotoa tomentosa (Malpighiaceae), treatment (Ant excluded) and not manipulated and hosting a group of ant species (Mix of ant species) or predominantly one ant species (Crematogaster sp, Ectatomma tuberculatum, Camponotus crassus), during 1999 (A, B; before fire occurrence), 2000 (C, D; immediately after fire occurrence), and 2001 (E, F; more than 1 year after
fire occurrence). Values are means 95% confidence intervals (markers = means; rectangles = SE; whiskers = SD). Different letters over boxes denote statistically significant differences (ANOVA, P < 0.001).
Fire- and Ant–Plant Interactions
single ant species (i.e., E. tuberculatum, Crematogaster sp., or Camponotus crassus) or a mix of ant species. A Tukey’s post-hoc test was
used to test for specific treatment differences on final leaf area
(logit transformed).
In the second set of analyses, our main question was whether
fire influenced the level of protection and benefit. We compared
the effects of the ant exclusion treatment versus the identity of the
naturally occurring ant species across all 3 years. Of the original
157 marked plants, we found sufficient numbers of plants that
were occupied by either E. tuberculatum or C. crassus in each of the
three study years. This allowed us to test for the effects of individual ant species across all 3 years. We tested for treatment, year, and
treatment 9 year effects on final leaf damage (logit transformed),
the square root of the number of thrips, log of bud number, fruit
number, and the square root of seed number.
In our second set of analyses, we also looked for a fire effect as
having three possible modes of action. First, there could be a significant increase or decrease (we predicted the former to occur because
we expected reduced ant protection immediately following the fire)
in the amount of leaf damage or number of thrips in the year just
subsequent to the fire (2000) compared to the other 2 years. Second,
there could be a fire effect on the strength of the interactions, due to
decreased ant individuals and increased vulnerable leaf tissue. A significant treatment 9 year interaction would be supportive of such
an effect, with an expected weaker treatment effect occurring in the
year of the fire. Third, we compared effect sizes (ln(response variableant-excluded / response variable control)) for final leaf area loss and
thrip number, and number of buds, fruits, and seeds for each of the
3 years (Denno et al. 2003). We considered year to be a fixed effect
in this case because we were interested in a very specific year effect
(before, during, and after fire). Our prediction was that effect size
would be higher for years 1999 and 2001 compared to year 2000
when ant abundance would be reduced, due to the fire effects. Preplanned comparisons (the two control treatments, one for each ant
species, vs. ant-reduced treatment) were made after calculating the
overall models as an estimate of the strength of the interaction,
which we predicted would be weaker in 2000.
Finally, we used ANOVA to test whether the initial leaf area
missing differed significantly by treatment, and found it not to be
so for either 1999 (P = 0.812) or 1999–2001 (P = 0.140). Thus,
the plants, at the time of assignment to treatment, did not vary
in their initial standing leaf damage for either analysis.
RESULTS
FIRE
AND THE PROTECTIVE EFFECT OF ANTS ON LEAF DAMAGE AND
1999, treatments affected both final leaf
area loss to chewing insects (F4,54 = 5.04, P = 0.0016) and the
number of thrips (v2 = 45.64, P < 0.0001, df = 4). For final leaf
damage, plants of the ant-excluded treatment lost the most leaf
area to insect herbivores, while plants with mixed species composition of ants suffered intermediate levels of leaf area loss
between ant-excluded plants and plants occupied by a single species of ant (Fig. 2A). Initial leaf damage predicted the amount of
final damage (F1,54 = 6.33, P = 0.0149); ant-excluded plants and
THRIP ABUNDANCE.—In
463
plants occupied by E. tuberculatum had 2–5 times more thrips
than plants occupied by C. crassus, Crematogaster sp., or a mix of
ant species (Fig. 2B).
From 1999 to 2001, there were significant treatment and
year effects on final leaf damage (treatment: F2,106 = 26.09,
P < 0.0001; year: F2,106 = 23.02, P < 0.0001) and on the number
of thrips (treatment: v2 = 12.37, P = 0.0021; year: v2 = 7.18,
P < 0.0276). Ant-excluded plants suffered greater leaf area loss
and had more thrips than plants occupied by either ant species in
all 3 years (Fig. 2). The two ant species that were abundant in all
3 years (Ectatomma tuberculatum and Camponotus crassus) did not
vary in their effect on leaf area missing or number of thrips (posthoc, P > 0.05).
Leaf damage (percent leaf area missing) was higher in the
year 2000 (mean S.E. = 24.8 percent leaf area missing 2.3)
for all three treatments compared to 1999 (mean S.E. =
9.1 2.0) and 2001 (mean S.E. = 8.4 0.5) (P < 0.05, Tukey’s post-hoc comparison, compare Fig. 2C vs. 2A and E). The
year 9 treatment interaction was not significant for thrip abundance (P = 0.5705), but was for leaf damage (F4,106 = 2.46,
P = 0.0499), because of a stronger difference between antexcluded and ant treatments in 1999 compared to 2000 and 2001
(Fig. 2).
FIRE
AND THE BENEFICIAL EFFECT OF PROTECTION BY ANTS ON
REPRODUCTIVE OUTPUT.—Analysis
of 1999 data showed a significant overall treatment effect on the number of flower buds
(v2 = 222.66, P = 0.0001), but not on fruits (P = 0.5884) or
seeds (P = 0.8718) (Fig. 3A). In 1999, there were fewer flower
buds on Crematogaster sp.-occupied plants and higher bud numbers on plants occupied by C. crassus, with ant-excluded plants
having intermediate levels.
There were significant treatment effects on fruit and seed
but not bud production across the 3 years. These treatment
effects were not influenced by fire, as indicated by the lack of significant treatment 9 year interactions for any of the three reproductive variables. There was a significant year effect for buds
(v2 = 20.08, P < 0.0001) with all years different from each other
but no treatment (P = 0.7573) or treatment 9 year effects
(P = 0.5724) (Fig. 3). Plants produced fewer buds with each successive year (mean S.E.: 1999: 92.1 10.7, 2000: 63.3 5.7;
2001: 44.6 5.3). There were significant ant exclusion effects on
fruit (v2 = 12.55, P = 0.0019) and seed production (v2 = 11.49,
P = 0.0032), but no significant year effects (P = 0.0843 and
P = 0.2819, respectively) or significant treatment 9 year interactions (P = 0.4731 and P = 0.6561, respectively). Plants occupied
by Ectatomma and Camponotus had higher fruit production than
ant-excluded plants (and no difference between ant species), while
plants occupied by Ectatomma had significantly higher seed production than ant-excluded plants (P < 0.05) (Fig. 3).
EFFECT SIZES OF ANT PROTECTION AND SUBSEQUENT BENEFIT.—For
all but one variable (final leaf damage), differences in effect size
were not in the direction or strength (P < 0.05 for pre-planned
contrasts) predicted by our hypothesis about the effect of fire.
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Del-Claro and Marquis
A
B
C
Fire- and Ant–Plant Interactions
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FIGURE 3. Outcomes of the ants–plant interaction: Number of buds (B), fruits (F), and seeds (S), in shrubs of the extrafloral nectar-bearing plant Peixotoa tomentosa (Malpighiaceae), treatment (ant excluded) and not manipulated and hosting a group of ant species (Mix of ant species) or one predominant ant species (Crematogaster sp, Ectatomma tuberculatum, Camponotus crassus), during 1999 (A, before fire occurrence), 2000 (B, immediately after fire occurrence), and 2001 (C, more than
1 year after fire occurrence). Values are means 95% confidence intervals (markers = means; rectangles = SE; whiskers = SD). In 1999, only the number of
buds was different among groups, with higher bud numbers in plants occupied by C. crassus (ANOVA, P < 0.05). Comparisons between years showed that plants
occupied by Ectatomma and Crematogaster had higher fruit production than ant-excluded plants, while only plants occupied by Ectatomma had significantly higher
seed production than ant-excluded plants (P < 0.05, Tukey’s post-hoc). Different letters over boxes denote statistically significant differences (ANOVA, P < 0.001).
FIGURE 4. Differences in effect size for ant exclusion versus the two control treatments on final percent leaf area loss compared among years. The differences between the two control treatments (plants occupied by either
Ectatomma tuberculatum—gray bars, or by Camponotus crassus—black bars) and
the ant-excluded treatment for final leaf damage were greater in 1999 and
2001 than in 2000. P-values are for pre-planned comparisons in ANOVA, ant
exclusion versus the two control treatments.
Only the differences between the two control treatments (plants
occupied by either Ectatomma tuberculatum or Camponotus crassus)
and the ant-excluded treatment for final leaf damage were greater
in 1999 and 2001 compared to 2000, and P-values were greater
in those years (Fig. 4). Both are consistent with a weakening of
the impact of ants on leaf damage immediately following the fire.
DISCUSSION
The source of contingencies in ant–plant protective systems can
be ‘internal’ (i.e., the result of the identity of the particular species
involved in the interaction) or ‘external’ (i.e., due to abiotic factors
that affect one or more of the species in the interaction). One
question that arises is whether the nature of ant–plant interactions is relatively immune to external influences. If so, continuous
interaction across generations may lead to the evolution of traits
to increase ant–plant interactions. By contrast, frequent disruption
of these interactions by external factors would decrease the evolution of traits that would lead to such predictability. In this study,
we investigated the role of the identity of the ant species occupying the host plant (an internal source of contingency) versus the
external influence of fire on the interaction. We found that the
identity of ants occupying the plant had a much greater effect on
the outcome of the interaction for the plant than did the occurrence of fire.
As predicted by our first hypothesis, we found that leaf
damage was highest in ant-excluded plants for all 3 years of the
study (Fig. 2). This result is consistent with the commonly demonstrated protective role of ants on plants with extrafloral nectaries (e.g., Rico-Gray & Oliveira 2007). A meta-analysis of the role
of ants as plant biotic defenses (Rosumek et al. 2009) showed
that on average ant exclusion increases the number of herbivores
on plants by almost 50 percent, doubling the amount of tissue
lost to herbivores. In another Cerrado Malpighiaceae (Byrsonima
crassifolia) plants from which ants were excluded had twice as
many chewing and sucking insects on inflorescences as plants visited by Camponotus ants (Fernandes et al. 2005). Vilela et al. (2014)
showed that sequential flowering among different Malpighiaceae
species favors the use of these plants by a similar herbivore guild
over time. The interactions with ants, mainly Camponotus, reduced
the abundance of shared herbivores and resulting foliar herbivory.
We also found that Camponotus ants reduced leaf area loss and
the number of thrips (Fig. 2).
Consistent with our second hypothesis, we found that both
protection (decrease in herbivore numbers and damage) and benefit (increase in fitness components) (De la Fuente & Marquis
1999) were variable depending on the associated ant species. Leaf
area loss and thrip abundance varied by ant species in 1999, and
seed production per plant across the three study years varied
depending on the occupying ant species. Different ant species
can provide different benefits to host plants, as seen for both
ant–plant–hemipteran (Del-Claro & Oliveira 2000, Moreira &
Del-Claro 2005) and defense systems with EFN-bearing plants
(Miller 2007, Palmer et al. 2008). Prior to fire occurrence, P. tomentosa plants hosting a mix of ant species lost more leaf area than
did those tended by a single dominant and aggressive species.
According to Miller (2007), EFN plants visited by multiple ant
species that vary in antiherbivore abilities saw reduced benefits
relative to an exclusive association with a single, high-quality
mutualist species (see also Mody & Linsenmair 2004).
The weak effects of the ‘mix of ant species’ treatment contradicts a prediction in the predator–prey literature that more
predator species, presumably with more diversified strategies for
acquiring prey (here, herbivores), result in lower herbivory com-
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Del-Claro and Marquis
pared to interactions with fewer predator species (Terbogh & Estes 2010). In the ‘mix of ant species’ treatment, the predominance
of Cephalotini ants, like Cephalotes pusillus, may be an answer to
the lack of protection against herbivory in this treatment.
Although common in EFN-bearing plants in the Neotropics
nesting inside dead trunks or stems of live trees (Powell 2008),
C. pusillus are ineffective against herbivores (Sendoya et al. 2009);
they collect nectar and pollen without protecting the plant and
act as a parasite (Byk & Del-Claro 2010). Similar results,
observed in ant–plant mutualisms in the African savanna, showed
that dominant ant species protected plants against herbivore
action, while an antagonistic species favored more attacks by
stem-boring beetles and doubled plant mortality (Palmer et al.
2008). The identity of the herbivore species is relevant. For
instance, Heterothrips peixotoa avoid attack by some ant predators,
destroying flowers, and reducing reproduction (Vilela et al. 2014),
whereas Crematogaster are more effective at removing thrips than
other ant species in Cerrado (Del-claro et al. 1997).
We found less evidence for our third hypothesis, namely that
fire modifies the interactions among ants, herbivores, and plants.
In support of this hypothesis, we found that leaf herbivory
increased in the year immediately following fire occurrence and
then decreased in the subsequent year. We tested for an effect of
fire on these interactions in two ways. First, we tested for a significant treatment 9 year interaction but it only occurred for one
response variable (leaf damage), and in that case, the interaction
was due to greater effect of ant presence in 1999 compared to
the other 2 years (Fig. 2). This result does not support our third
hypothesis. Second, we compared effect sizes for each of the
response variables. We found that exclusion of ants had a weaker
effect in 2000 compared to the other 2 years, but only for final
leaf damage (Fig. 4). It appears that fire results in higher leaf tissue loss produced subsequent to burning, and that this effect is
the result of decreased ant protection (Fig. 4). We suspect that
the high quality of resprouting plant tissue (Vieira et al. 1996, Alves-Silva et al. 2014) also contributed to higher damage. However,
no similar effect occurred for numbers of thrips, nor did
increased leaf area loss translate into decreased impact of ants on
reproductive output in the year immediately following the fire.
Our conclusion is that ants provided both protection and benefit
to plants in all 3 years. Fire weakened protection against herbivores following fire, but there was no change in the benefit provided by ants to plants as a result of the fire. We do not know if
the reduced protection was due to fewer ants visiting the plants,
higher palatability of leaf tissue for herbivores, or some combination of these factors.
In our experiment, subjects were randomly assigned to control and ant exclusion treatments, and all plants suffered the
effects of fire. Thus, our experiment was a ‘before-after’ experiment with respect to the fire treatment (Rasmussen et al. 2001).
Our experiment has the advantage that it was conducted at a scale
large enough to potentially affect not just individual focal plants,
but also associated herbivore, ant, and pollinator populations.
Our results suggest that the final outcome of ant–plant interactions following fire in Cerrado is linked to specific details of the
relationship. These details include the characteristics of associated
ant species (both aggressiveness toward herbivores, and the ability
to survive fires or to re-colonize fire-affected areas); plant response
to fire (quickly resprouting, providing a valuable food and locally
abundant resource that may be less defendable by ants); and fire
intensity (variation in the geographical extent, duration, and reoccurrence). In the years that we studied these interactions, fire
reduced the protective role of ants on P. tomentosa, but did not
modify the benefits accrued by the plant as a result of its attraction
to ants. This result provides evidence that the interaction between
the ants and plant is influenced by fire, but not to the degree that
the benefit to the plant associating with aggressive ants is
decreased. Thus, our study provides evidence that a major disturbance factor in this system is not enough to weaken the selective
forces that maintain the association between P. tomentosa and its
more aggressive ant species.
ACKNOWLEDGMENTS
The authors thank L. Kaminski and J. C. Santos and the three
anonymous reviewers for their valuable comments on previous
versions of the manuscript. KDC thanks CNPq (473055/2012-0
and 301605/2013-0) and Fapemig and RJM thank NSF-BSR
0842144 for financial support.
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