Download 1 Are invasive ants better plant-defense mutualists? A comparison of

Are invasive ants better plant-defense mutualists? A comparison of foliage patrolling and
herbivory in sites with invasive yellow crazy ants and native weaver ants in northern
Australia
Lori Lach1 and Benjamin D Hoffmann2
1
School of Plant Biology, M090, The University of Western Australia, 35 Stirling Highway,
Crawley, WA 6009, Australia
2
CSIRO Sustainable Ecosystems, Tropical Ecosystems Research Centre, Darwin, NT 0822,
Australia
Corresponding author information:
Email: [email protected]
Phone: +61 8 6488 4565
Fax: +61 8 6488 7461
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Abstract
Benefits arising from facultative mutualisms between ants and plants vary with the identity of
the ant partner. Invasive and native ants are both attracted to plants that offer extrafloral
nectar, but few studies have compared their abilities to displace herbivores and benefit plants.
Yellow crazy ants (Anoplolepis gracilipes) have invaded eucalypt woodlands of Arnhem
Land, northern Australia, where they displace the native dominant weaver ant (Oecophylla
smaragdina). We compared the plant defense services provided by weaver ants and yellow
crazy ants on trees with (Acacia lamprocarpa) and without (Eucalyptus tetrodonta)
extrafloral nectar rewards through surrogate herbivore (termite) addition experiments and
surveys of herbivore damage. Yellow crazy ants were more likely than weaver ants to
discover termites on A. lamprocarpa, but the likelihood of termite discovery on E. tetrodonta
did not vary with ant species. Yellow crazy ants were also more thorough in their attacks of
termites, recruited 3.4-4 times more workers to termites, and were 3.4 times quicker at
discovering termites on A. lamprocarpa than were weaver ants. Discovery of termites by
other predators did not vary significantly between trees in yellow crazy and weaver ant sites.
Repeating the experiment on the same trees resulted in greater discovery of termites at the
locations where they had previously been discovered. Herbivory scores did not reflect the
foliage patrolling pattern by the ants. Old A. lamprocarpa leaves and both new and old leaves
and branches on E. tetrodonta in yellow crazy ant sites had higher chewing herbivory scores
than their counterparts in weaver ant sites, but there were no differences between yellow
crazy ant and weaver ant sites in the total branch scores or chewing scores on new A.
lamprocarpa leaves. Our results reveal that the more aggressive and efficient foliar patrolling
by yellow crazy ants does not translate to increased plant protection.
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Introduction
The magnitude of benefits acquired by a partner in a mutualistic interaction is dependent on
multiple factors, including the characteristics of the other partner (Stadler and Dixon 2008).
For example, the facultative mutualism between plants and ants, in which ants are often
attracted to the plant by the presence of extrafloral nectaries (EFNs), is on balance beneficial
for plants (Chamberlain and Holland 2009, Rosumek, et al. 2009), but the level of benefit
varies greatly depending on which ant species are attracted to the plant and their relative
ability to displace harmful herbivores (Cronin 1998, Ness, et al. 2006). Ant attraction
depends on the quality and quantity of available nectar (Blüthgen and Fiedler 2004), and
plants can optimize their investment in nectar by varying its availability temporally and
spatially to coincide with susceptibility to herbivory (Heil, et al. 2000). Whether attracted
ants will displace herbivores will further depend on their aggression, abundance, and ability
to rapidly recruit other colony members (Di Giusto, et al. 2001, Ness, et al. 2010).
Abundance, aggression, and the ability to rapidly recruit colony members are common traits
among several invasive ant species (Krushelnycky, et al. 2010). As with other invasive
species, invasive ants can create novel or disrupt existing interactions within the ecological
community with the potential for changes in ecosystem functioning to ensue (e.g., O'Dowd,
et al. 2003). Invasive ants often displace a large proportion of native ant assemblages, thereby
disrupting mutualisms native ants may have had with plants and prompting the question: are
invasive ants also effective plant mutualists? (Lach 2003). If abundance, aggression, and
attraction to carbohydrate-rich resources are key attributes of plant guards, then we would
expect the invaders to be particularly effective plant mutualists. To date, benefits to
facultatively ant-associated plants from native ants and invasive ants in their introduced range
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have been compared in only two published studies. Both focus on the red imported fire ant,
Solenopsis invicta and their results are inconsistent; S. invicta provides greater protection
from herbivores on some plant species (Ness 2003, Stiles and Jones 2001) but it has a net
negative effect on others (Stiles and Jones 2001). Other invasive ant species are known to
visit EFNs in some contexts (Anoplolepis gracilipes: Lester and Tavite 2004, Savage, et al.
2009; Linepithema humile: Freitas, et al. 2000, Lach, et al. 2009; Pheidole megacephala:
Lach, et al. 2009; Technomyrmex albipes, Lach, et al. 2010) but it is unknown how their
herbivore-deterring abilities compare to native ants. The high densities often achieved by
invasive ants may translate to higher visitation rates to EFNs and better protection against
herbivores.
The provision of extrafloral nectar by plants and biotic defense by native or invasive ants are
not always tightly linked. For example, ants that are highly predaceous and have a high
foraging efficiency, such as the red imported fire ant, Solenopsis invicta, may displace
herbivores in the absence of plant-based rewards (Lach and Hooper-Bùi 2010) or without
consuming those that are offered (Ness 2003). Thus, though dominant ants, native or
otherwise, are attracted to and fuelled by carbohydrate-rich resources on plants (Blüthgen and
Fiedler 2004, Davidson 1998, Krushelnycky, et al. 2010), their foraging patterns also reflect
the availability of prey and their colony needs at the time of foraging (Ness 2003). Herbivore
deterrence can also be a result of territorial behavior by dominant ants. Arboreal ants, such as
weaver ants (Oecophylla spp.), are highly effective at deterring would-be herbivores in trees
in which they nest (Peng and Christian 2005), even if those trees do not have carbohydraterich resources. Pheromones produced by the weaver ants, rather than direct predation, may be
the predominant deterrent to herbivores in Oecophylla territories (Offenberg, et al. 2004).
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The native dominant weaver ant, Oecophylla smaragdina, is among the native ants that
become outnumbered and displaced by invasive yellow crazy ants, Anoplolepis gracilipes, in
intact Eucalyptus woodland in northern Australia (Hoffmann and Saul 2010). Yellow crazy
ants are aggressive, predaceous, and often very abundant ants that have invaded numerous
tropical and subtropical oceanic islands, continental Asia and parts of northern Australia
(Hoffmann and Saul 2010, Wetterer 2005). The ant is capable of displacing other ants from
EFNs (Savage, et al. 2009) and when conditions are favourable for arboreal foraging, it can
be an effective predator of herbivores in agroecosystems (Lach 2003 and references therein).
A. gracilipes are perhaps most notorious for their interactions with multiple trophic levels and
ensuing dramatic ecosystem changes on Christmas Island (O'Dowd, et al. 2003).
We undertook this study to answer the questions: 1) does invasion by A. gracilipes and
subsequent displacement of O. smaragdina result in higher rates of foliage patrolling? and 2)
if so, does this lead to greater plant protection? We answered these questions through
surrogate herbivore placement experiments and surveys of herbivore damage on trees that
had only been exposed to one or the other dominant ant species. To determine the effect of
carbohydrate resources on ant patrolling behavior, we conducted these experiments and
observations on a plant that offers an extrafloral nectar reward (Acacia lamprocarpa) and a
paired plant that does not (Eucalyptus tetrodonta), and varied the placement of the surrogate
herbivore on the plants. We hypothesized that the greater abundance of the A. gracilipes
would result in greater foliage patrolling and anti-herbivore defense of the reward-offering
plant but that A. gracilipes would be no more likely than the native O. smaragdina to patrol
or defend the plant that did not offer a reward.
Methods
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Habitat and species descriptions
We conducted the study in Eucalyptus savanna woodlands in northeast Arnhem Land in
Australia’s Northern Territory. The regional climate is tropical monsoonal with high
temperatures (17-330C) throughout the year and an annual rainfall of approximately 1200 mm
falling predominantly during the summer wet season from December to July. The woodlands
have an open canopy (20% cover; Williams, et al. 1996) with an overstorey dominated by
eucalypts, predominantly E. tetrodonta, to heights of approximately 15 m, and an understorey
of predominantly acacias, especially A. lamprocarpa and A. leptocarpa, as well as grasses.
Fires occur annually to bi-annually within this region. The regularity of fires ensured that
experimental trees at each site were of identical age and therefore had had similar potential
exposure to ants and herbivores.
We selected saplings of the two experimental plant species, E. tetrodonta and A. lamprocarpa
because of their dominance in this system and their similarity in height and form and because
one of them has EFNs and the other does not. Phyllodes of A. lamprocarpa (hereafter called
leaves for simplicity) have a single EFN located at the base of each petiole, with those EFNs
on the newest growth at the extremities of the branches or stem attracting the most visits by
ants (L. Lach, B. Hoffmann, pers. obs.), most likely because they produce the most nectar
(sensu Heil, et al. 2000). In contrast, as with other eucalypts, E. tetrodonta does not have
EFNs. Eucalypts instead employ constitutive means of defense such as sclerophylly and
surface waxes (Ohmart and Edwards 1991).
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The two ant species we compared, the yellow crazy ant, Anoplolepis gracilipes, and the
weaver ant, Oecophylla smaragdina, are considered dominant species and have mutually
exclusive distributions in these woodlands and elsewhere they have been studied (Greenslade
1971, Hoffmann and Saul 2010, Majer 1993). Both species readily survive the regional highfrequency fire regime. A. gracilipes has been present within northern Australia for at least 30
years and has spread to at least 50 locations throughout a 25,000 ha region (Hoffmann and
Saul 2010). Tuna baits placed on the ground at our sites regularly attracted > 100 A.
gracilipes within 5 minutes (Lach and Hoffmann, unpublished data). In this region, A.
gracilipes nest almost exclusively on the ground, but readily forage in trees. The native O.
smaragdina is widespread throughout northern Australia’s Eucalyptus woodlands and nests
and forages arboreally (Crozier, et al. 2009).
Little is known about specific folivores in these Eucalyptus woodlands, as is true for most
savanna-type habitats in Australia (Andersen and Lonsdale 1990). Chewing folivores likely
include lepidopteran and sawfly larvae, and chrysomelid beetles (Andersen and Lonsdale
1990, Ohmart and Edwards 1991), and folivory is likely to peak in the wet season between
October and April (Andersen and Lonsdale 1990). Newly flushed foliage is high in nitrogen
and is often most vulnerable to folivory (Ohmart and Edwards 1991).
Experiments
2006: Foliage patrolling
In May 2006 we selected three sites invaded by A. gracilipes (GPS -12.353S 136.816E, 12.365S 136.766E, -12.353S 136.748E) and two sites without A. gracilipes that were
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dominated by O. smaragdina (GPS -12.380°S 136.818°E, -12.391°S, 136.820°E). All of the
sites had been burned in the previous fire season (September-December). All sites were
greater than 600 m apart, and the closest distance between an A. gracilipes site and an O.
smaragdina site was 1100 m. At each site we selected up to 12 pairs of A. lamprocarpa and
E. tetrodonta saplings that were approximately the same size (pairs were 1.0-1.7m tall) and
within 5 m of each other. We chose trees in which the focal ant species was seen foraging in
the near vicinity and that lacked sap-sucking insects to ensure that there was no other
carbohydrate-rich attractant on the tree. In O. smaragdina sites, we also chose trees without
O. smaragdina nests so that both ant species would have to travel the same distance to reach
the surrogate herbivore; nearest O. smaragdina nests were usually not more than 1-2m away.
The majority of trees in O. smaragdina sites lack nests (LL and BH pers. obs.) and therefore
the behavior of O. smaragdina on trees without nests is more representative of their behavior
at the site. At two A. gracilipes sites we found only five and six pairs of trees to meet these
criteria, resulting in 23 pairs of trees across the three invaded sites, and 22 pairs in two noninvaded sites.
On each tree we selected two opposite leaves near new growth at the top and two near the
bottom of the tree. A single live termite (Amitermes viteosus) was glued to each of the
selected A. lamprocarpa leaves so that it was within 2 cm of the EFN. On the selected E.
tetrodonta leaves, termites were placed within 2 cm of the petiole. The mean distance from
the ground to the termites did not significantly differ between A. gracilipes and O.
smaragdina sites for either tree species for either upper (meanAl-Ag = 132.2 cm SD = 33.,
meanAl-Os = 138.5 cm SD = 28.9; meanEt-Ag = 132.3 cm SD= 22.1, mean Et-Os = 128.9 cm
SD= 23.5) or lower termites (meanAl-Ag. = 80.4 cm SD = 30.4, meanAl-Os = 73.9 cm SD = 22.9;
meanEt-Ag = 73.4 cm SD= 22.4, mean Et-Os = 75.7 cm SD= 29.4). Termites were glued dorsal
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side down with a non-toxic, quick-drying adhesive that did not deter ants. To ensure
consistency in placement, all termites were placed by the same person (LL). Termites have
previously been used as a readily available, abundant surrogate herbivore to compare ant
patrolling on plants (e.g., Oliveira 1997). Placing multiple termites allowed us to compare
how many termites were discovered and attacked by the two ant species, hereafter referred to
as thoroughness of attack.
We observed each termite every 10 minutes for 90 minutes and recorded presence or absence
along with the number and species of ants present, and the presence of any other predator. If
neither the termite nor ants were present, we searched the tree to determine what had taken
the termite. If the termite disappeared within 10 minutes and no ants were present on the tree,
the termite was replaced. Termites that disappeared after 10 minutes of observation and in
which no ant or other predator was seen were considered to have an unknown fate. All
observations were done between 0700 and 1700.
We compared foliage patrolling between A. gracilipes and O. smaragdina with three types of
response variables from this experiment: likelihood of attack, thoroughness of attack, and
recruitment. Likelihood of attack refers to the odds that a termite would be attacked by a
focal ant (A. gracilipes in invaded sites, or O. smaragdina in non-invaded sites).
Thoroughness of attack comprised two different comparisons: likelihood that both of the
lower or both of the upper termites would be attacked if at least one of the pair had been
attacked and likelihood that all four termites would be attacked if at least one had been
attacked. We modelled the binomial attack/no attack variables with generalized linear models
using the Genmod procedure in SAS v 9.2. Genmod uses a logit transformation and corrects
for any covariance from the multiple observations on the same tree and trees at the same site.
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We used tree species, ant species, termite position (upper or lower) and their interactions in
the model and back-transformed the least-square means (and their standard errors) from these
analyses to determine the odds ratios. Finally, we used Mann-Whitney U tests to compare the
mean maximum number of ants recruited by each ant species to termites on A. lamprocarpa
that were not removed, including only those in which the termite was discovered in the first
60 minutes so that the ants would have had at least 30 minutes to recruit. Recruitment to
termites on E. tetrodonta was too infrequent to analyze.
The likelihood of attack by predators other than the focal ants (i.e., spiders, other ant species)
was analyzed with the same generalized linear model as likelihood of attack by the focal ants.
2007: Foliage patrolling and herbivory
In June 2007, we selected two additional pairs of sites (GPS -12.336°S 136.852°E, -12.270°S
136.793°E). Sites used in 2006 were unavailable due to ant control efforts. The invaded and
non-invaded paired sites were 50 m and 400 m from each other, and the two pairs were
approximately 7 km apart. All four sites had been burned the previous year. At each of the
four sites, we selected 20 pairs of A. lamprocarpa and E. tetrodonta, and the 2006
Experiment 1 was repeated as described above except that the trees were examined every 20
minutes for 60 minutes. The mean distance from the ground to the termites did not differ
significantly between A. gracilipes and O. smaragdina sites for either tree species for either
upper (meanAl-Ag = 143.4 cm SD = 26.8, meanAl-Os = 152.3 cm SD = 30.9; mean Et-Ag = 139.9
SD= 29.4, mean Et-Os = 140.4 SD= 30.5) or lower termites (meanAl-Ag = 83.9 cm SD = 23.5,
meanAl-Os = 72.3 cm SD = 21.6; mean Et-Ag = 75.5 cm SD= 28.3, mean Et-Os = 68.0 cm SD=
21.1). We obtained odds ratios for likelihood of attack and thoroughness of attack by ants and
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the likelihood of attack by other predators by modelling the attack/no attack binomial
variables with Genmod as described above.
To determine whether the two ant species differed in the speed at which an herbivore is
discovered, in June 2007 at the two pairs of sites above and at a third pair of sites (GPS - 12.373S 136.830E) we selected an additional 20 A. lamprocarpa trees of similar size. We
glued one termite to a leaf on the lower to middle part of each tree and watched continuously
for 15 minutes (900 seconds) until an ant or other predator attacked it. As in 2006, all
observations were made between 0700 and 1700. Mean distance from the ground to the
termite did not significantly differ between A. gracilipes sites (mean = 78.4 cm SD=18.9) and
O. smaragdina sites (mean = 84.0 cm SD=15.6). For statistical analysis, we modelled speed
of attack as 900 minus the number of seconds it took for the termite to be discovered with a
generalized linear model using the GLMMIX macro in SAS v.9.2. GLMMIX allows for an
overdispersed Poisson distribution with the use of a log-link function and corrects for any
covariance from multiple observations at the same site. We back-transformed the least-square
means and standard errors for presentation in Results.
We compared herbivory on the 60 A. lamprocarpa and 40 E. tetrodonta trees at our three
pairs of invaded and non-invaded sites. We arbitrarily selected three branches from different
heights and protruding from different directions on each experimental tree. On each of the
three branches the ten most terminal leaves were examined and the number with any
herbivore damage was recorded. The third (new) and ninth (old) leaves from the terminus
were collected and scored in the lab for percent area missing due to chewing damage (0=0%,
1=1-10%, 2=11-25%, 3=26-50%, 4=51-75%, 5=76-100%) as well as mining and gall
presence or absence. To ensure consistency, all damage assessments were conducted by the
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same person (BH) and were conducted after the surrogate herbivore addition experiment
above. We added the categorical scores for the three branches, as well as for the third and
ninth leaves for each tree and compared these mean totals for A. gracilipes and O.
smaragdina sites with Mann-Whitney U tests.
RESULTS
Likelihood, speed, and thoroughness of attack
In 2006, A. gracilipes attacked at least one termite on 82.6% and 73.9% of A. lamprocarpa
and E. tetrodonta, respectively. O. smaragdina attacked at least one termite on 77.3% of A.
lamprocarpa and on 45.5% of E. tetrodonta. The likelihood that an individual termite would
be attacked by the focal ant were explained by the focal ant species, the tree species, the
position of the termite on the tree, and their three-way interaction (Fig. 1a, Table 1:
Experiment 1:1). On A. lamprocarpa likelihood of discovery by a focal ant was greater in A.
gracilipes sites and for the lower termites; on E. tetrodonta lower termites were more likely
than upper termites to be attacked by either focal ant, but the difference was especially great
in A. gracilipes sites (Fig. 1a, Table 1: Experiment 1: 2, 3).
A. gracilipes were more thorough in their attacks, being more likely to attack both of the
lower, both of the upper, and all four of the termites on A. lamprocarpa than O. smaragdina
(Fig. 1b, Table 1: Experiment 1: 4, 5). They were also more likely to attack all four termites
on E. tetrodonta, though this did not remain statistically significant after Bonferroni
correction (Fig. 1b). A. gracilipes also recruited more workers to A. lamprocarpa termites
than did O. smaragdina (Table 2).
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Results in 2007 were similar. Termites were more likely to be attacked by the focal ant if they
were in A. gracilipes sites and lower on the tree, but attack likelihood only marginally
significantly differed between tree species (Fig. 2, Table 3:1). A. gracilipes were more likely
than O. smaragdina to attack termites on lower and upper A. lamprocarpa leaves, though
only the lower comparison remained statistically significant after Bonferroni correction (Fig.
2). Unlike 2006,, thoroughness of attack was not explained by ant species, tree species, or
termite position (Table 3: 2,3). Termites on A. lamprocarpa were discovered more quickly by
A. gracilipes than by O. smaragdina (mean number of seconds under attack: Ag = 73.6 ±
20.7, Os =22.0 ± 12.6, Table 3:4). Speed of attack was not explained by the distance of the
termite from the ground (Table 3:4).
Discovery by other predators
In both years, attack by predators other than the focal ant species (spiders, other ant species)
was rare, occurring on 2.5-5.0% of termites on A. lamprocarpa trees and on 2.8-6.1% of
termites on E. tetrodonta trees. Predation by other predators did not differ significantly with
focal ant or tree species in 2006 (data not shown, all p >0.05). In 2007, termites on A.
lamprocarpa were slightly more likely to be attacked by predators other than the focal ants
than termites on E. tetrodonta (odds of attack Al. = 1.06 ± 0.009, odds of attack Et. = 1.02 ±
0.016) regardless of whether they were in invaded sites or where they were on the tree (Table
3:5).
Herbivory
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Old A. lamprocarpa leaves and both new and old leaves and branches on E. tetrodonta in A.
gracilipes sites had higher chewing herbivory scores than their counterparts in O. smaragdina
sites (Fig. 4). There were no differences between A. gracilipes and O. smaragdina sites in the
total branch scores or chewing scores on new A. lamprocarpa leaves, or for mining or galling
scores on either tree species for either new or old leaves.
DISCUSSION
Our hypothesis that A. gracilipes would be more effective foliage patrollers on A.
lamprocarpa (with EFNs) than O. smaragdina was supported in both years of the study, with
A. gracilipes ants being more likely to attack termites in both the upper and lower positions
on the tree. In 2006, A. gracilipes were also more likely to be more thorough in their attacks
than O. smaragdina (this was not confirmed in 2007, possibly because of the shorter
observation period). A. gracilipes also recruited more workers to attacked termites and were
faster at finding the termites on A. lamprocarpa than O. smaragdina . As expected, there
were few differences between yellow crazy ants and weaver ants in the likelihood and
thoroughness of attacks on termites on the non-reward offering E. tetrodonta.
Our inclusion of different tree species and different locations on the tree allowed us to test the
effect of extrafloral nectar and foraging distance (cost) on rates of foliage patrolling by the
ants. Termites placed on the upper leaves of A. lamprocarpa were near to an extrafloral
nectar reward, whereas those on E. tetrodonta were not, but new foliage of both species is
more likely to have herbivore prey (Ohmart and Edwards 1991). A. gracilipes were equally
likely to attack upper and lower termites on A. lamprocarpa, but not on E. tetrodonta,
suggesting that the extra cost in foraging (the extra ~70 cm travelled) is compensated for by
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the nectar reward, not by the greater likelihood of finding herbivore prey near new foliage. In
contrast, O. smaragdina were less likely to attack upper termites than lower termites on A.
lamprocarpa indicating that their foraging is not as influenced by the nectar reward. Our
findings differ from studies in Brazil with native ants (primarily Camponotus spp.) and
extrafloral nectaried plants that have found no difference in termite discovery on different
aged leaves (Oliveira, et al. 1987) or greater likelihood of discovery of termites placed on
apical leaves than lower leaves (Oliveira 1997).
Herbivory scores did not reflect the foliage patrolling pattern by the ants. Galling and mining
scores did not differ across sites; herbivores that consume internal plant tissues are largely
protected from predation by ants (but see Altfeld and Stiling 2009). However, chewing scores
for old A. lamprocarpa leaves were greater for trees in A. gracilipes sites than in O.
smaragdina sites, and chewing scores for both old and new E. tetrodonta as well as overall
branch herbivory were greater in yellow crazy ant sites than in weaver ant sites. Moreover,
termites were no more likely to be attacked by other predators in O. smaragdina than in A.
gracilipes sites. Thus, O. smaragdina appear to be associated with decreased herbivory
despite not being as effective as A. gracilipes at foliage patrolling. Both ant species have
previously been reported as being effective at controlling plant pests and reducing herbivory
(Peng and Christian 2010, Way and Khoo 1992), although the behavior and effects of A.
gracilipes appears context-dependent, perhaps due to characteristics of the herbivores or the
attraction of the plant (Lach 2003, Peng and Christian 2010). A. gracilipes attracted to trees
by honeydew-producing insects on Bird Island (Seychelles) were associated with increased
insect chewing damage on one tree species, but decreased damage on another (Hill, et al.
2003). Relationships between folivory and other invasive ant species attracted to facultatively
ant-associated plants by insects or nectar are similarly equivocal with some studies showing
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declines in folivores or folivory on plants with invasive ants relative to no ants or other ants
(Linepithema humile: Koptur 1979; Technomyrmex albipes: Lach, et al. 2010; Solenopsis
invicta: Ness 2003, Stiles and Jones 2001, Styrsky, et al. 2006) and others showing increases
(Pheidole megacephala: Djiéto-Lordon, et al. 2004; Solenopsis invicta: Styrsky, et al. 2006).
Thus our hypothesis that A. gracilipes would be superior plant mutualists for A. lamprocarpa
was not supported. Our findings are consistent with the pheromone avoidance hypothesis,
which posits that some herbivores (e.g., leaf beetles and fruit flies) have evolved traits to
detect and avoid ant pheromones, such as those deposited by O. smaragdina to mark their
territory (Offenberg, et al. 2004). These pheromones are deposited throughout the territory
(Offenberg 2007), long-lasting (9 weeks to 10 months if reinforced), and resistant to rain
(Beugnon and Dejean 1992) and therefore regular patrolling of foliage would not be
necessary to effect plant defense. Inherent to this hypothesis is the shared evolutionary
history between O. smaragdina and herbivores at our sites, which would have allowed
sufficient time for the evolution of pheromone detection and avoidance among herbivores. A.
gracilipes are not known to produce territorial pheromones nor has there likely been enough
time since their introduction for the herbivores to evolve any similar short-cuts to avoid the
invasive yellow crazy ant. That the plants produce volatile organic compounds in response to
herbivory to which the native O. smaragdina are primed to respond (as has been shown with
myrmecophytic plants and their resident ants (e.g., Bruna, et al. 2008) is also possible, but not
likely considering the relatively large spatial distances between the experimental plants and
weaver ants on other trees.
It is also possible that the foliage patrolling activity that we measured is not indicative of ant
activity at the time of peak herbivory or of responses to real herbivores. Despite the many
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hours we spent observing our experimental trees, we observed only one herbivore, a
lepidopteran larva. However, we did observe evidence of chewing herbivory on new leaves,
so we suspect that most chewing herbivores are nocturnally active. A decrease in A.
gracilipes activity at nightfall combined with nocturnally active herbivores would also
explain the higher chewing scores in A. gracilipes sites. However, in northeast Arnhem Land,
A. gracilipes are as or more active at night than during the day (Hoffmann, unpublished data)
so we consider this an unlikely explanation of our results. Alternatively, the observed anttermite interactions may not be representative of ant-herbivore interactions. Surrogate
herbivores have proven useful in other studies of ant-plant interactions (Cronin 1998, Ness, et
al. 2006, Oliveira 1997). We used termites as surrogate herbivores in our experiments
because they were readily available in large quantities and provided a means of standardizing
treatments across many replicates. The real herbivores likely have mechanisms for escape
(e.g., dropping, flying away) in response to the approach of aggressive ants. Even so,
displaced herbivores are no longer feeding and therefore foliage patrolling behaviour of the
ants as measured by attacks of ants on termites is likely indicative of some level of plant
protection.
Our study did not test for any site-level differences in herbivore populations and these might
also contribute to our observed differences in chewing damage. We are very confident that
our yellow crazy and weaver ant sites were practically identical in all readily observable
characteristics that might affect herbivore populations (e.g., plant species composition, plant
density, abiotic conditions) except their ant fauna. The ants, however, likely have site-level
effects on the arthropod communities, including both herbivores and other natural enemies of
herbivores. Weaver ants’ aggressive defense of their territories (Crozier, et al. 2009) may
leave few opportunities for herbivores to feed, mate, oviposit, and develop undisturbed. A.
17
gracilipes are also capable of displacing insects and other invertebrates, and they have been
associated with declines of herbivores and scavengers including hermit crabs (McNatty, et al.
2009), red land crabs (O'Dowd, et al. 2003) amphipods, isopods and chewing insects (Hill, et
al. 2003). Aside from their ability to displace other ants (e.g., Gerlach 2004, Savage, et al.
2009), their effects on other arthropod predators are undocumented. Our findings of reduced
chewing herbivory in O. smaragdina sites, could be explained by either lower populations of
chewing herbivores in O. smaragdina sites or lower populations of natural enemies of
herbivores in yellow crazy ant sites.
Conclusion
To date, A. gracilipes have been studied almost exclusively in island ecosystems that have
relatively depauperate and poorly competitive native ant faunas (e.g., Lester and Tavite 2004,
O'Dowd, et al. 2003, Savage, et al. 2009), thereby providing little opportunity for comparison
of their behaviors and interactions to native dominant ants. Moreover, their interactions with
plants are also relatively unstudied (but see O'Dowd, et al. 2003, Savage, et al. 2009). Here
we compared A. gracilipes to the native dominant O. smaragdina they displace and found
that A. gracilipes are better foliage patrollers on the tree that offers a reward, but not on the
tree that does not, yet for both tree species, O. smaragdina appear to be the better plant
defender. If the increased herbivory of plants in A. gracilipes sites translates to diminished
plant growth or fitness, displacement of O. smaragdina is only one of many changes that
occur as a result of A. gracilipes invasion.
Acknowledgements
18
We thank John Edgar, Daryl Lacey and Arian Pearson for field assistance. Funding was
provided by the Australian National Heritage Trust. LL was supported by the Australian
Research Council. Comments by A. Andersen, J. Offenberg, and T. Evans improved the
manuscript.
References
Altfeld, L. and Stiling, P. 2009. Effects of aphid-tending Argentine ants, nitrogen enrichment
and early-season herbivory on insects hosted by a coastal shrub. - Biol Invasions 11: 183191.
Andersen, A. N. and Lonsdale, W. M. 1990. Herbivory by insects in Australian tropical
savannas: a review. - J Biogeog 17: 433-444.
Beugnon, G. and Dejean, A. 1992. Adaptative properties of the chemical trail system of the
African weaver ant Oecophylla longinoda Latreille (Hymenoptera, Formicidae,
Formicinae). - Ins Soc 39: 341-346.
Blüthgen, N. and Fiedler, K. 2004. Preferences for sugars and amino acids and their
conditionality in a diverse nectar-feeding ant community. - J Anim Ecol 73: 155-166.
Bruna, E. M., Darrigo, M. R., Pacheco, A. M. F. and Vasconcelos, H. L. 2008. Interspecific
variation in the defensive responses of ant mutualists to plant volatiles. - Biol J Linn Soc
94: 241-249.
Chamberlain, S. A. and Holland, J. N. 2009. Quantitative synthesis of context dependency in
ant–plant protection mutualisms. - Ecology 90: 2384-2392.
Cronin, G. 1998. Between-species and temporal variation in Acacia-ant-herbivore
interactions. - Biotropica 30: 135-139.
Crozier, R. H., Newey, P. S., Schlüns, E. A. and Robson, S. K. A. 2009. A masterpiece of
evolution--Oecophylla weaver ants (Hymenoptera: Formicidae). - Myrm News 13: 57-71.
19
Davidson, D. W. 1998. Resource discovery versus resource domination in ants: a functional
mechanism for breaking the trade-off. - Ecol Entom 23: 484-490.
Di Giusto, B., Anstett, M.-C., Dounias, E. and McKey, D. B. 2001. Variation in the
effectiveness of biotic defence: the case of an opportunistic ant-plant protection
mutualism. - Oecologia 129: 367-375.
Djiéto-Lordon, C., Dejean, A., Gibernau, M., Hossaert-McKey, M. and McKey, D. 2004.
Symbiotic mutualism with a community of opportunistic ants: protection, competition,
and ant occupancy of the myrmecophyte Barteria nigritana (Passifloraceae). - Acta
Oecologia 26: 109-116.
Freitas, L., Galetto, L., Bernardello, G. and Paoli, A. A. S. 2000. Ant exclusion and
reproduction of Croton sarcopetalus (Euphorbiaceae). - Flora 195: 398-402.
Gerlach, J. 2004. Impact of the invasive crazy ant Anoplolepis gracilipes on Bird Island,
Seychelles. - J Insect Cons 8: 15-25.
Greenslade, P. J. M. 1971. Interspecific competition and frequency changes among ants in
Solomon Islands coconut plantations. - J Appl Ecol 8: 323-352.
Heil, M., Fiala, B., Baumann, B. and Linsenmair, K. E. 2000. Temporal, spatial and biotic
variations in extrafloral nectar secretion by Macaranga tanarius. - Func Ecol 14: 749757.
Hill, M., Holm, K., Vel, T., Shah, N.-J. and Matyot, P. 2003. Impact of the introduced yellow
crazy ant Anoplolepis gracilipes on Bird Island, Seychelles. - Biodiv Cons 12: 19691984.
Hoffmann, B. D. and Saul, W.-C. 2010. Yellow crazy ant (Anoplolepis gracilipes) invasions
within undisturbed mainland Australian habitats: no support for biotic resistance
hypothesis.- Biol Invasions 10.1007/s10530-010-9701-3.
20
Koptur, S. 1979. Facultative mutualism between weedy vetches bearing extrafloral nectaries
and weedy ants in California. - Am J Bot 66: 1016-1020.
Krushelnycky, P. D., Holway, D. A. and LeBrun, E. G. 2010. Invasion processes and causes
of success. - In: Lach, L., Parr, C. L. and Abbott, K. L. (eds.), Ant Ecology. Oxford
University Press, pp. 245-260.
Lach, L. 2003. Invasive ants: Unwanted partners in ant-plant interactions? - Ann Miss Bot
Gard 90: 91-108.
Lach, L., Hobbs, R. J. and Majer, J. D. 2009. Herbivory-induced extrafloral nectar increases
native and invasive ant worker survival. - Pop Ecol.
Lach, L. and Hooper-Bùi, L. M. 2010. Consequences of ant invasions. - In: Lach, L., Parr, C.
L. and Abbott, K. L. (eds.), Ant Ecology. Oxford University Press, pp. 261-286.
Lach, L., Tillberg, C. V. and Suarez, A. V. 2010. Contrasting effects of an invasive ant on a
native and an invasive plant.- Biol Invasions 10.1007/s10530-010-9703-1.
Lester, P. J. and Tavite, A. 2004. Long-legged ants, Anoplolepis gracilipes (Hymenoptera:
Formicidae), have invaded Tokelau, changing composition and dynamics of ant and
invertebrate communities. - Pac Sci 58: 391-401.
Majer, J. D. 1993. Comparison of the arboreal ant mosaic in Ghana, Brazil, Papua New
Guinea and Australia--its structure and influence on arthropod diversity. - In: LaSalle, J.
and Gauld, I. D. (eds.), Hymenoptera and Biodiversity. C-A-B International, pp. 115-141.
McNatty, A., Abbott, K. L. and Lester, P. J. 2009. Invasive ants compete with and modify the
trophic ecology of hermit crabs on tropical islands. - Oecologia 160: 187-194.
Ness, J., Mooney, K. and Lach, L. 2010. Ants as mutualists. - In: Lach, L., Parr, C. L. and
Abbott, K. L. (eds.), Ant Ecology. Oxford University Press, pp. 97-114.
Ness, J. H. 2003. Contrasting exotic Solenopsis invicta and native Forelius pruinosus ants as
mutualists with Catalpa bignonioides, a native plant. - Ecol Entom 28: 247-251.
21
Ness, J. H., Morris, W. F. and Bronstein, J. L. 2006. Integrating quality and quantity of
mutualistic service to contrast ant species protecting Ferocactus wislizeni. - Ecology 87:
912-921.
O'Dowd, D. J., Green, P. T. and Lake, P. S. 2003. Invasional 'meltdown' on an oceanic island.
- Ecol Lett 6: 812-817.
Offenberg, J. 2007. The distribution of weaver ant pheromones on host trees. - Ins Soc 54:
248-250.
Offenberg, J., Nielsen, M. G., MacIntosh, D. J., Havanon, S. and Aksornkoae, S. 2004.
Evidence that insect herbivores are deterred by ant pheromones. - Proc Roy Soc Lond B
271: S433-S435.
Ohmart, C. P. and Edwards, P. B. 1991. Insect herbivory on eucalyptus. - Annu Rev Entom
36: 637-657.
Oliveira, P. S. 1997. The ecological function of extrafloral nectaries: Herbivore deterrence by
visiting ants and reproductive output in Caryocar brasiliense (Caryocaraceae). - Func
Ecol 11: 323-330.
Oliveira, P. S., Silva, A. F. d. and Martins, A. B. 1987. Ant foraging on extrafloral nectaries
of Qualea grandiflora (Vochysiaceae) in cerrado vegetation: ants as potential
antiherbivore agents. - Oecologia 74: 228-230.
Peng, R. K. and Christian, K. 2005. Integrated pest management in mango orchards in the
Northern Territory Australia, using the weaver ant, Oecophylla smaragdina,
(Hymenoptera: Formicidae) as a key element. - Int J Pest Mgmt 51: 149-155.
Peng, R. K. and Christian, K. 2010. Ants as biological-control agents in the horticultural
industry. - In: Lach, L., Parr, C. L. and Abbott, K. L. (eds.). Oxford University Press, pp.
123-125.
22
Rosumek, F., Silveira, F., de S. Neves, F., de U. Barbosa, N., Diniz, L., Oki, Y., Pezzini, F.,
Fernandes, G. and Cornelissen, T. 2009. Ants on plants: a meta-analysis of the role of
ants as plant biotic defenses. - Oecologia 160: 537-549.
Savage, A. M., Rudgers, J. A. and Whitney, K. D. 2009. Elevated dominance of extrafloral
nectary-bearing plants is associated with increased abundances of an invasive ant and
reduced native ant richness. - Div Distr 15: 751-761.
Stadler, B. and Dixon, T. 2008. Mutualism: Ants and their insect partners -Cambridge
University Press.
Stiles, J. H. and Jones, R. H. 2001. Top-down control by the red imported fire ant (Solenopsis
invicta). - Am Midl Nat 146: 171-185.
Styrsky, J. D., Kaplan, I. and Eubanks, M. D. 2006. Plant trichomes indirectly enhance
tritrophic interactions involving a generalist predator, the red imported fire ant. - Biol
Contr 36: 375-284.
Way, M. J. and Khoo, K. C. 1992. Role of ants in pest management. - Annu Rev Entom 37:
479-503.
Wetterer, J. K. 2005. Worldwide distribution and potential spread of the long-legged ant,
Anoplolepis gracilipes (Hymenoptera: Formicidae). - Sociobiol 45: 1-21.
Williams, R. J., Duff, G. A., Bowman, D. M. J. S. and Cook, G. D. 1996. Variation in the
composition and structure of tropical savannas as a function of rainfall and soil texture
along a large scale climatic gradient in the Northern Territory, Australia. - J Biogeog 23:
747-756.
23
Figure 1. a) Odds (± SE) of the invasive yellow crazy ants (shaded bars) and native weaver
ants (open bars) attacking a termite in 2006 for each tree species and termite position on the
tree. b) Odds (± SE) of the invasive yellow crazy ants and native weaver ants attacking either
both termites on the lower or upper part of the tree or all termites on the tree in 2006 for each
tree species and the position of the termites on the tree. * denotes comparisons between
yellow crazy ants and weaver ants with p <0.05; ** indicates p less than the Bonferroni
corrected alpha of 0.0125 for the four inter-specific comparisons of interest in a) and p
<0.008 for the six comparisons in b). Lines with p-values indicate significant differences for
within species comparisons of position effect (Bonferroni alpha = 0.0125). There were no
significant intraspecies comparisons by position for (b). See Table 1: 1-5 for results of the
analyses of variance.
Figure 2. Odds (± SE) of the invasive yellow crazy ants (shaded bars) and native weaver ants
(open bars) attacking a termite by termite position and tree species in 2007 (see Table 3:1 for
analyses of variance). * denotes comparisons between yellow crazy ants and weaver ants with
p <0.05; ** indicates p less than the Bonferroni corrected alpha of 0.0125 for the four interant species comparisons of interest. Lines with p-values indicate significant differences for
within species comparisons of position effect (Bonferroni alpha = 0.0125).
Figure 3. Boxplots of the median summed herbivory scores for a) A. lamprocarpa and b) E.
tetrodonta trees in A. gracilipes (shaded bars) and O. smaragdina (open bars) sites. For the
chewing, mining and galling categories, the patterned bars indicate damage on the older
leaves and the unpatterned bars indicate damage on newer leaves. Closed circles represent the
5th and 95th percentiles. P-values are shown for significant inter-species comparisons using
the Mann-Whitney U test.
24
a)
b)
p = 0.0127
*
Odds of attack
2
3
p < 0.0001
**
Odds of thorough attack
2.5
*
1.5
1
0.5
**
**
*
upper
all
lower
2.5
2
1.5
1
0.5
0
0
lower
upper
lower
A. lamprocarpa
upper
lower
E. tetrodonta
A. lamprocarpa
Figure 1.
p<0.0001
2.5
odds of attack
**
2
**
p=0.0035
p<0.0001
p=0.0247
*
1.5
1
0.5
0
lower
upper
A. lamprocarpa
lower
upper
E. tetrodonta
Figure 2.
25
upper
E. tetrodonta
all
Median summed herbivory score
a)
p =0.031
Total
branch
Median summed herbivory score
b)
p =0.030
p <0.001 p =0.001
Total
branch
Figure 3.
26
Table 1. Chi-square results from type III GEE analysis of ant attacks of termites in 2006.
Response variable and sources of variation
Df
Χ2
P
1. Both tree species, any termite (N=360)
Ant species
1
7.52
0.0061
Tree species
1
11.93
0.0006
Termite position (upper or lower)
1
20.68
<0.00001
Ant * tree
1
0.36
0.55
Ant * position
1
0.27
0.60
Tree * position
1
3.25
0.0714
Ant * tree* position
1
7.56
0.0060
Ant species
1
4.49
0.0340
Termite position
1
5.84
0.0156
Ant * position
1
3.10
0.0785
Ant species
1
3.00
0.0832
Termite position
1
14.36
0.0002
Ant * position
1
4.44
0.0351
Ant species
1
9.98
0.0016
Tree species
1
0.63
0.43
Termite position (upper or lower)
1
0.17
0.68
Ant * tree
1
0.30
0.58
Ant * position
1
0.03
0.86
Tree * position
1
2.27
0.13
2. A. lamprocarpa (N=180)
3. E. tetrodonta (N=180)
4. Ant attack of both termites (N=85)
27
Ant * tree* position
1
0.44
0.51
Ant species
1
12.30
0.0005
Tree species
1
8.12
0.0044
Ant*tree
1
7.85
0.0051
5. Ant attack of all termites (N=57)
Table 2. A Mann-Whitney U comparison of the median number of yellow crazy and weaver
ants recruited to lower and upper termites on A. lamprocarpa in 2006..
Yellow crazy ants
Weaver ants
M-W U
P
N Median (range)
N Median (range)
lower
9
12.0 (3-15)
8
3.0 (1-7)
58.5
0.0020
upper
9
8.5 (1-12)
7
2.5 (1-7)
64.5
0.0266
28
Table 3. Chi-square results from type III GEE analysis and F statistics from type III analysis
of variance for binomial and continuous variables, respectively, in 2007.
Response variables and sources of variation
Df
Χ2
P
1. Both tree species, any termite (N=632)
Ant species
1
7.96
0.0048
Tree species
1
3.67
0.0554
Termite position
1
38.51
<0.0001
Ant * tree
1
2.12
0.15
Ant * position
1
0.24
0.63
Tree * position
1
4.60
0.0320
Ant * tree * position
1
0.03
0.86
Ant species
1
0.11
0.74
Tree species
1
2.11
0.15
Termite position (upper or lower)
1
2.50
0.11
Ant * tree
1
0.71
0.40
Ant * position
1
1.06
0.30
Tree * position
1
2.04
0.15
Ant * tree* position
1
0.01
0.90
Ant species
1
1.55
0.21
Tree species
1
0.99
0.32
Ant*tree
1
0.39
0.53
2. Ant attack of both termites (N=107)
3. Ant attack of all termites (N=73)
4. Speed of attack A. lamprocarpa (N=120)
29
Ant species
1, 113
4.39
0.0383
Termite height
1, 111
1.16
0.28
Ant species
1
0.09
0.76
Tree species
1
4.64
0.0312
Termite position
1
0.00
1.00
5. Attack by other predators (N=632)
30