Interspecific Aggression and Resource Monopolization of the Invasive Ant Anoplolepis gracilipes in Malaysian Borneo

Simple SummaryInvasive insects, particularly ants, pose a significant threat to biodiversity due to their strong reproductive capabilities and environmental adaptability. Tropical regions are especially vulnerable, as favorable climatic and ecological conditions facilitate the proliferation of invasive ant species. Yellow crazy ants (Anoplolepis gracilipes), as one of the IUCN’s top 100 most destructive invasive species, also the first documented chimeric ant species, may exploit this unique genetic trait to survive in adverse environments. Therefore, investigating their interactions with native species is both scientifically meaningful and practically essential. Existing studies and our preliminary observations indicate that yellow crazy ants frequently coexist with weaver ants (Oecophylla smaragdina), a dominant native species. This study employed field-controlled experiments and species distribution models to predict the distribution dynamics of two ant species. Our results suggest that the yellow crazy ant has a strong competitive ability in the wild, and the overlapping area between the two species is shrinking. Under future climate change scenarios, yellow crazy ants are projected to expand their range, while weaver ants are expected to decline. Our findings suggest that it is imperative to closely monitor the spread of this invasive ant species.Global climate change, coupled with the escalating severity of species invasions, has profoundly impacted and continues to influence species distribution patterns across multiple spatial scales. The invasive ant species Anoplolepis gracilipes (yellow crazy ants) and the dominant species Oecophylla smaragdina (weaver ants) share a significant overlapping distribution in tropical Asia and Oceania. The changes in their distribution areas, particularly in the overlapping regions, under future climate change scenarios remain inadequately explored. By integrating field behavioral experiments conducted on two ant species with climate and topographic datasets, we evaluated the extent of overlapping ranges and predicted the future dynamics of both species. Our results show that yellow crazy ants are more efficient at finding food and mobilizing workers, indicating stronger collaborative abilities than weaver ants. Under food and water deprivation conditions, yellow crazy ants exhibit a higher survival rate than weaver ants. Climatic factors exert a greater influence on the potential distribution of yellow crazy ants compared to topographic factors. Regions with consistently high suitability for yellow crazy ants primarily include southern China, Myanmar, India, Thailand, Malaysia, and Australia. The potential distribution area for weaver ants has constricted due to climate change, while that for yellow crazy ants has expanded. Initially, these two ant species had highly overlapping suitable habitats. However, this overlap is projected to diminish under future climate conditions. Mitigating future climate change could substantially reduce the expansion of yellow crazy ants. This discovery underscores the importance of monitoring and managing the dynamic changes in the distribution areas of both invasive and native species against the backdrop of climate change.

The Argentine ant (Linepithema humile) is a widespread invasive species that competitively displaces native ants throughout its introduced range. Although this pattern of displacement is well known, its underlying mechanisms remain little studied. To gain a more detailed understanding of this widespread competitive displacement, I compared the exploitative and interference abilities of the Argentine ant with those of seven species of native ants it displaces in riparian woodlands in northern California. I performed four different manipulative field experiments; each measured different aspects of the competitive ability of the eight species of ants in this study. The main goals of this study were to identify the mechanisms responsible for the Argentine ant's strong competitive ability, to determine if native ants are subject to species-specific trade-offs in exploitative and interference ability typically present among coexisting ants, and if so, to assess whether Argentine ants are subject to this trade-off as well. Argentine ants located and recruited to baits as quickly or more quickly than did native ants—both in areas where Argentine ants and native ants occurred together (i.e., at the edge of invasion fronts) and where they occurred separately (i.e., away from invasion fronts). Along the edge of invasion fronts, Argentine ants also controlled a greater proportion of baits than did native ants. In one-on-one interactions, individual Argentine ant workers experienced mixed success in overcoming individual workers of the seven native ant species. When fighting against native ants, Argentine ants used both physical aggression and chemical defensive compounds, although the latter mechanism was more often successful in deterring opponents. Chemical defensive compounds produced by Argentine ants were repellent but appeared no more so than those of native ants. Although Argentine ant workers were not able to overcome native ant workers consistently, Argentine ant colonies succeeded in displacing most native ant colonies from baits. The discrepancy between worker-level and colony-level interference ability suggests that numerical advantages are key to the Argentine ant's proficiency at interference competition. Like ants in other communities, the native ants in this study were subject to a competitive trade-off in which interference ability and exploitative ability were negatively correlated. In contrast, Argentine ants were proficient at both exploitative and interference competition relative to the native ants they displaced and are thus removed from this trade-off. These findings imply that Argentine ants secure a majority of available food resources where this species comes into contact with native ants. Argentine ants may be able to break the competitive trade-off constraining native ants because of their unique colony structure and because they have escaped their natural enemies. The observation that Argentine ants are uncoupled from the competitive trade-off constraining native ants may provide a general explanation for patterns of dominance within ant communities and for the success of other introduced species.

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 A. gracilipes and O. smaragdina ants on trees with ( Acacia lamprocarpa ) and without ( Eucalyptus tetrodonta ) extrafloral nectar rewards through surrogate herbivore (termite) addition experiments and surveys of herbivore damage. Anoplolepis gracilipes were more likely than O. smaragdina to discover termites on A. lamprocarpa , but the likelihood of termite discovery on E. tetrodonta did not vary with ant species. Anoplolepis gracilipes 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 O. smaragdina . Discovery of termites by other predators did not vary significantly between trees in A. gracilipes and O. smaragdina sites. 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 A. gracilipes sites had higher chewing herbivory scores than their counterparts in O. smaragdina sites. Our results reveal that the more aggressive and efficient foliar patrolling by A. gracilipes does not translate to increased plant protection. Ant invasions can disrupt native ant–plant mutualisms despite invasive ants possessing many traits associated with effective plant guarding.

Similarity of ant communities increases with isolation from natural habitat and abundance of invasive ants in oil palm plantations of Central Borneo

The emission of volatile organic compounds (VOCs) depends on temperature and light. Other factors such as insect herbivory also may modify VOC emission. In particular, aphid feeding promotes the release of new compounds and changes the composition of plant volatile blends. Given that some aphids are tended by ants, we investigated whether ants change the emission of VOCs indirectly through attendance on aphids. The effect of Lachnus roboris aphids and two different tending ant species on terpene emission rates of 4-year-old holm oak (Quercus ilex) saplings was investigated during a field experiment. There were five treatments: saplings alone (T1), saplings infested with L. roboris aphids (T2), saplings infested with aphids tended by the local ant Lasius grandis (T3), those tended by small colonies of the invasive ant Lasius neglectus (T4), and those tended by large colonies of the same invasive ant species (T5). The infestation by L. roboris elicited the emission of Delta(3)-carene and increased the emission of myrcene and gamma-terpinene. Terpene emissions were modified depending on the tending ant species. Attendance by the local ant L. grandis increased alpha and beta-pinene and sabinene. Attendance by the invasive ant L. neglectus only decreased significantly the emission of myrcene, one of the major compounds of the Q. ilex blend. Aphid abundance decreased with time for all treatments, but there was no difference in aphid abundance among treatments. Total terpene emission rates were not correlated with aphid abundance. These results highlight that aphids and tending ants may change terpene emission rates, depending on the ant species.

Invasive ant species represent a serious threat to the integrity of many ecological communities, often causing decreases in the abundance and species richness of both native ants and other arthropods. One of the most in-depth and well-known studies of this type documented a severe impact of the imported red fire ant, Solenopsis invicta, on the native ant and arthropod fauna of a biological field reserve in central Texas (USA) during the initial invasion in the late 1980s. I sampled the community again in 1999, 12 years later, utilizing the same methodology, to compare the short- and long-term impacts of this invasion. Pitfall traps and baits were used to obtain quantitative measures of the ant and arthropod community, and hand collecting was additionally employed to determine the overall ant species composition. Although the abundance and species richness of native ants and several other arthropod groups decreased precipitously immediately after the S. invicta invasion, all measures of native ant and arthropod species diversity had returned to preinvasion levels after 12 years. Solenopsis invicta was still the most abundant ant species, but not nearly as abundant as it was during the initial phase of the invasion. The results of this study indicate that the impact of such invasive ants may be greatest during and shortly after the initial phase of an invasion.

The Argentine ant (Linepithema humile) is a widespread invasive species that competitively displaces native ants throughout its introduced range. Although this pattern of displacement is well known, its underlying mechanisms remain little studied. To gain a more detailed understanding of this widespread competitive displacement, I compared the exploitative and interference abilities of the Argentine ant with those of seven species of native ants it displaces in riparian woodlands in northern California. I performed four different manipulative field experiments; each measured different aspects of the competitive ability of the eight species of ants in this study. The main goals of this study were to identify the mechanisms responsible for the Argentine ant’s strong competitive ability, to determine if native ants are subject to species-specific trade-offs in exploitative and interference ability typically present among coexisting ants, and if so, to assess whether Argentine ants are subject to this trade-off as well. Argentine ants located and recruited to baits as quickly or more quickly than did native ants—both in areas where Argentine ants and native ants occurred together (i.e., at the edge of invasion fronts) and where they occurred separately (i.e., away from invasion fronts). Along the edge of invasion fronts, Argentine ants also controlled a greater proportion of baits than did native ants. In one-on-one interactions, individual Argentine ant workers experienced mixed success in overcoming individual workers of the seven native ant species. When fighting against native ants, Argentine ants used both physical aggression and chemical defensive compounds, although the latter mechanism was more often successful in deterring opponents. Chemical defensive compounds produced by Argentine ants were repellent but appeared no more so than those of native ants. Although Argentine ant workers were not able to overcome native ant workers consistently, Argentine ant colonies succeeded in displacing most native ant colonies from baits. The discrepancy between worker-level and colony-level interference ability suggests that numerical advantages are key to the Argentine ant’s proficiency at interference competition. Like ants in other communities, the native ants in this study were subject to a competitive trade-off in which interference ability and exploitative ability were negatively correlated. In contrast, Argentine ants were proficient at both exploitative and interference competition relative to the native ants they displaced and are thus removed from this trade-off. These findings imply that Argentine ants secure a majority of available food resources where this species comes into contact with native ants. Argentine ants may be able to break the competitive trade-off constraining native ants because of their unique colony structure and because they have escaped their natural enemies. The observation that Argentine ants are uncoupled from the competitive trade-off constraining native ants may provide a general explanation for patterns of dominance within ant communities and for the success of other introduced species.

1. Non‐native species may be the cause of native species declines or an effect of habitat degradations that promote the former and damage the latter. Social insects are extraordinarily successful organisms, and non‐native social insects, such as ants, often are very successful invaders of novel habitats. 2. The red imported fire ant ( Solenopsis invicta ) and the Asian needle ant ( Brachyponera chinensis ) are widespread non‐native invaders in the eastern United States that bring ecological, economic and social impacts, but their putative effects on native ant communities may be concomitant with habitat degradation rather than inherent in their invasion. 3. Given this gap in understanding, our goal was to examine how experimental warming influenced native and non‐native ant communities. Specifically, we hypothesised that (a) non‐native ant abundance and species richness will increase in warmed plots and that (b) warming‐induced increases in the non‐native ant populations will correspond with decreases in native ant abundance and species richness. 4. To test these hypotheses, we used three levels of experimental forest edge soil warming (warming targeted at +0, +3, and +5°C above ambient soil temperature) in a mixed deciduous forest in the Georgia (USA) Piedmont. We used repeated pitfall trapping to investigate how the experimental warming influenced the composition of native and non‐native ant communities with a focus on how warming combined with S. invicta and B. chinensis invasion impacted native ant communities. 5. Our results suggest that experimental warming promoted the non‐native invasive ants, particularly S. invicta and B. chinensis . We also found that B. chinensis somewhat inhibited native ant communities, but not because of warming. The warmed environment benefited both non‐native invasive ants at the expense of native ants but, given that B. chinensis negatively impacted the native ants with far fewer workers than S. invicta , B. chinensis may pose a greater threat to native ant communities than S. invicta in a warmer world.

Biological invasions are typically associated with disturbance, which often makes their impact on biodiversity unclear—biodiversity decline might be driven by disturbance, with the invader just being a ‘passenger’. Alternatively, an invader may act as a ‘back-seat driver’, being facilitated by disturbance that has already caused some biodiversity decline, but then causing further decline. Here we examine the interactive effects of anthropogenic fire and invasive ant species (Anoplolepis gracilipes or Wasmannia auropunctata) on native ant diversity in New Caledonia, a globally recognized biodiversity hotspot. We first examined native ant diversity at nine paired burnt and unburnt sites, with four pairs invaded by Anoplolepis, 5 years after an extensive fire. In the absence of invasion, native epigaeic ants were resilient to fire, but native ant richness and the abundance of Forest Opportunists were markedly lower in invaded burnt sites. Second, we examined native ant diversity along successional gradients from human-derived savanna to natural rainforest in the long-term absence of fire, where there was a disconnection between disturbance-mediated variation in microhabitat and the abundance of the disturbance specialist Wasmannia. All native ant diversity responses (total abundance, richness, species composition, functional group richness and the abundance of Forest Opportunists) declined independently of microhabitat variables but in direct association with high Wasmannia abundance. Our results indicate that invasive ants are acting as back-seat drivers of biodiversity decline in New Caledonia, with invasion facilitated by disturbance but then causing further biodiversity decline.

Impacts of highly invasive ants in new ecosystems are well documented, but many more ant species are establishing in new ranges for which there is little or no information. We studied the effects of the recently discovered Australian ant, Monomorium sydneyense Forel, on the ant community of Sulphur Point in Tauranga, New Zealand. At the community scale, the species composition in invaded areas was significantly different from that in areas free of M. sydneyense. However, no single ant species was significantly more or less abundant in the presence of M. sydneyense. Some resident ant species categorised in the same functional group as the invader appeared to be scarcer when sympatric with M. sydneyense, but the local abundances of these species were always spatially variable, so the effects were not statistically significant Patchy distribution of M. sydneyense, and other aspects of its behaviour, such as poor foraging abilities and a lack of unicoloniality (where there is little or no aggression between conspecific ants from spatially separate nests), appear to allow resident ant species to coexist with M. sydneyense at Sulphur Point.

Invasive ant species can have dramatic impacts on native ants, through direct predation and by usurping common resources. Most invasive ants and many native ants use honeydew, produced by phloem-sucking hemipterans. Because colonies of invasive ants can become very large after establishment, these ants may facilitate greater hemipteran trophobiont population growth compared with their sympatric native ant counterparts. We examined the population growth of an aphid mutualist, Aphis gossypii, and a nonmutualist, Myzus persicae, exposed to two Dolichoderine ants, Linepithema humile, a globally widespread invasive species, and Tapinoma sessile, a widespread co-occurring native ant, in North America in an enemy-free laboratory study. L. humile worker foraging activity was at least twice that of T. sessile, and populations of the myrmecophile, A. gossypii, were greater when exposed to L. humile than T. sessile, possibly caused, in part, by more frequent encounters with L. humile. L. humile ignored M. persicae when A. gossypii was absent, whereas T. sessile preyed on it. Both ant species preyed on M. persicae when A. gossypii was also present. This suggested that both ants may assess nutritional gains from aphid species (i.e., honeydew versus body tissue), eliminating less productive aphids competing for host plant space. Through their impact on populations of hemipteran mutualists, we suggest that colonies of L. humile and perhaps other invasive ants may acquire more honeydew than native ants, thereby fueling colony growth that leads to numerical dominance and widespread success in introduced environments.

Biological invasions represent communities in flux. Although stasis is never the rule in nature, biological interactions in communities usually occur within a framework of shared ecological and evolutionary history. Consequently, biological invasions represent unique opportunities to study dynamics that can otherwise be difficult to observe (Elton 1958). Invasive ants are excellent organisms with which to pursue this goal, in part because ants as a group play a variety of important ecological roles within biological communities (Holldobler and Wilson 1990). Ant invasions hold much potential for improving an understanding of ecological processes in general, as well as of interactions more specific to myrmecology. For instance, the dynamics that exist during ant invasions may reveal the traits that promote behavioural or ecological dominance. Furthermore, highly successful invasive ant species are often less conspicuous in their native ranges, so identification of the factors responsible for their greater prominence in introduced areas can provide insight into the more typical workings of ant communities, may highlight intrinsic differences between communities that have formed in separate biogeographic regions, and may suggest possible methods of control. Understanding ant invasion processes and causes of success thus has both basic and applied relevance. Human activities have introduced many ant species to new biogeographic regions (McGlynn 1999b; Chapter 13). While most introduced ant species have limited success in spreading away from the human-modified habitats in which they usually first arrive, a subset can invade nearby, or even distant, undisturbed natural habitats. This distinction is not always hard and fast, as virtually all species exhibit different degrees of invasiveness and ecological dominance in different locales, and sometimes even under similar conditions in the same locale (e.g. Abbott et al. 2007). Notwithstanding, the following species are the most consistent in their ability to penetrate natural ecosystems and affect the composition or abundance of native species within them (Holway et al. 2002a): the Argentine ant (Linepithema humile), the red imported fire ant (Solenopsis invicta), the tropical fire ant (S. geminata), the big-headed ant (Pheidole megacephala), the little fire ant (Wasmannia auropunctata), and the yellow crazy ant (Anoplolepis gracilipes) (see Plate 14). We focus on these species because of their pronounced invasive tendencies, and also because they have been studied the most intensively, especially with regard to the factors that control spread and underlie dominance. In fact, the majority of our knowledge on these topics comes from studies on just two species: S. invicta and L. humile. This represents both a major weakness and a clear avenue for progress in the field, pointing to a need for more complete information on the ecologies of other invasive ant species, as well as their close relatives that fail to become invasive (Chapter 13; see also Section 14.4). This limitation must be acknowledged at the outset, as it impinges on our under-

Mutualism can facilitate the colonization, establishment, and spread of invasive species. By modifying interactions with third parties, mutualisms can have cascading community-wide effects. Both native and invasive ants are capable of forming mutualisms with hemipteran insects, preying on non-hemipteran herbivores and indirectly affecting primary production. Comparative research on the effects of both native and invasive ant exclusions on multitrophic interactions is therefore crucial for understanding the invasive potential of ants, along with any ecological consequences that invasions may have. We performed a quantitative review of the multitrophic effects of invasive and native ants on insect–plant food webs. Herbivorous insects are the most common food source for both invasive (comprising 56% of prey species caught) and native ants (55% of the prey species caught), followed by predators (31% for invasive ants, 45% for native ants). Excluding both invasive and native ants significantly reduced hemipteran abundance, and excluding invasive ants had a greater negative impact on hemipteran abundance than native ants. Native ant predation significantly reduced herbivore abundance, but excluding invasive ants had no effect. Cascading effects of native ants on plant fitness were significantly positive, but there was no significant impact of invasive ants. These findings suggest a weak relationship between the presence of invasive ants and non-hemipteran herbivore abundance. We suggest that the hemipteran–ant mutualism could represent a ‘symbiotic invasion’. The ecological dominance of invasive ants is often facilitated by hemipteran insects. This association requires invasive ant control strategies to expand beyond ants to consider mutualists.

The islands of remote Polynesia (east of Rotuma, Samoa, Tonga and New Zealand) have long been thought to contain few, if any, native ants. The findings of recent sediment core studies, however, challenge this conventional wisdom and indicate some species may be native. The majority of ant species in remote Polynesia, however, are introductions from tropical and subtropical regions around the world. Despite this diversity of origins, and the lack of a common coevolutionary history in the region, patterns of organization in remote Polynesian ant communities are generally similar to those observed in coevolved continental areas. The distribution of ant species across Polynesia is consistent with a primary mechanism of anthropogenic introductions, with the availability of suitable habitat as a secondary mechanism. The species-area relationship for better-collected Polynesian islands reveals these islands are depauperate compared to Melanesian islands with endemic species. Four out of five of the “world’s worst” invasive ant species are present in remote Polynesia. Recent studies have documented how range expansions of such ant species have had detrimental effects on native arthropod populations, although the overall effects of introduced ants per se on naive endemic island arthropods may never be known with certainty. Considering the relatively fragile nature of island ecosystems, and the potential transformative effects of invasive ants on arthropod communities, vigilance is required to prevent the spread of invasive ant species across Polynesia.

The services provided within a community can change as the species composition of that community changes. For example, ant–seed dispersal mutualisms can be disrupted in habitats dominated by invasive ants. We propose that this disruption is related to changes in mean ant body size, given that invasive ants are smaller than most native seed-dispersing ants. We demonstrate that the mean and maximum distances that ants transport seeds adapted for ant dispersal increase with worker body size, and that this relationship is an accelerating power function. This pattern is consistent among three ant subfamilies that include most seed-dispersing ants as well as most invasive ant species, is generalizable across ant species and communities, and is independent of diaspore mass. Using a case study, we demonstrate that both the mean body size of seed-collecting ants and seed dispersal distances are decreased in sites invaded by Solenopsis invicta, the imported red fire ant. Furthermore, we demonstrate that the mean size of seed-collecting ants at a seed depot or within a community is a useful predictor of mean seed dispersal distances at those sites. Last, we show that small seed-collecting ants and decreased seed dispersal distances are common features of sites occupied by invasive ants. The link between ant body size and seed dispersal distance, combined with the dominance of invaded communities by typically small ants, predicts the disruption of native ant–seed dispersal mutualisms in invaded habitats.