Behaviourally and genetically distinct populations of an invasive ant provide insight into invasion history and impacts on a tropical ant community

Populations of invasive species often exhibit a high degree of spatial and temporal variability in abundance and hence their effects on resident communities. Here, we examine behavioural, genetic and environmental factors that influence variation in populations of the yellow crazy ant, Anoplolepis gracilipes, on the remote Nukunonu Atoll of Tokelau, Pacific Ocean. Behavioural assays revealed high levels of aggression between two groups of yellow crazy ants from different islands, and genetic analysis confirmed the presence of two distinct populations with unique mitochondrial (mt)DNA haplotypes, designated A and D. The two populations likely resulted from two separate invasion events. The populations exhibited significant differences in abundance of A. gracilipes, with a mean sevenfold difference in relative abundance between the two main haplotypes. The higher density haplotype D population coexisted with 50% fewer other ant species and altered ant community composition. Vegetation composition was also significantly different on islands harbouring the two populations. The results suggest genetic differences could play a role in the spatial and temporal variation in the effect of the yellow crazy ant on a small oceanic atoll. We could not differentiate between genetic effects and effects of vegetation. However, our results indicate that spatial variability in behaviour and impacts within populations of invasive species could be in part due to genetic differences, and play a substantial role in influencing the outcome of biological invasions.     

Community level impacts of an ant invader and food mediated coexistence

Ant invasions exert a range of effects on recipient communities, from displacement of particular species to more complex community disruption. While species loss has been recorded for a number of invasion events, a little examined aspect of these invasions is the mechanisms for coexistence with resident ant species.The yellow crazy ant, Anoplolepis gracilipes (Smith), is considered one of the world’s worst ant invaders and has recently undergone rapid population growth in Tokelau. We surveyed the ground-dwelling ant fauna in two plots on each of five invaded and three uninvaded islands across two atolls in Tokelau to examine community characteristics of the ant fauna in areas with and without yellow crazy ants. We also used three types of food bait (tuna, jam and peanut butter) to experimentally test if species are able to coexist by consuming different food resources. Anoplolepis gracilipes was found to coexist with two to six other ant species at any one site, and coexisted with a total of 11 ant species. Four species never co-occurred with A. gracilipes. Non-metric multidimensional scaling showed significant differences in community composition and the relative abundance of species between areas that had, and had not, been invaded by A. gracilipes. The number of other ant species was significantly lower in communities invaded by the yellow crazy ant, but did not decline with increasing A. gracilipes abundance, indicating that impacts were independent of population density. The yellow crazy ant dominated all tuna and jam baits, but had a low attendance on peanut butter, allowing four other ant species to access this resource. Our results demonstrate community level impacts of an ant invader on a tropical oceanic atoll and suggest that differing use of food resources can facilitate coexistence in ant communities. Received 11 September 2006; revised 15 January 2007; accepted 22 February 2007.     

Invasive ants compete with and modify the trophic ecology of hermit crabs on tropical islands

Invasive species can dramatically alter trophic interactions. Predation is the predominant trophic interaction generally considered to be responsible for ecological change after invasion. In contrast, how frequently competition from invasive species contributes to the decline of native species remains controversial. Here, we demonstrate how the trophic ecology of the remote atoll nation of Tokelau is changing due to competition between invasive ants (Anoplolepis gracilipes) and native terrestrial hermit crabs (Coenobita spp.) for carrion. A significant negative correlation was observed between A. gracilipes and hermit crab abundance. On islands with A. gracilipes, crabs were generally restricted to the periphery of invaded islands. Very few hermit crabs were found in central areas of these islands where A. gracilipes abundances were highest. Ant exclusion experiments demonstrated that changes in the abundance and distribution of hermit crabs on Tokelau are a result of competition. The ants did not kill the hermit crabs. Rather, when highly abundant, A. gracilipes attacked crabs by spraying acid and drove crabs away from carrion resources. Analysis of naturally occurring N and C isotopes suggests that the ants are effectively lowering the trophic level of crabs. According to δ¹⁵ N values, hermit crabs have a relatively high trophic level on islands where A. gracilipes have not invaded. In contrast, where these ants have invaded we observed a significant decrease in δ¹⁵ N for all crab species. This result concurs with our experiment in suggesting long-term exclusion from carrion resources, driving co-occurring crabs towards a more herbivorous diet. Changes in hermit crab abundance or distribution may have major ramifications for the stability of plant communities. Because A. gracilipes have invaded many tropical islands where the predominant scavengers are hermit crabs, we consider that their competitive effects are likely to be more prominent in structuring communities than predation. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Supercolonies of the invasive yellow crazy ant, Anoplolepis gracilipes, on an oceanic island: Forager activity patterns, density and biomass

Summary. Ants have the capacity to reach unusually high densities, mostly in their introduced ranges. Numerical dominance is often cited as key to the ability of exotic ants to displace native ant species, reduce the abundance of invertebrates and negatively impact upon bird, land crab and other vertebrate populations. On Christmas Island, Indian Ocean, the yellow crazy ant, Anoplolepis gracilipes (Jerdon), forms supercolonies, where extremely high densities of foraging ants have contributed to ‘invasional meltdown’ in rainforest areas. Densities of up to 2254 foraging ants per m² and a biomass of 1.85 g per m² were recorded, and nest densities reached 10.5 nest entrances per m². Populations of A. gracilipes can overcome and kill red endemic land crabs (Gecarcoidea natalis) over 100 times their own biomass. This is the highest recorded density of foraging ants, and adds another element to the definition of ‘supercolony’ of unicolonial ants. This paper documents one extreme in a continuum of densities of unicolonial, invasive ant species and highlights the need to incorporate forager densities into invasive ant research.Received 17 November 2004; revised 14 February 2005, accepted 21 February 2005.     

Habitat disturbance modifies dominance,coexistence, and competitive interactions in tropical ant communities

1. Interspecific competition is a major structuring principle in ecological communities. Despite their prevalence, the outcome of competitive interactions is hard to predict, highly context-dependent, and multiple factors can modulate such interactions. 2. We tested predictions concerning how competitive interactions are modified by anthropogenic habitat disturbance in ground-foraging ant assemblages inhabiting fragmented Inter-Andean tropical dry forests in southwestern Colombia, and investigated ant assemblages recruiting to baits in 10 forest fragments exposed to varying level of human disturbance. 3. Specifically, we evaluated how different components of competitive interactions (patterns of species co-occurrence, resource partitioning, numerical dominance, and interspecific trade-offs between discovery and dominance competition) varied with level of habitat disturbance in a human-dominated ecosystem. 4. Multiple lines of evidence suggest that the role of competitive interactions in structuring ground-foraging ant communities at baits varied with respect to habitat disturbance. As disturbance increased, community structure was more likely to exhibit random co-occurrence patterns, higher levels of monopolization of food resources by dominant ants, and disproportionate dominance of a single species, the little fire ant (Wasmannia auropunctata). At a regional scale, we found evidence for a trade-off between dominance and discovery abilities of the 15 most common species at baits. 5. Together, these results suggest that human disturbance modifies the outcome of competitive interactions in ground-foraging ant assemblages and may promote dominant species that reduce diversity and coexistence in tropical ecosystems.     

Impact of the invasive crazy ant Anoplolepis gracilipes on Bird Island, Seychelles

The crazy ant (Anoplolepis gracilipes) invaded Bird island, Seychelles, in the 1980s. In 1997, its range expanded and population densities increased. The impacts of this change were studied in 2001 using a combination of arthropod collecting methods. The ant population excluded larger invertebrates (principally the large ant Odontomachus simillimus and the crabs, principally Ocypode spp.). Cockroaches, however, remained abundant in ant-infested areas and tree-nesting birds (Lesser Noddy Anous tenuirostris) appear to be able to breed successfully in the presence of the crazy ant. The ants are only abundant in areas of deep shade which provide cool nesting areas, yet enabling them to forage in the open when ground temperatures fall. The expansion of the ants was correlated with the regeneration of woodland on the island. Recommendations are made for the management of the woodland which may reduce the impacts of the crazy ant.     

Honeydew collection by the invasive garden ant Lasius neglectus versus the native ant L. grandis

Honeydew collection performed by the invasive ant Lasius neglectus and by the native ant L. grandis was compared. The invasive ant collected 2.09 kg of honeydew per tree while the native ant collected 0.82 kg. The aphid Lachnus roboris was visited by both ant species. In holm oaks colonized by L. neglectus, aphid abundance tended to increase and its honeydew production increased twofold. The percentage of untended aphids was lower in holm trees occupied by L. neglectus. As tending ants also prey on insects, we estimated the percentage of carried insects. The native ant workers carried more insects than the invasive ant. Both ant species preyed mainly on Psocoptera and the rarely tended aphid, Hoplocallis picta. We conclude that the higher honeydew collection achieved by L. neglectus was the consequence of (1) its greater abundance, which enabled this ant to tend more Lachnus roboris and (2) its greater level of attention towards promoting an increase of honeydew production. Handling editor: Heikki Hokkanen     

Interference and exploitation competition of three nectar-thieving invasive ant species

Summary. Plant and insect exudates are known to play a key role in structuring tropical ant communities, but less is known about the utilization of these resources in communities dominated by invasive ants. Invasive ants are thought to require large amounts of carbohydrates such as honeydew or nectar to maintain their high abundances. Invasive ants that consume floral nectar may compete with legitimate floral visitors through interference or exploitation competition. I compared the nectar-thieving behavior of three widespread invasive ant species: long-legged ants (Anoplolepis gracilipes), Argentine ants (Linepithema humile), and big-headed ants (Pheidole megacephala) in inflorescences of the native Hawaiian ‘ōhi’a tree, an important food source for native fauna. A. gracilipes was least likely to leave inflorescences unvisited and visited inflorescences in higher numbers than both L. humile and P. megacephala. A. gracilipes and L. humile visited more flowers in an inflorescence and were less likely to retreat from a flower with a competitor than P. megacephala. A. gracilipes was able to take 5.5 and 11.3 times the amount of nectar than L. humile and P. megacephala, respectively. Thus, A. gracilipes may be effective at both interference and exploitation competition against other nectarivores, L. humile may be effective at interference competition, and P. megacephala may be relatively weak at both types of competition against other nectarivores. Ascertaining the competitive abilities of invasive ants against legitimate floral visitors will be especially important in agricultural and other systems that are nectar or pollinator limited.Received 6 December 2004; revised 13 January 2005; accepted 14 January 2005.     

Nest raiding by the invasive Argentine ant on colonies of the harvester ant, Pogonomyrmex subnitidus

Invasive ants often displace native ants, and published studies that focus on these interactions usually emphasize interspecific competition for food resources as a key mechanism responsible for the demise of native ants. Although less well documented, nest raiding by invasive ants may also contribute to the extirpation of native ants. In coastal southern California, for example, invasive Argentine ants (Linepithema humile) commonly raid colonies of the harvester ant, Pogonomyrmex subnitidus. On a seasonal basis the frequency and intensity of raids vary, but raids occur only when abiotic conditions are suitable for both species. In the short term these organized attacks cause harvester ants to cease foraging and to plug their nest entrances. In unstaged, one-on-one interactions between P. subnitidus and L. humile workers, Argentine ants behaved aggressively in over two thirds of all pair-wise interactions, despite the much larger size of P. subnitidus. The short-term introduction of experimental Argentine ant colonies outside of P. subnitidus nest entrances stimulated behaviors similar to those observed in raids: P. subnitidus decreased its foraging activity and increased the number of nest entrance workers (many of which labored to plug their nest entrances). Raids are not likely to be the result of competition for food. As expected, P. subnitidus foraged primarily on plant material (85% of food items obtained from returning foragers), but also collected some dead insects (7% of food items). In buffet-style choice tests in which we offered Argentine ants food items obtained from P. subnitidus, L. humile only showed interest in dead insects. In other feeding trials L. humile consistently moved harvester ant brood into their nests (where they were presumably consumed) but showed little interest in freshly dead workers. The raiding behavior described here obscures the distinction between interspecific competition and predation, and may well play an important role in the displacement of native ants, especially those that are ecologically dissimilar to L. humile with respect to diet. Received 15 July 2005; revised 19 October 2005; accepted 26 October 2005.     

Aggressive interactions between the introduced Argentine ant, Linepithema humile and the native odorous house ant, Tapinoma sessile

The Argentine ant (Linepithema humile) is an invasive species that disrupts the balance of natural ecosystems by displacing indigenous ant species throughout its introduced range. The mechanisms by which Argentine ants effectively compete against native ant species have been previously addressed in field studies that centered on interference and exploitation competition at baits and mainly examined the colony-level performance of Argentine ants. Detailed behavioral observations explaining the basis for the strong competitive ability of L. humile are comparatively rare. To gain a better understanding of the mechanisms by which Argentine ants displace native ants we examined the aggressive interactions between the Argentine ants and the odorous house ant, Tapinoma sessile in four different aggression assays: (1) worker dyad interactions, (2) symmetrical group interactions, (3) intruder introductions into an established resident colony, and (4) a resource competition assay which focused on competition for food and nesting space. Our results demonstrate a clear disparity between worker-level and colony-level fighting ability of Argentine ants and provide behavioral evidence to explain the superior interference ability of Argentine ants in group assays. Argentine ants experienced mixed success in fighting against odorous house ants in dyad interactions, but gradually gained a numerical advantage in symmetrical group interactions by active cooperation among nestmates. Results of the resource competition assay indicate that Argentine ants recruit rapidly, numerically dominate food and nesting sites, and aggressively displace T. sessile from baits. Taken together, the results of these assays allow us to pinpoint the behavioral mechanisms responsible for the remarkable competitive ability of Argentine ants.     

Seed dispersal curve of a Mediterranean myrmecochore: Influence of ant size and the distance to nests

Euphorbia characias is a Mediterranean spurge with a diplochorous dispersal system: after a ballistic dispersal that scatters the seeds, some ant species find and retrieve the seeds to their nest (myrmecochorous dispersal). The seed dispersal curve generated by ants in an abandoned field was described and partitioned according to ant size and to the distance to nest entrance from where seeds fell after ballistic dispersal. Both variables (ant size, distance to nest) affected dispersal distance. The seed dispersal curve showed a peak at short distance (median=1m) and a tail extending to 9.4m. The peak and the tail are explained differently. Short distances are usually generated by small ants (Pheidole pallidula and Tapinoma nigerrimum; 0.56±0.41m [n=48]) both from the nearest or farther nest entrances. The tail of the curve is generated disproportionately by big ants (Aphaenogaster senilis and Messor barbarus; 2.09±1.71m [n=61]) from farther nests. Seeds have a much greater probability (P=0.734) of being transported to nests which are not the nearest. This effect is largely due to transportation by big ants.     

Detrimental effects of two widespread invasive ant species on weight and survival of colonial nesting seabirds in the Hawaiian Islands

Invasive ants are a significant conservation concern and can have far-reaching effects in ecosystems they invade. We used the experimental control of ant numbers on two pairs of small (<5 ha) offshore islets dominated by either the big-headed ant, Pheidole megacephala or the tropical fire ant, Solenopsis geminata to investigate the influence of these species on seabird hatching success, fledging success and weight. Limited unpublished observations of both ant species attacking nesting seabirds exist, but the frequency of attacks or how they affect seabird growth and survival are unknown. Island-wide treatments with hydramethylnon resulted in the eradication of P. megacephala and the temporary reduction of S. geminata densities. No difference in hatching success, growth, or fledging success of Wedge-tailed Shearwaters (Puffinus pacificus), a common colonial nesting seabird in the Hawaiian Islands was observed on the pair of islets dominated by P. megacephala. On islets dominated by S. geminata, ant control resulted in a temporary increase in fledging success. Injury frequency increased dramatically on the untreated islet (8.3–100%) while remaining the same on the treated islet (27–38%). Severely injured chicks (i.e., chicks that lost >20% of tissue on their feet) weighed significantly less than uninjured chicks and did not fledge. It is unclear if the chicks were being preyed upon or stung in defense of nearby ant colonies. Radical changes in invasive ant populations have been noted, and booming ant populations could cause short-term, but widespread damage to seabird colonies. The negative effects of invasive ants on seabirds may be difficult to detect, and therefore unknown or underestimated throughout the world where the two groups overlap.     

Foraging behavior as a determinant of asymmetric competitive interaction between two ant species in a tropical agroecosystem

This work is concerned with elucidating competitive interactions between two neotropical ants, Solenopsis geminata and Pheidole radoszkowskii, focusing on their foraging behavior. When released from competition from P. radoszkowskii, S. geminata increased its foraging activity. On the other hand, when released from competition from S. geminata, P. radoszkowskii did not respond, demonstrating asymmetric competition between the two species. Foraging experiments showed that P. radoszkowskii is more efficient at finding food resources, whereas S. geminata is better at defending the resources once they are encountered. These differences in foraging behavior appear to permit the coexistence of these two species. The practical implications of the results for the management of ant communities in tropical agroecosystems is discussed with respect to the potential use of ants as natural enemies.     

Habitat complexity reduces aggression and enhances consumption in crayfish

We tested the hypothesis that increased habitat complexity would reduce intraspecific interactions among crayfish (Orconectes propinquus), and result in an increase in the consumption rate of prey at different crayfish densities. The effect of crayfish density, food level (trout eggs), and habitat complexity on prey consumption by crayfish was quantified in the laboratory. There was a significant difference in the consumption rate between different food levels. When food was scarce, almost all trout eggs were consumed regardless of crayfish density or habitat complexity. When food was unlimited, there was a significant positive linear relationship between eggs consumed per crayfish and habitat complexity. However, the relationship was not significant when trials without habitat were deleted from the analysis. We found that habitat complexity significantly reduced intraspecific aggression. Our findings suggest that a minimal amount of habitat complexity can reduce interactions among predators, ultimately resulting in increased prey consumption.     

Phylogenetic relationships among species groups of the ant genus Myrmecia

Phylogenetic relationships among the nine species groups of the predominately Australian ant genus Myrmecia were inferred using 38 Myrmecia species and an outgroup using DNA sequences from two nuclear genes (622nt from 28S rRNA and 1907nt from the long-wave opsin gene). Nothomyrmecia macrops was selected as the most appropriate outgroup based on recent reliable studies showing monophyly of Myrmecia with Nothomyrmecia. The four species groups with an occipital carina (those of gulosa, nigrocincta, urens, and picta) were found to form a paraphyletic and basal assemblage out of which the five species groups lacking an occipital carina (those of aberrans, mandibularis, tepperi, cephalotes, and pilosula) arise as a strongly supported monophyletic assemblage. Monophyly was supported for four groups (those of gulosa, nigrocincta, picta, and mandibularis) but the situation is unclear for four others (those of urens, aberrans, tepperi, and pilosula). The aberrans group appears to be basal within the group lacking an occipital carina; a previous suggestion that it is the sister group to the rest of the genus is thus not supported.     

Genetic vulnerability of a remnant ant population

Ants are ecologically important species in many environments forming a big proportion of the total animal biomass. However, their special features such as sociality and complementary sex-determination system have received little attention in conservation discussions. We examined the social and genetic structure of the hairy wood ant Formica lugubris in Ireland to evaluate factors relevant to the vulnerability of small, isolated wood ant populations. We also clarified the conservation status of the species in Ireland. Our results showed that the populations are mainly monogynous (one queen per nest). Consequently the effective population size is very low (<100 individuals) in Ireland. We found extremely little genetic variation, signs of inbreeding and inbreeding depression, which can be a consequence of the small effective population size and of the restricted gene flow due to strong isolation of populations. Putatively high genetic diversity at the sex-determining locus can reflect a larger population in the past. The study shows that even though the population may seem stable because of the long life span of queen ants, sociality can impact species conservation by keeping the effective population size small. According to our results, the hairy wood ant can be considered native to Ireland. Hence it needs urgent protection and the genetic issues need to be considered in the future management strategies.     

Host-plant selection,diet diversity,and optimal foraging in a tropical leafcutting ant

Summary A month-long study was conducted on the comparative foraging behavior of 20 colonies of the leafcutting ant, Atta cephalotes L. in Santa Rosa National Park, Guanacaste Province, Costa Rica. The study was conducted during the middle of the wet season, when trees had mature foliage and the ants were maximally selective among species of potential host plants. The colonies always gathered leaves from more than a single tree species but on average one species constituted almost half the diet with the remaining species being of geometrically decreasing importance. Colonies exhibited greater diversity in their choice of leaves and lower constancy of foraging when the average quality of resource trees was lower, as predicted by elementary optimal foraging theory. Furthermore, the ants were more selective of the species they attacked at greater distances from the nest. However, the ants sometimes did not attack apparently palatable species, and often did not attack nearby individuals of species they were exploiting at greater distances.A classical explanation for why leafcutting ants exploit distant host trees when apparently equally good trees are nearer, is that the ants are pursuing a strategy of conserving resources to avoid long-term overgrazing pressure on nearby trees. We prefer a simpler hypothesis: (1) Trees of exploited species exhibit individual variation in the acceptability of their leaves to the ants. (2) The abundance of a species will generally increase with area and radial distance from the nest, so the probability that at least one tree of the species will be acceptable to the ants also increases with distance. (3) The ants forage using a system of trunk-trails cleared of leaf litter, which significantly reduces their travel time to previously discovered, high-quality resource trees (by a factor of 4- to 10-fold). (4) Foragers are unware of the total pool of resources available to the colony. Therefore once scouts have chanced upon a tree which is acceptable, the colony will concentrate on harvesting from that tree rather than searching for additional sources of leaves distant from the established trail.     

Microclimatic changes and the indirect loss of ant diversity in a tropical agroecosystem

Recent changes in the coffee agroecosystem of Costa Rica were used to study the mechanism of biodiversity loss in transforming agroecosystems, focusing on the ground-foraging ant community. Coffee farms are being transformed from vegetationally diverse shaded agroforestry systems to unshaded coffee monocultures. We tested the hypothesis that the high-light environment and lack of leaf litter cover in the unshaded system are the determinants of the differences in ground-foraging ant diversity. Four treatments were established within the light gaps of a shaded plantation: shade, leaf litter, shade plus leaf litter, and a control (no shade or leaf litter added). Ants were sampled using tuna fish baits and light and temperature were measured. Shade and leaf litter had a significant effect on the ant fauna but probably for indirect reasons having to do with species interactions. In both shade treatments, Solenopsis geminata, the tropical fire ant, decreased significantly while the other species increased. The possibility that the physical factor changes the nature of competitive interactions between the most abundant species is discussed.     

Chaff piles of harvester ant (Messor spp.) nests in a desert ecosystem

Summary The plant species composition of the chaff piles of three species of harvester ant (Messor spp.) and the contribution of the chaff to the organic pool were studied from August 1985 to July 1987. There were distinct differences in the plant species composition of the chaff of the three species. We attribute this to the different diets of the three species, which reflect the relative sizes of their individuals and their foraging strategies. The amount of chaff accumulated varies greatly between the three species (Messor rugossus: 127–196 g · ha^–1 · y^–1;Messor ebeninus: 2823–4437 g · ha^–1 · y^–1;Messor arenarius: 2165–2535 g · ha^–1 · y^–1), although the number of nests per hectare is virtually the same. We found that the amount of chaff is related to the rate of activity and the size of the individuals of each of the three ant species. The total chaff accumulated during the study period was 19.2 kg · ha^–1, which is an important contribution to the organic matter in the soil in the Negev desert ecosystem.     

Queen polymorphism in the North American harvester ant,Ephebomyrmex imberbiculus

Summary In the North American harvester antEphebomyrmex imberbiculus, in addition to dealate queens, wingless female reproductives occur that have greatly reduced ocelli and thoracic sutures (intermorphic queens). Both queen types are equivalent in function, and do not differ in ovarian morphology. Colonies may contain several inseminated and egg-laying intermorphic queens. We discuss queen polymorphism in respect to the biology of this desert-dwelling species.