An invasion revisited: the African big-headed ant (<Emphasis Type="Italic">Pheidole megacephala</Emphasis>) in northern Australia

Long-term studies provide the best information for invasion ecology as multi-temporal sampling can illuminate invasion dynamics, such as population changes and rate of spread. In 1996, Hoffmann et al. surveyed an infestation of African big-headed ant, Pheidole megacephala, in semi-natural rainforest of northern Australia. Here, we re-survey this infestation 9 years later to assess the dynamics of this invasion against the key finding of the initial study. Importantly, we re-sample a site sampled prior to invasion to demonstrate the causal link between an infestation and changes in a native invertebrate community. We found the area infested had almost doubled, and P. megacephala biomass in infested sites was up to 18 times greater than that of native ants in uninfested sites. In the two sites with the youngest infestations in 1996, P. megacephala abundance had increased more than 20-fold than that measured in 1996. Native ant abundance and species richness in infested sites were notably lower in 2005, with only one native ant specimen found in the most recently infested site. The abundance of other macro-invertebrates was the lowest in the three oldest infested sites. Coleoptera and Orthoptera were less abundant in infested sites. This study supports the findings of the first ‘snapshot’ study, and has shown that the area infested has almost doubled, populations are increasing, and ecological impacts remain severe. It reinforces that P. megacephala represents a serious ecological threat, and we argue that there is a greater need for management of this ant globally.     

Ecological restoration following the local eradication of an invasive ant in northern Australia

There have been many management programs for invasive ants, yet few have achieved eradication. Of those that were successful, none have documented the subsequent recovery of the affected ecological system. Here I document the ecological impact and eradication of a 5 ha infestation of the African big headed ant Pheidole megacephala from an intact habitat in northern Australia, as well as the subsequent recovery of the native ant fauna. Pre-treatment, the impact of P. megacephala on the native ant fauna was clear. Native ant abundance and species richness were almost always significantly lower in infested compared to uninfested samples. Multivariate analysis statistically separated sample grids from infested and uninfested areas. Following treatment, no P. megacephala individuals were detected for 2 years and it was therefore declared eradicated. Ecological recovery post treatment was also clear. Twenty-one months post-treatment, native ant abundance and species richness within the treated (infested) area were always almost always significantly greater than in the pre-treatment sample, corresponding with no change in the control area (uninfested area). Total species richness from plots in the treated area was identical to that from plots in the control area. Multivariate analysis showed no statistical separation of the treated or control plots. Species richness within lure plots displayed no trend within the treated area relative to the treatment boundary or locations away from the treated area. This project demonstrates the feasibility of eradicating this ant, and that ecological systems are capable of recovering following removal of an exotic invader.     

Yellow crazy ant (<Emphasis Type="Italic">Anoplolepis gracilipes</Emphasis>) invasions within undisturbed mainland Australian habitats: no support for biotic resistance hypothesis

Ants are highly successful invaders, especially on islands, yet undisturbed mainland environments often do not contain invasive ants, and this observation is largely attributed to biotic resistance. An exception is the incursion of Yellow crazy ant Anoplolepis gracilipes within northeast Arnhem Land. The existence of A. gracilipes within this landscape’s intact environments containing highly competitive ant communities indicates that biotic resistance is not a terminally inhibitory factor mediating this ant’s distribution at the regional scale. We test whether biotic resistance may still operate at a more local scale by assessing whether ecological impacts are proportional to habitat suitability for A. gracilipes, as well as to the competitiveness of the invaded ant community. The abundance and species richness of native ants were consistently greater in uninfested than infested plots but the magnitude of the impacts did not differ between habitats. The abundance and ordinal richness of other macro-invertebrates were consistently lower in infested plots in all habitats. A significant negative relationship was found for native ant abundance and A. gracilipes abundance. No relationships were found between A. gracilipes abundance and any measure of other macro-invertebrates. The relative contribution of small ants (<2.5 mm) to total abundance and relative species richness was always greater in infested sites coinciding with a reduction of the contribution of the larger size classes. Differences in the relative abundance of ant functional groups between infested and uninfested sites reflected impacts according to ant size classes and ecology. The widespread scale of these incursions and non-differential level of impacts among the habitats, irrespective of native ant community competitiveness and abiotic suitability to A. gracilipes, does not support the biotic resistance hypothesis.     

Response of an open‐forest ant community to invasion by the introduced ant,Pheidole megacephala

The introduced tramp ant, Pheidole megacephala, is a well‐known pest of urban areas and coastal dune ecosystems in eastern Australia. Until recently, establishment and spread of P. megacephala colonies has been regarded as likely only in disturbed areas. Here we describe the extent of an established colony of P. megacephala in a long undisturbed open forest near Maryborough in southeast Queensland and compare ant community structure with those of nearby uninfested sites. Tuna baiting revealed three distinct zones: (i) a zone totally dominated by P. megacephala (at least 10 ha) where few other ant ant species were detected; (ii) a zone where P. megacephala was absent and many other ant species were found; and (iii) a zone where opportunists (species of Ochetellus and Paratrechina) competed with P. megacephala at baits. Pitfall trapping over a 9‐month period resulted in 12 species being recorded at the infested site, compared with a mean of 25 species recorded at adjoining uninfested forest. Over 94% of ants recorded in pitfalls at the infested site were P. megacephala. Most notably, P. megacephala had completely displaced dominant Dolichoderines (species of Iridomyrmex), subordinate Camponotini (species of Camponotus, Opisthopsis and Polyrhachis) and other species of Pheidole which are common at forest sites.     

Invasive African big-headed ants, Pheidole megacephala, on coral cays of the southern Great Barrier Reef: distribution and impacts on other ants

Infestation of islands by exotic ants is widespread and increasing due to human activities throughout the world. Exotic ants, particularly the invasive African big-headed ant, Pheidole megacephala (Fabricius), are of great conservation concern for coral cays at the southern end of the Great Barrier Reef, Australia. Little is known, however, about the distribution and ecological impacts of invasive ants in this insular system. We surveyed the ants of 14 vegetated coral cays recording a total of 24 ant species, including at least nine exotics. Pheidole megacephala was by far the most abundant and widespread species, occurring on 11 of 14 islands, often in very large numbers. The inter-island distribution of P. megacephala was best explained by human activities, with frequently visited, and to a lesser degree disturbed islands, more likely to be infested. On large islands (≥?10?ha) P. megacephala exhibited distinct habitat preferences, occurring in significantly lower abundances within heavily-shaded Pisonia grandis forest in the centre of islands, compared to more open, fringing woodland or shrubland. On smaller islands (<10?ha) with less extensive Pisonia stands, P. megacephala penetrated throughout the forest where its abundance was similar to that in open woodland. Despite considerable differences in biotic (floristic composition) and abiotic factors (e.g. island size) as well as the spatial configuration among islands, the severity of infestation by P. megacephala best explained variation in species richness, abundance and assemblage composition of other ants. We suggest a number of strategies to manage P. megacephala infestations on these islands.     

Factors governing rate of invasion: a natural experiment using Argentine ants

Predicting the success of biological invasions is a major goal of invasion biology. Determining the causes of invasions, however, can be difficult, owing to the complexity and spatio-temporal heterogeneity of the invasion process. The purpose of this study was to assess factors influencing rate of invasion for the Argentine ant (Linepithema humile), a widespread invasive species. The rate of invasion for 20 independent Argentine ant populations was measured over 4 years in riparian woodlands in the lower Sacramento River Valley of northern California. A priori predictors of rate of invasion included stream flow (a measure of abiotic suitability), disturbance, and native ant richness. In addition, baits were used to estimate the abundance of Argentine ants and native ants at the 20 sites. A multiple regression model accounted for nearly half of the variation in mean rate of invasion (R ² = 0.46), but stream flow was the only significant factor in this analysis. Argentine ants spread, on average, 16 m year⁻¹ at sites with permanent stream flow and retreated, on average, −6 m year⁻¹ at sites with intermittent stream flow. Rate of invasion was independent of both disturbance and native ant richness. Argentine ants recruited to more baits in higher numbers in invaded areas than did native ants in uninvaded areas. In addition, rate of invasion was positively correlated with the proportion of baits recruited to by native ants in uninvaded areas. Together, these findings suggest that abiotic suitability is of paramount importance in determining rate of invasion for the Argentine ant. Received: 16 September 1997 / Accepted: 8 February 1998     

Effect of Argentine ant invasions on ground-dwelling arthropods in northern California riparian woodlands

Although the Argentine ant (Linepithema humile) is a widespread invasive species that displaces native ants throughout its introduced range, the effects of these invasions on arthropods other than ants remain poorly known. This study documents the consequences of Argentine ant invasions on ants and other ground-dwelling arthropods in northern California riparian woodlands. Baits and unbaited pitfall traps were used to sample different components of the arthropod communities at five pairs of uninvaded and invaded sites. Sites occupied by Argentine ants supported almost no native epigeic ants except for the winter-active Prenolepis imparis. Sites with Argentine ants averaged four to ten times more ant workers than did sites with native ants, but ant worker biomass did not differ between uninvaded and invaded sites. Argentine ants recruited to baits in invaded areas, on average, in less than half the time of native ants in uninvaded areas. Despite the loss of epigeic native ants, higher Argentine ant worker abundance, and faster recruitment by Argentine ants at invaded sites, pitfall trap samples from uninvaded and invaded areas contained similar abundances and diversities of non-ant arthropods. These findings suggest that Argentine ants and the native ants they displace interact with the ground-dwelling arthropods of these habitats in a similar manner. Received: 24 February 1997 / Accepted: 9 November 1997     

Limiting Spread of a Unicolonial Invasive Insect and Characterization of Seasonal Patterns of Range Expansion

Limiting dispersal is a fundamental strategy in the control of invasive species, and in certain situations containment of incipient populations may be an important management technique. To test the feasibility of slowing the rapid spread of two Argentine ant (Linepithema humile) supercolonies in Haleakala National Park, Hawaii, we applied ant bait and toxicant within an experimental plot situated along a supercolony boundary. The 120×260 m plot simulated a small section of what could potentially be a 120 m wide treatment encompassing the entire expanding boundaries of both supercolonies. Foraging ant numbers at baited monitoring stations decreased sharply within two weeks after treatment, and ant spread was completely halted within the plot for at least one year. In contrast, an adjacent untreated colony boundary advanced an average of 65.2 m over the course of 1 year. Most of this spread took place in the summer and fall, at the time of highest ant abundance at bait monitoring stations, while no outward dispersal occurred during the spring and early summer. These patterns are consistent with the hypothesis that local budding dispersal in this unicolonial species stems from density dependent pressure rather than inherent founding behavior associated with mating. Based on results from this experiment, we are investigating the effectiveness of annual boundary treatments in slowing the Argentine ant invasion at Haleakala National Park. The goals of this program are to protect populations of native arthropods and to keep options open for eventual attempts at eradication. This revised version was published online in July 2006 with corrections to the Cover Date.     

Impact of a mutualism between an invasive ant and honeydew-producing insects on a functionally important tree on a tropical island

Mutualisms between invasive ants and honeydew-producing Hemiptera have the potential to result in unusually high population levels of both partners, with subsequent major changes to ecosystem composition and dynamics. We assessed the relationship between the invasive ant, Pheidole megacephala, and its hemipteran mutualists, Dysmicoccus sp. and Pulvinaria urbicola, on Cousine Island, Seychelles. We also assessed the impacts of the mutualism on the condition of the hemipteran host plant, Pisonia grandis, a native and functionally important tree species. There was a strong positive relationship between Ph. megacephala activity and hemipteran abundance, and the exclusion of ants from Pi. grandis resulted in a significant decline in Pu. urbicola abundance. High abundance of the mutualists was strongly associated with damage to the Pi. grandis forest. This indicates that the mutualism is contributing to the massive increase in the population levels of the mutualist species, and is intensifying their impacts on the island. The widespread trophobiosis and its associated high densities of mutualists pose serious threats to the ecosystem, highlighting the need to control the ant and associated hemipteran populations.     

Impacts of the invasive Argentine ant on native ants and other invertebrates in coastal scrub in south‐eastern Australia

Invasive ants threaten native biodiversity and ecosystem function worldwide. Although their principal direct impact is usually the displacement of native ants, they may also affect other invertebrates. The Argentine ant, Linepithema humile (Dolichoderinae), one of the most widespread invasive ant species, has invaded native habitat where it abuts peri‐urban development in coastal Victoria in south‐eastern Australia. Here we infer impacts of the Argentine ant on native ants and other litter and ground‐dwelling invertebrates by comparing their abundance and taxonomic composition in coastal scrub forest either invaded or uninvaded by the Argentine ant. Species composition of native ants at bait stations and extracted from litter differed significantly between Argentine ant‐invaded and uninvaded sites and this was consistent across years. Argentine ants had a strong effect on epigeic ants, which were either displaced or reduced in abundance. The native ant Rhytidoponera victoriae (Ponerinae), numerically dominant at uninvaded sites, was completely absent from sites invaded by the Argentine ant. However, small hypogeic ants, including Solenopsis sp. (Myrmicinae) and Heteroponera imbellis (Heteroponerinae), were little affected. Linepithema humile had no detectable effect upon the abundance and richness of other litter invertebrates. However, invertebrate group composition differed significantly between invaded and uninvaded sites, owing to the varied response of several influential groups (e.g. Collembola and Acarina). Floristics, habitat structure and measured environmental factors did not differ significantly between sites either invaded or uninvaded by Argentine ants, supporting the contention that differences in native ant abundance and species composition are related to invasion. Changes in the native ant community wrought by Argentine ant invasion have important implications for invertebrate communities in southern Australia and may affect key processes, including seed dispersal.     

Exotic thistles increase native ant abundance through the maintenance of enhanced aphid populations

Exotic species change the structure and composition of invaded communities in multiple ways, but the sign of their impact on native species is still controversial. We evaluated the effects of the thistles Carduus thoermeri and Onopordum acanthium—two of the most abundant exotic plant species in disturbed areas of the Patagonian steppe—on the native tending ant assemblage. Exotic thistles showed an increased number of plants with aphids and had greater aphid density than native plants. Since native tending ants were present only in plants with aphids, their abundance was higher in infested thistles than in native plants. Path analyses confirmed that ant activity depended more on aphid density than on thistle traits. Our results suggest that the presence of exotic thistles in disturbed areas of NW Patagonia indirectly benefit the native ant assemblage through the maintenance of an increased aphid population. This illustrates how the impact of exotic on native species can depend on the ecological context.     

Non-native plantation forests as alternative habitat for native forest beetles in a heavily modified landscape

The once extensive native forests of New Zealand’s central North Island are heavily fragmented, and the scattered remnants are now surrounded by a matrix of exotic pastoral grasslands and Pinus radiata plantation forests. The importance of these exotic habitats for native biodiversity is poorly understood. This study examines the utilisation of exotic plantation forests by native beetles in a heavily modified landscape. The diversity of selected beetle taxa was compared at multiple distances across edge gradients between each of the six possible combinations of adjacent pastoral, plantation, clearfell and native forest land-use types. Estimated species richness (Michaelis–Menten) was greater in production habitats than native forest; however this was largely due to the absence of exotic species in native forest. Beetle relative abundance was highest in clearfell-harvested areas, mainly due to colonisation by open-habitat, disturbance-adapted species. More importantly, though, of all the non-native habitats sampled, beetle species composition in mature P. radiata was most similar to native forest. Understanding the influence of key environmental factors and stand level management is important for enhancing biodiversity values within the landscape. Native habitat proximity was the most significant environmental correlate of beetle community composition, highlighting the importance of retaining native remnants within plantation landscapes. The proportion of exotic beetles was consistently low in mature plantation stands, however it increased in pasture sites at increasing distances from native forest. These results suggest that exotic plantation forests may provide important alternative habitat for native forest beetles in landscapes with a low proportion of native forest cover.     

Brood parasitism by mites (Uropodidae) in an invasive population of the pest-ant Pheidole megacephala

We report that in Okinawa Island (southern Japan) the pupae of the invasive ant Pheidole megacephala were parasitized by an undescribed Uropodidae mite species. Our observations suggest that by sucking the hemolymph of the ant pupae during its own development, mites induce some conspicuous morphological changes and the death of parasitized P. megacephala pupae. Of the 75 collected nests, 69 (92%) were infested by the mite species. The prevalence of parasitism varied strongly among the worker and sexual castes, with the soldier and male pupae being the most attacked, followed by the minor and queen pupae. Our data represent the first case of such high parasitism in an invasive ant population. Received 2 August 2005; revised 29 September 2005; accepted 7 November 2005.     

The interaction between the coastal brown ant,Pheidole megacephala (Fabricius), and other invertebrate fauna of Mt Coot-tha (Brisbane,Australia)

Abstract Sampling of invertebrate fauna was carried out on Mt Coot-tha, between 25 February and 11 March 1992, to determine whether the presence of the coastal brown ant, Pheidole megacephala (Fabricius), affected die abundance and diversity of other invertebrates. Four plots were sampled; in two the coastal brown ant was present and in two it was absent. The overall abundance of invertebrates did not differ significantly between plots. The abundance of Collembola did vary significantly, but intraplot variability was large. Mite numbers did not differ significantly between plots, except for the family Laelapidae. Abundance of other ant species did not differ significantly between plots, but there was a difference in species composition; Dolichoderinae and Ponerinae were absent or poorly represented in plots where the coastal brown ant was present.     

Foraging activity of an Amazonian leaf-cutting ant: responses to changes in the availability of woody plants and to previous plant damage

This study investigates some factors affecting the foraging activity of the leaf-cutting ant Atta laevigata. The study was conducted on an abandoned farm near Manaus, in Brazilian Amazonia, where forest was beginning to regenerate. I determined how temporal changes in the structure of the woody plant community (linked to the regeneration process) affected the leaf harvesting activity of this Neotropical ant. A 0.39-ha plot was established, and woody plants existing and emerging in this plot were identified, tagged, measured and mapped. At intervals of approximately 14 days all marked plants were checked to determine which had been attacked by A. laevigata during that period. Woody plant density doubled over the 18-months of the study. However, the number of plants attacked by A. laevigata, controlling for seasonal differences in ant activity, was independent of the number available for attack. The number of plants attacked was also independent of the number of new plants emerged in the previous 2 months. Apart from changes in total plant density, the relative abundance of most species also changed with time. Data on the composition of the plant community and on the composition of the ants' diet at different time intervals were subject to ordination analysis. The analyses revealed that successional changes in the composition of the woody plant community did significantly affect the composition of the leaf diet of A. laevigata. Variations in the ants' diet correlated most strongly with variations in the abundance of Bellucia imperialis. As this increased in abundance its relative contribution to the ants' diet increased. In addition, I observed a small decline in the diversity of plant resources exploited by the ants. B. imperialis was one of the species most preferred by A. laevigata, suggesting that variations in the diversity of its diet were related to the abundance of preferred species. The chances of an individual plant being attacked was independent of whether or not the plant had been attacked during the previous 5 months. This suggests that plant resistance against ant herbivory is not affected by previous damage. Two other lines of evidence also pointed to this. First, there was no significant difference in the leaf area cut (as measured during assays of leaf preference with A. laevigata) between leaves of plants that had been subject to artificial defoliation 3 mo earlier and those of control plants. Second, the number of attacks by A. laevigata on individuals of a given species usually followed a random distribution, even though a uniform or clumped distribution would have been expected under the hypothesis that damage by leaf-cutting ants affects the plant's resistance to further damage. Received: 3 March 1997 / Accepted: 23 June 1997     

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.     

Seeds and seedlings of two neotropical montane understory shrubs respond differently to anthropogenic edges and treefall gaps

Edges resulting from forest clear-cutting and treefall gaps can affect plant populations and consequently the distribution of species across landscapes. These two types of disturbance might interact to exacerbate or ameliorate “edge effects”, a rarely tested possibility. We focused on the effects of distance from forest edge (0–10, 30–40, 60–70, and 190–200 m) and habitat within forest fragments (treefall gaps and intact forest) on the early stages of development of Palicourea gibbosa and Faramea affinis, two common shrubs of montane forests in southwest Colombia. Seed germination and seedling growth did not change with distance from forest edge. Within forest fragments, however, seed germination and seedling growth were higher in treefall gaps than in intact forest understory for both species. In contrast, seed predation was influenced by distance from forest edge and in P gibbosa it depended on habitat. Seed predation was highest in the forest interior (190–200 m from forest edge) and in P. gibbosa this was true only in treefall gap habitats. These results suggest that animal mediated processes such as post-dispersal seed predation are more likely than physiological processes to be affected by anthropogenic edges. Our results provide some evidence that treefall gaps may interact with “edge effects”, however, they are inconclusive as to whether they exacerbate or ameliorate them. Received: 31 August 1998 / Accepted: 18 February 1999     

Resource competition assays between the African big‐headed ant,Pheidole megacephala (Fabricius) and the invasive Argentine ant,Linepithema humile (Mayr): mechanisms of inter‐specific displacement

1. The spread of Argentine ants, Linepithema humile (Mayr), in introduced areas is mainly through the displacement of native ant species owing to high inter‐specific competition. In South Africa, L. humile has not established in the climatically suitable eastern and northern escarpments dominated by the African big headed ant, Pheidole megacephala (Fabricius), probably owing to local biotic resistance. 2. Inter‐specific aggression, at the individual and colony level, and competition for a shared resource were evaluated in the laboratory. 3. Aggression between the two ant species was very high in all of the assays. Both species suffered similar mortality rates during one‐on‐one aggression assays, however, during symmetrical group confrontations, L. humile workers showed significantly higher mortality rates than P. megacephala workers. During asymmetrical group confrontations both species killed more of the other ant species when they had numeric advantage. Both ant species located the shared resource at the same time; however, once P. megacephala discovered the bait, they displaced L. humile from the bait through high inter‐specific aggression, thereafter dominating the bait for the remainder of the trial. 4. The results demonstrate the potential of P. megacephala to prevent the establishment and survival of incipient L. humile colonies through enhanced resource competition and high inter‐specific aggression. This is the first study to indicate potential biotic resistance to the spread of L. humile in South Africa.     

Monopolization of litter processing by a dominant land crab on a tropical oceanic island

Litter processing by macroinvertebrates typically involves suites of species that act together to determine rates of breakdown and decomposition. However, tropical oceanic islands and coastal fringes on continents are often dominated by one or a few species of omnivorous land crabs that consume leaf litter. We used an exclusion experiment, together with other leaf removal and litter decomposition studies, to assess the role of a single dominant species, the red crab (Gecarcoidea natalis), in litter dynamics in rain forest on Christmas Island, Indian Ocean. In the presence of red crabs, litter cover and biomass varied seasonally, from almost complete cover and high biomass at the end of the dry season to almost total absence of litter at the end of each wet season. When crabs were excluded from both the shaded understory and light gaps in rain forest, litter increased rapidly to almost complete cover, which was then maintained year round. Leaf tether experiments, and measures of litter input and standing crops, indicated that red crabs monopolize litter processing, removing between 39 and 87% of the annual leaf fall from the forest floor. Rates of litter turnover were over twice as high in the presence of land crabs: the decomposition constant, k, was 2.6 in the understory exclusion plots, but rose to 6.0 in the presence of crabs. Red crabs occur at biomass densities (114 g m⁻²) far greater than those reported elsewhere for entire litter faunas. They significantly reduced the abundance of other litter invertebrates, but we did not detect any change in the relative frequencies of the major invertebrate groups (mites, collembolans, pulmonate snails, ants, psocopterans, and spiders). Wherever omnivorous land crabs are abundant, their activities may be paramount in litter decomposition and in regulating the rate of nutrient cycling. In monopolizing litter processing, they may also be important physical “ecosystem engineers”, translocating organic matter and nutrients into the soil and reducing available habitat for other animals. Received: 19 August 1998 / Accepted: 11 January 1999     

Immediate impacts of invasion by Wasmannia auropunctata (Hymenoptera: Formicidae) on native litter ant fauna in a New Caledonian rainforest

Abstract For the last 30 years, Wasmannia auropunctata (the little fire ant) has spread throughout the Pacific and represents a severe threat to fragile island habitats. This invader has often been described as a disturbance specialist. Here we present data on its spread in a dense native rainforest in New Caledonia. We monitored by pitfall trapping the litter ant fauna along an invasive gradient from the edge to the inner forest in July 1999 and March 2000. When W. auropunctata was present, the abundance and richness of native ants drops dramatically. In invaded plots, W. auropunctata represented more than 92% of all trapped ant fauna. Among the 23 native species described, only four cryptic species survived. Wasmannia auropunctata appears to be a highly competitive ant that dominates the litter by eliminating native ants. Mechanisms involved in this invasive success may include predation as well as competitive interactions (exploitation and interference). The invasive success of W. auropunctata is similar to that of other tramp ants and reinforces the idea of common evolutionary traits leading to higher competitiveness in a new environment.