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.     

Genetic diversity is positively associated with fine‐scale momentary abundance of an invasive ant

Many introduced species become invasive despite genetic bottlenecks that should, in theory, decrease the chances of invasion success. By contrast, population genetic bottlenecks have been hypothesized to increase the invasion success of unicolonial ants by increasing the genetic similarity between descendent populations, thus promoting co‐operation. We investigated these alternate hypotheses in the unicolonial yellow crazy ant, Anoplolepis gracilipes, which has invaded Arnhem Land in Australia's Northern Territory. We used momentary abundance as a surrogate measure of invasion success, and investigated the relationship between A. gracilipes genetic diversity and its abundance, and the effect of its abundance on species diversity and community structure. We also investigated whether selected habitat characteristics contributed to differences in A. gracilipes abundance, for which we found no evidence. Our results revealed a significant positive association between A. gracilipes genetic diversity and abundance. Invaded communities were less diverse and differed in structure from uninvaded communities, and these effects were stronger as A. gracilipes abundance increased. These results contradict the hypothesis that genetic bottlenecks may promote unicoloniality. However, our A. gracilipes study population has diverged since its introduction, which may have obscured evidence of the bottleneck that would likely have occurred on arrival. The relative importance of genetic diversity to invasion success may be context dependent, and the role of genetic diversity may be more obvious in the absence of highly favorable novel ecological conditions.     

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.     

Invasive ants as back-seat drivers of native ant diversity decline in New Caledonia

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.     

Booms,busts and population collapses in invasive ants

The abundance of many invasive species can vary substantially over time, with dramatic population declines and local extinctions frequently observed in a wide range of taxa. We highlight population crashes of invasive ants, which are some of the most widespread and damaging invasive animals. Population collapse or substantial declines have been observed in nearly all of the major invasive ant species including the yellow crazy ant (Anoplolepis gracilipes), Argentine ants (Linepithema humile), big-headed or coastal brown ant (Pheidole megacephala), the tropical fire ant (Solenopsis geminata), red imported fire ants (Solenopsis invicta), and the little fire ant or electric ant (Wasmannia auropunctata). These declines frequently attract little attention, especially compared with their initial invasion phase. Suggested mechanisms for population collapse include pathogens or parasites, changes in the food availability, or even long-term effects of the reproductive biology of invasive ants. A critical component of the collapses may be a reduction in the densities of the invasive ant species, which are often competitively weak in low abundance. We propose that mechanisms causing a reduction in invasive ant abundance may initiate a local extinction vortex. Declines in abundance likely reduce the invasive ant’s competitive ability, resource acquisition and defense capability. These reductions could further reduce the abundance of an invasive ant species, and so on. Management of invasive ants through the use of pesticides is expensive, potentially ecologically harmful, and can be ineffective. We argue that pesticide use may even have the potential to forestall natural population declines and collapses. We propose that in order to better manage these invasive ants, we need to understand and capitalize on features of their population dynamics that promote population collapse.     

Elevated dominance of extrafloral nectary-bearing plants is associated with increased abundances of an invasive ant and reduced native ant richness

Aim Invasive ants can have substantial and detrimental effects on co‐occurring community members, especially other ants. However, the ecological factors that promote both their population growth and their negative influences remain elusive. Opportunistic associations between invasive ants and extrafloral nectary (EFN)‐bearing plants are common and may fuel population expansion and subsequent impacts of invasive ants on native communities. We examined three predictions of this hypothesis, compared ant assemblages between invaded and uninvaded sites and assessed the extent of this species in Samoa. Location The Samoan Archipelago (six islands and 35 sites). Methods We surveyed abundances of the invasive ant Anoplolepis gracilipes, other ant species and EFN‐bearing plants. Results Anoplolepis gracilipes was significantly more widely distributed in 2006 than in 1962, suggesting that the invasion of A. gracilipes in Samoa has progressed. Furthermore, (non‐A. gracilipes) ant assemblages differed significantly between invaded and uninvaded sites. Anoplolepis gracilipes workers were found more frequently at nectaries than other plant parts, suggesting that nectar resources were important to this species. There was a strong, positive relationship between the dominance of EFN‐bearing plants in the community and A. gracilipes abundance on plants, a relationship that co‐occurring ants did not display. High abundances of A. gracilipes at sites dominated by EFN‐bearing plants were associated with low species richness of native plant‐visiting ant species. Anoplolepis gracilipes did not display any significant relationships with the diversity of other non‐native ants. Main conclusions Together, these data suggest that EFN‐bearing plants may promote negative impacts of A. gracilipes on co‐occurring ants across broad spatial scales. This study underscores the potential importance of positive interactions in the dynamics of species invasions. Furthermore, they suggest that conservation managers may benefit from explicit considerations of potential positive interactions in predicting the identities of problematic invaders or the outcomes of species invasions.     

Habitat complexity facilitates coexistence in a tropical ant community

The role of habitat complexity in the coexistence of ant species is poorly understood. Here, we examine the influence of habitat complexity on coexistence patterns in ant communities of the remote Pacific atoll of Tokelau. The invasive yellow crazy ant, Anoplolepis gracilipes (Smith), exists in high densities on Tokelau, but still coexists with up to seven other epigeic ant species. The size-grain hypothesis (SGH) proposes that as the size of terrestrial walking organisms decreases, the perceived complexity of the environment increases and predicts that: (1) leg length increases allometrically with body size in ants, and (2) coexistence between ant species is facilitated by differential habitat use according to body size. Analysis of morphological variables revealed variation inconsistent with the morphological prediction of the SGH, as leg length increased allometrically with head length only. We also experimentally tested the ability of epigeic ants in the field to discover and dominate food resources in treatments of differing rugosity. A. gracilipes was consistently the first to discover food baits in low rugosity treatments, while smaller ant species were consistently the first to discover food baits in high rugosity treatments. In addition, A. gracilipes dominated food baits in planar treatments, while smaller ant species dominated baits in rugose treatments. We found that the normally predictable outcomes of exploitative competition between A. gracilipes and other ant species were reversed in the high rugosity treatments. Our results support the hypothesis that differential habitat use according to body size provides a mechanism for coexistence with the yellow crazy ant in Tokelau. The SGH may provide a mechanism for coexistence in other ant communities but also in communities of other terrestrial, walking insects that inhabit a complex landscape.Electronic Supplementary Material Supplementary material is available to authorised users in the online version of this article at .     

Do invasive ants respond more strongly to carbohydrate availability than co‐occurring non‐invasive ants? A test along an active Anoplolepis gracilipes invasion front

Invasions by non‐native insects can have important ecological impacts, particularly on island ecosystems. However, the factors that promote the success of invaders relative to co‐occurring non‐invasive species remain unresolved. For invasive ants, access to carbohydrate resources via interactions with both extrafloral nectary‐bearing plants and honeydew‐excreting insects may accelerate the invasion process. A first step towards testing this hypothesis is to determine whether invasive ants respond to variation in the availability of carbohydrate resources, and whether this response differs from that of co‐occurring, non‐invasive ants. We investigated the effect of carbohydrate subsidies on the short‐term foraging and hemipteran‐tending behaviours of the invasive ant Anoplolepis gracilipes (Formicidae) and co‐occurring ant species on an extrafloral nectary‐bearing plant by experimentally manipulating carbohydrate levels and tracking ant recruitment. We conducted experiments in 2 years at two sites: one site was invaded by A. gracilipes prior to 2007 and the other became invaded during the course of our study, allowing pre‐ (2007) and post‐invasion (2009) comparisons. Short‐term increases in carbohydrate availability increased the density of A. gracilipes workers on plants by as much as 400% and reduced tending of honeydew‐excreting insects by this species by up to 89%, with similar responses across years. In contrast, ants at the uninvaded site in 2007 showed a weak and non‐significant forager recruitment response. Across all sites, A. gracilipes workers were the only ants that responded to carbohydrate manipulations in 2009. Furthermore, ant–carbohydrate dynamics at a site newly invaded by A. gracilipes quickly diverged from dynamics at uninvaded sites and converged on those of the site with an established invasion. These findings suggest that carbohydrate resources may be particularly important for A. gracilipes invasions, and underscore the importance of species interactions, particularly putative mutualisms, in facilitating exotic species invasions.     

Invasive alien plants affect grassland ant communities, colony size and foraging behaviour

Ants are dominant members of many terrestrial ecosystems and are regarded as indicators of environmental changes. However, little is known about the effects of invasive alien plants on ant populations, particularly as regards the density, spatial distribution and size of ant colonies, as well as their foraging behaviour. We addressed these questions in a study of grassland ant communities on five grasslands invaded by alien goldenrods (Solidago sp.) and on five non-invaded grasslands without this plant. In each grassland, seven 100 m² plots were selected and the ant colonies counted. Ant species richness and colony density was lower in the plots on the invaded grasslands. Moreover, both of these traits were higher in the plots near the grassland edge and with a higher number of plant species in the grasslands invaded by goldenrods but not in the non-invaded ones. On average, ant colony size was lower on the invaded grasslands than the non-invaded ones. Also, ant workers travelled for longer distances to collect food items in the invaded areas than they did in the non-invaded ones, even after the experimental removal of some ant colonies in order to exclude the effect of higher colony density in the latter. Our results indicate that invasive alien goldenrods have a profound negative effect on grassland ant communities which may lead to a cascade effect on the whole grassland ecosystem through modification of the interactions among species. The invasion diminishes a major index of the fitness of ants, which is a colony’s size, and probably leads to increased foraging effort of workers. This, in turn, may have important consequences for the division of labour and reproductive strategies within ant colonies.     

Targeted Research to Improve Invasive Species Management: Yellow Crazy Ant Anoplolepis gracilipes in Samoa

Lack of biological knowledge of invasive species is recognised as a major factor contributing to eradication failure. Management needs to be informed by a site-specific understanding of the invasion system. Here, we describe targeted research designed to inform the potential eradication of the invasive yellow crazy ant Anoplolepis gracilipes on Nu’utele island, Samoa. First, we assessed the ant’s impacts on invertebrate biodiversity by comparing invertebrate communities between infested and uninfested sites. Second, we investigated the timing of production of sexuals and seasonal variation of worker abundance and nest density. Third, we investigated whether an association existed between A. gracilipes and carbohydrate sources. Within the infested area there were few other ants larger than A. gracilipes, as well as fewer spiders and crabs, indicating that A. gracilipes is indeed a significant conservation concern. The timing of male reproduction appears to be consistent with places elsewhere in the world, but queen reproduction was outside of the known reproductive period for this species in the region, indicating that the timing of treatment regimes used elsewhere are not appropriate for Samoa. Worker abundance and nest density were among the highest recorded in the world, being greater in May than in October. These abundance and nest density data form baselines for quantifying treatment efficacy and set sampling densities for post-treatment assessments. The number of plants and insects capable of providing a carbohydrate supply to ants were greatest where A. gracilipes was present, but it is not clear if this association is causal. Regardless, indirectly controlling ant abundance by controlling carbohydrate supply appears to be promising avenue for research. The type of targeted, site-specific research such as that described here should be an integral part of any eradication program for invasive species to design knowledge-based treatment protocols and determine assessment benchmarks to achieve eradication.     

Yellow crazy ants (<Emphasis Type="Italic">Anoplolepis gracilipes</Emphasis>) reduce numbers and impede development of a burrow-nesting seabird

Yellow crazy ants (Anoplolepis gracilipes) are a significant threat to biodiversity due to a rapidly expanding range and the potential to disrupt ecosystem interactions at multiple trophic levels. Extirpation of ground-nesting seabirds subsequent to yellow crazy ant invasion has been reported anecdotally. Yellow crazy ant control is difficult and resulting positive effects on nesting seabirds is undocumented. We report the effects of ant invasion and subsequent control on burrow-nesting seabirds following the invasion of more than half of a 1.25-ha wedge-tailed shearwater (Ardenna pacifica) colony located on eastern O‘ahu. The number of active seabird burrows in invaded areas dropped from 125 in 2006 to 6 in 2010, with no corresponding decline in active burrows in adjacent, uninvaded areas. Ant control efforts in 2011 reduced ant densities by more than 97% and resulted in a substantial increase in active burrows (43 in 2011). In invaded areas, burrows appeared to be abandoned by adults prior to egg-laying. Chicks surviving in invaded areas exhibited mild to severe developmental abnormalities, and overall had shorter culmens, tarsi and wingchords, smaller eye diameters, and lower weights than chicks outside invaded areas. We conclude that yellow crazy ants constitute a significant, and likely underestimated, risk to ground-nesting seabirds. Loss of seabird nesting colonies can have significant effects on nutrient inputs, and can bring about shifts in plant communities and faunal composition. Range expansion of yellow crazy ants is expected and ant/seabird interactions are likely to increase.     

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.     

Soil pathogen communities associated with native and non‐native Phragmites australis populations in freshwater wetlands

Soil pathogens are believed to be major contributors to negative plant–soil feedbacks that regulate plant community dynamics and plant invasions. While the theoretical basis for pathogen regulation of plant communities is well established within the plant–soil feedback framework, direct experimental evidence for pathogen community responses to plants has been limited, often relying largely on indirect evidence based on above‐ground plant responses. As a result, specific soil pathogen responses accompanying above‐ground plant community dynamics are largely unknown. Here, we examine the oomycete pathogens in soils conditioned by established populations of native noninvasive and non‐native invasive haplotypes of Phragmites australis (European common reed). Our aim was to assess whether populations of invasive plants harbor unique communities of pathogens that differ from those associated with noninvasive populations and whether the distribution of taxa within these communities may help to explain invasive success. We compared the composition and abundance of pathogenic and saprobic oomycete species over a 2‐year period. Despite a diversity of oomycete taxa detected in soils from both native and non‐native populations, pathogen communities from both invaded and noninvaded soils were dominated by species of Pythium. Pathogen species that contributed the most to the differences observed between invaded and noninvaded soils were distributed between invaded and noninvaded soils. However, the specific taxa in invaded soils responsible for community differences were distinct from those in noninvaded soils that contributed to community differences. Our results indicate that, despite the phylogenetic relatedness of native and non‐native P. australis haplotypes, pathogen communities associated with the dominant non‐native haplotype are distinct from those of the rare native haplotype. Pathogen taxa that dominate either noninvaded or invaded soils suggest different potential mechanisms of invasion facilitation. These findings are consistent with the hypothesis that non‐native plant species that dominate landscapes may “cultivate” a different soil pathogen community to their rhizosphere than those of rarer native species.     

The changing nature of plant–microbe interactions during a biological invasion

Invasive plant species can alter belowground microbial communities. Simultaneously, the composition of soil microbial communities and the abundance of key microbes can influence invasive plant success. Such reciprocal effects may cause plant–microbe interactions to change rapidly during the course of biological invasions in ways that either inhibit or promote invasive species growth. Here we use a space-for-time substitution to illustrate how effects of soil microbial communities on the exotic legume Vicia villosa vary across uninvaded sites, recently invaded sites, and sites invaded by V. villosa for over a decade. We find that soil microorganisms from invaded areas increase V. villosa growth compared to sterilized soil or live soils collected from uninvaded sites, likely because mutualistic nitrogen-fixing rhizobia are not abundant in uninvaded areas. Notably, the benefits resulting from inoculation with live soils were higher for soils from recently invaded sites compared to older invasions, potentially indicating that over longer time scales, soil microbial communities change in ways that may reduce the success of exotic species. These findings suggest that short-term changes to soil microbial communities following invasion may facilitate exotic legume growth likely because of increases in the abundance of mutualistic rhizobia, but also indicate that longer term changes to soil microbial communities may reduce the growth benefits belowground microbial communities provide to exotic species. Our results highlight the changing nature of plant–microbe interactions during biological invasions and illustrate how altered biotic interactions could contribute to both the initial success and subsequent naturalization of invasive legume species.     

Effects of invasive Typha × glauca on wetland nutrient pools,denitrification, and bacterial communities are influenced by time since invasion

Invasive plants dramatically shift the structure of native wetland communities. However, less is known about how they affect belowground soil properties, and how those effects can vary depending on time since invasion. We hypothesized that invasion of a wetland by a widespread invasive plant (Typha × glauca) would result in changes in soil nutrients, denitrification, and bacterial communities, and that these effects would increase with time since invasion. We tested these hypotheses by sampling Typha-invaded sites of different ages (~40, 20, and 13 years), a Typha-free, native vegetation site, and a restored site (previously invaded ~30–40 years ago) but that had Typha return within 2 years of the restoration. At each site, we measured Typha stem density, plant species richness, soil nutrients, denitrification rates, and the abundance and composition of bacterial denitrifier communities. All Typha-dominated sites had the least plant species richness regardless of time since invasion. Additionally, sites that were invaded the longest exhibited significantly higher concentrations of soil organic matter, nitrate, and ammonium than the native site. In contrast, denitrification was higher in sites invaded more recently. Denitrifier diversity for the nirS gene was also significantly different, with highest nirS diversity in sites invaded the longest. Interestingly, the denitrifier communities within the restored site were most similar to the ones in T. × glauca sites, suggesting a legacy effect. Our study suggests this invader can alter important ecosystem properties, such as native species richness, nutrient pools, and transformations, as well as bacterial community composition depending on time since invasion.     

Living at the edge of the front; reduced predation risk to invasive round goby in a Great Lakes tributary

The objective of this research was to determine if the highly invasive round goby (Neogobius melanostomus) experiences lower predation risk during early stages of invasion. We compared round goby predation rates between a recently invaded area (occupied for ≈1 year) and a longer established area (≈7 years) of the Trent River, Ontario, Canada. Tethering trials were conducted in three habitat types, and comparable habitats in the two areas were similar in water temperature, velocity and depth. Predation rates of tethered round gobies were on average 27% lower in the recently invaded area. Reduced predation in the recently invaded area may be due to the short duration of round goby occupancy and/or differences in predator communities between the two study areas. Data before the round goby invasion suggest that predator communities were similar between the two range areas, but differences in predator abundance cannot be ruled out as a potential mechanism. Other possible mechanisms include a numerical or learned response by predators over time to a novel prey item. Reduced predation rate during the initial stages of invasion may contribute to the fitness of individuals that migrate into areas not previously occupied, and thus facilitate successful range expansion.     

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.     

Overview of the Distribution,Habitat Association and Impact of Exotic Ants on Native Ant Communities in New Caledonia

Ants are among the most ubiquitous and harmful invaders worldwide, but there are few regional studies of their relationships with habitat and native ant communities. New Caledonia has a unique and diverse ant fauna that is threatened by exotic ants, but broad-scale patterns of exotic and native ant community composition in relation to habitat remain poorly documented. We conducted a systematic baiting survey of 56 sites representing the main New Caledonian habitat types: rainforest on ultramafic soils (15 sites), rainforest on volcano-sedimentary soils (13), maquis shrubland (15), Melaleuca-dominated savannas (11) and Acacia spirorbis thickets (2). We collected a total of 49 species, 13 of which were exotic. Only five sites were free of exotic species, and these were all rainforest. The five most abundant exotic species differed in their habitat association, with Pheidole megacephala associated with rainforests, Brachymyrmex cf. obscurior with savanna, and Wasmannia auropunctata and Nylanderia vaga present in most habitats. Anoplolepis gracilipes occurred primarily in maquis-shrubland, which contrasts with its rainforest affinity elsewhere. Multivariate analysis of overall ant species composition showed strong differentiation of sites according to the distribution of exotic species, and these patterns were maintained at the genus and functional group levels. Native ant composition differed at invaded versus uninvaded rainforest sites, in the absence of differences in habitat variables. Generalised Myrmicinae and Forest Opportunists were particularly affected by invasion. There was a strong negative relationship between the abundance of W. auropunctata and native ant abundance and richness. This emphasizes that, in addition to dominating many ant communities numerically, some exotic species, and in particular W. auropunctata, have a marked impact on native ant communities.     

Does Argentine ant invasion affect prey availability for foliage-gleaning birds?

Food availability during the breeding season plays a critical role in reproductive success of insectivorous birds. Given that the invasive Argentine ant (Linepithema humile) is known to alter arthropod communities, we predicted that its invasion may affect the availability of food resources for coexisting foliage-gleaning birds. With this aim we studied, for 3 years, foliage arthropods occurring on cork oaks (Quercus suber) and tree heaths (Erica arborea) in invaded and non-invaded secondary forests of the northeastern Iberian Peninsula. Our results show that Argentine ants interact with arboreal foliage arthropods in a different manner than the native ants they displace do. The invasive ant impacted the arthropod community by reducing order diversity and ant species richness and by causing extirpation of most native ant species. Arthropod availability for foliage gleaners’ nestlings diminished in invaded cork oaks, mainly responding to the abundance and biomass depletion of caterpillars. Results suggest that the reproduction of canopy-foraging foliage-gleaning species that mostly rely on caterpillars to feed their young could be compromised by the Argentine ant invasion. Thus, the Argentine ant could be promoting bottom-up effects in the trophic web through its effects on the availability of arthropod preys for insectivorous birds.