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.     

Population structure and intraspecific aggression in the invasive ant species Anoplolepis gracilipes in Malaysian Borneo

Invasive species are one of the main sources of the ongoing global loss of biodiversity. Invasive ants are known as particularly damaging invaders and their introductions are often accompanied by population-level behavioural and genetic changes that may contribute to their success. Anoplolepis gracilipes is an invasive ant that has just recently received increased attention due to its negative impact on native ecosystems. We examined the behaviour and population structure of A. gracilipes in Sabah, Malaysia. A total of 475 individuals from 24 colonies were genotyped with eight microsatellite markers. Intracolonial relatedness was high, ranging from 0.37 to 1 (mean +/- SD: 0.82 +/- 0.04), while intercolonial relatedness was low (0.0 +/- 0.02, range -0.5-0.76). We compared five distinct sampling regions in Sabah and Brunei. A three-level hierarchical F-analysis revealed high genetic differentiation among colonies within the same region, but low genetic differentiation within colonies or across regions. Overall levels of heterozygosity were unusually high (mean H(O) = 0.95, mean H(E) = 0.71) with two loci being entirely heterozygous, indicating an unusual reproductive system in this species. Bioassays revealed a negative correlation between relatedness and aggression, suggesting kinship as one factor facilitating supercolony formation in this species. Furthermore, we genotyped one individual per nest from Sabah (22 nests), Sarawak (one nest), Brunei (three nests) and the Philippines (two nests) using two mitochondrial DNA markers. We found six haplotypes, two of which included 82.1% of all sequences. Our study shows that the sampled area in Sabah consists of a mosaic of differently interrelated nests in different stages of colony establishment. While some of the sampled colonies may belong to large supercolonies, others are more likely to represent recently introduced or dispersed propagules that are just beginning to expand.     

Characterization of microsatellite markers for the invasive ant species Anoplolepis gracilipes

We present primer sequences for eight polymorphic microsatellite loci for the formicine ant Anoplolepis gracilipes, a serious pest species in South‐East Asia and Pacific islands and still spreading on all continents. Microsatellite loci were isolated with a highly efficient method of enrichment. The number of alleles ranged from two to 19 with an observed heterozygosity ranging from 0.842 to 1.0. The markers were designed for a sociogenetic study as well as for population genetics.     

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.     

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

The introduced yellow crazy ant or long-legged ant Anoplolepis gracilipes was first reported in Seychelles in 1969 and now occurs on at least nine islands in the Central Seychelles. We describe the yellow crazy ant's effects on vegetation and invertebrate communities on one of these, Bird Island; in 2000, Anoplolepis (first reported in 1991) occurred there at densities at least 80 times higher than on other islands in the Central Seychelles. They were associated with high densities of coccid scale insects on foliage, especially of the native tree Pisonia grandis, in some instances causing tree death. Yellow crazy ants on Bird Island also significantly affected invertebrate communities on foliage and on the ground, both in terms of taxonomic composition and the density of specific taxa, apparently causing the local exclusion of some invertebrates.     

Supercolony mosaics: two different invasions by the yellow crazy ant, Anoplolepis gracilipes, on Christmas Island, Indian Ocean

Invasive species are one of the main reasons for the ongoing global loss of biodiversity. Anoplolepis gracilipes is an invasive ant that has recently received significant attention due to its negative effect on the native fauna and flora of Christmas Island, Indian Ocean. This species has contributed to a drastic change in the structure of the Christmas Island rainforest through its negative impact on the island’s endemic red land crab, the dominant consumer on the islands forest floor. In this study, we investigate the population structure of A. gracilipes on Christmas Island in order to determine whether multiple introductions occurred on the island and how they correspond to known infestations. We genotyped 578 individuals collected from 50 nests across the Island. We identify two distinct subgroups in the population that represent two different supercolonies. These supercolonies are interspersed across the island, however both nuclear (microsatellites) and mitochondrial markers strongly suggest that there is no gene flow between the two colonies. Significant heterozygote excess within the entire sampling area, with all but one worker examined being heterozygous for all seven microsatellite loci, suggests an unusual reproductive system in these ants. Our results are consistent with recent sociogenetic findings in a population of A. gracilipes in Northern Borneo.     

Discovery of independent‐founding solitary queens in the yellow crazy ant Anoplolepis gracilipes in East Java,Indonesia (Hymenoptera: Formicidae)

We collected four solitary queens of the invasive ant Anoplolepis gracilipes under stones in East Java, Indonesia. They produced nanitic workers by claustral colony foundation. This is the first report of independent colony foundation by queens in this species. The discovery may give an important insight into discussion on the origin of this invasive ant.     

Prevalence and genetic diversity of three bacterial endosymbionts (Wolbachia, Arsenophonus, and Rhizobiales) associated with the invasive yellow crazy ant (Anoplolepis gracilipes)

Invasive species carry pathogens, parasites and mutualistic microorganisms into their new range. We examined the prevalence and genetic diversity of three bacterial endosymbionts (Wolbachia, Arsenophonus, and Rhizobiales) in four Anoplolepis gracilipes (Smith) populations in the Pacific region, four populations from mainland Australia and one population from Christmas Island in the Indian Ocean. Wolbachia was found in eight of the nine sampling sites, with between 31.8 and 100% of ants infected. These infection rates are substantially higher than those previously observed for other invasive ants such as Linepithema humile and Solenopsis invicta. All sequences of Wolbachia were genetically identical. Arsenophonus was observed in five of the nine sampling sites, with infection rates ranging between 4.5 and 50.8%. Like Wolbachia, Arsenophonus can modify the sex-ratio of its hosts via male-killing. Arsenophonus was found to co-occur with Wolbachia in the same ants in five of the nine sampling sites. Rhizobiales is a clade of symbiotic bacteria mostly found in herbivorous ants. These bacteria help provide nitrogen to their hosts and were found in only three of the nine sampling sites with an infection rate of 1.6–11.8%. It also co-occurred with the other bacteria. There was no genetic variation in Arsenophonus and Rhizobiales samples, with the exception of a sequence from one Arsenophonus sample in Samoa that differed by a single base pair. These bacterial endosymbionts may contribute to the population variability in A. gracilipes and may be manipulated for the purpose of pest management.     

Genetic and behavioural evidence for a city-wide supercolony of the invasive Argentine ant Linepithema humile (Mayr) (Hymenoptera: Formicidae) in southeastern Australia

The success of invasive ants is frequently attributed to genetic and behavioural shifts in colony structure during or after introduction. The Argentine ant ( Linepithema humile ), a global invader, differs in colony genetic structure and behaviour between native populations in South America and introduced populations in Europe, Japan, New Zealand and North America. However, little is known about its colony structure in Australia. We investigated the genetic structure and behaviour of L. humile across Melbourne, Victoria by quantifying variation at four microsatellite loci and assaying intraspecific aggression at neighbourhood (30–200 m), fine (1–3.3 km) and regional (5–82 km) spatial scales. Hierarchical analyses across these scales revealed that most genetic variation occurred among workers within nests (∼98%). However, although low genetic differentiation occurred among workers between nests at the fine and regional scales (∼2%), negligible differentiation was detected among workers from neighbouring nests. Spatial genetic autocorrelation analysis confirmed that neighbouring nests were genetically more similar to each other. Lack of aggression within and across these scales supported the view that L. humile is unicolonial and forms a large supercolony across Melbourne. Comparisons of genetic structure of L. humile among single nests sampled from Adelaide, Brisbane, Hobart and Perth with Melbourne showed no greater levels of genetic differentiation or dissimilar spatial structure, suggesting an Australia-wide supercolony.     

The role of abiotic conditions in shaping the long-term patterns of a high-elevation Argentine ant invasion

Analysis of long‐term patterns of invasion can reveal the importance of abiotic factors in influencing invasion dynamics, and can help predict future patterns of spread. In the case of the invasive Argentine ant (Linepithema humile), most prior studies have investigated this species’ limitations in hot and dry climates. However, spatial and temporal patterns of spread involving two ant populations over the course of 30 years at a high elevation site in Hawaii suggest that cold and wet conditions have influenced both the ant's distribution and its rate of invasion. In Haleakala National Park on Maui, we found that a population invading at lower elevation is limited by increasing rainfall and presumably by associated decreasing temperatures. A second, higher elevation population has spread outward in all directions, but rates of spread in different directions appear to have been strongly influenced by differences in elevation and temperature. Patterns of foraging activity were strongly tied to soil temperatures, supporting the hypothesis that variation in temperature can influence rates of spread. Based on past patterns of spread, we predicted a total potential range that covers nearly 50% of the park and 75% of the park's subalpine habitats. We compared this rough estimate with point predictions derived from a degree‐day model for Argentine ant colony reproduction, and found that the two independent predictions match closely when soil temperatures are used in the model. The cold, wet conditions that have influenced Argentine ant invasion at this site are likely to be influential at other locations in this species’ current and future worldwide distribution.     

Spatial Characteristics of the Invasion of Acer platanoides on a Temperate Forested Island

We examined the spatial pattern of an introduced population of Norway maple (Acer platanoides L.) on a temperate forested island in order to quantify the influence of landscape context on invasion pattern. The spatial location of every Norway maple tree and sapling (≥0.5 m tall) that had invaded the island forest (n = 4496) was mapped using a global positioning system. The influence of landscape context was examined with the aid of a geographic information system and indices of spatial association. We found that the coniferous forest type was the most heavily invaded (71.9% of all Norway maple stems) when compared to either the hardwood or mixed conifer–hardwood forest types (5.4% and 19.3%, respectively). Across all forest types (excluding urban trees), the population was highly aggregated around roads and other Norway maple trees. For example, 90% of the population was within 40.8 m of a road with an average distance from road of 21.02 ± 0.40 m. This association around roads was significantly greater than would be predicted by chance alone (P < 0.001). Similarly, nearest neighbor distances averaged 4.5 ± 0.2 m with 90% of individuals within 8.3 m of another Norway maple. Measures of spatial association indicated that the invasion was significantly aggregated at both the stand and island scale. Nevertheless, a comparatively small but potentially influential set of individuals were observed at relatively long distances from the main invasion front. Ramifications of these disjunct establishments and other observed patterns are discussed in the context of current spread pattern theory, invasive species monitoring, and control efforts.     

Observations on the Reproductive Biology of Miconia calvescens DC (Melastomataceae), an Alien Invasive Tree on the Island of Tahiti (South Pacific Ocean)1

Miconia calvescens DC (Melastomataceae) is a dominant invasive species in the tropical oceanic island of Tahiti (French Polynesia, South Pacific Ocean), where it was introduced as an ornamental plant. Whereas this small tree is sparse in its native range of Central America, it has spread in Tahiti into a wide variety of habitats including native wet forests. The remarkable success of this invasion is due in great part to prolific reproduction: a mature tree can bear up to 220 inflorescences with an average of 1330 flowers/inflorescence, 208 fruits/infructescence and 195 seeds/fruit. Two and a half years of phenological observations in a highly invaded site indicated that three major peaks of flowering occur/year over brief periods: flower anthesis lasted a few days and pollen grain germination suggested a short stigmatic receptivity of only a few hours; no pollinators were observed foraging on flowers during our survey; the production of fruits containing viable seeds in bagged inflorescences showed that self-fertilization can occur; pollen-ovule ratio (log P/O = 2.68) suggested facultative xenogamy; bagged isolated flowers to test for autogamy and style cutting to learn whether apomixis occured or not were not conclusive. The flowering phenology and the breeding system of M. calvescens enable this plant to build up rapidly successful populations from even a single propagule on the island of Tahiti and on other sites of introduction. The vegetation structure of Polynesian native forests compared to Neotropical rain forests probably plays an important role in determining the reproductive success of M. calvescens and could provide a complementary explanation of the biological invasion processes in tropical oceanic islands.     

Red crabs in rainforest on Christmas Island,Indian Ocean: No effect on the soil seed bank

Red land crabs (Gecarcoidea natalis) are both abundant and widespread in intact rainforest on Christmas Island, Indian Ocean, occurring at densities of ~ 1 crab m^–2. We used an on-going exclusion experiment to test the hypothesis that through their activities as seed predators and soil disturbers, these omnivores are important determinants of the density and relative species abundance of seeds in the soil seed bank of undisturbed primary rainforest. After 6.5 y of continuous crab exclusion, there was no significant difference in the density of seeds in the top 3 cm of soil between the control (3671 ± 517 seeds/0.19 m²) and exclusion plots (3285 ± 801 seeds/0.19 m²), nor was there any significant treatment difference in relative species composition. There were also no significant treatment differences when Muntingia calabura, which accounted for 91.9% of all seeds in the seed bank, was excluded from the analyses. We attribute the lack of a significant impact of red crabs to their probable inability to handle the very small seeds which occurred in the seed bank, and the low level of soil disturbance at the study site. We conclude that the observed impact of red crabs on the abundance and diversity of successional species in natural light gaps on Christmas Island is due to their impact on germinating seedlings alone, rather than any additional impact on the soil seed bank.     

The origin of Rattus rattus on the Îles Éparses,Western Indian Ocean

Tollenaere et al. (Journal of Biogeography, 2010, 37 , 398–410) present a phylogeographic analysis of Rattus rattus for the Western Indian Ocean, with particular emphasis on Madagascar, but do not include samples from three island groups centrally located in the Mozambique Channel. Haplotypes from these islands provide additional information on the colonization pathways of R. rattus in the Western Indian Ocean region. For each of the three Îles Éparses groups in the Mozambique Channel, we test the competing hypotheses that colonization by R. rattus was most likely: (1) from the Arabian Peninsula, (2) from East Africa, (3) from Madagascar, or (4) from independent shipping. These results are combined with historical observations of the presence of R. rattus on these islands to give stronger inference on the colonization pathways. Additionally, more accurate colonization dates provide guidance for contemporary conservation management.     

Phylogeography of the introduced species Rattus rattus in the western Indian Ocean,with special emphasis on the colonization history of Madagascar

Aim To describe the phylogeographic patterns of the black rat, Rattus rattus, from islands in the western Indian Ocean where the species has been introduced (Madagascar and the neighbouring islands of Réunion, Mayotte and Grande Comore), in comparison with the postulated source area (India). Location Western Indian Ocean: India, Arabian Peninsula, East Africa and the islands of Madagascar, Réunion, Grande Comore and Mayotte. Methods Mitochondrial DNA (cytochrome b, tRNA and D‐loop, 1762 bp) was sequenced for 71 individuals from 11 countries in the western Indian Ocean. A partial D‐loop (419 bp) was also sequenced for eight populations from Madagascar (97 individuals), which were analysed in addition to six previously published populations from southern Madagascar. Results Haplotypes from India and the Arabian Peninsula occupied a basal position in the phylogenetic tree, whereas those from islands were distributed in different monophyletic clusters: Madagascar grouped with Mayotte, while Réunion and Grand Comore were present in two other separate groups. The only exception was one individual from Madagascar (out of 190) carrying a haplotype that clustered with those from Réunion and South Africa. ‘Isolation with migration’ simulations favoured a model with no recurrent migration between Oman and Madagascar. Mismatch distribution analyses dated the expansion of Malagasy populations on a time‐scale compatible with human colonization history. Higher haplotype diversity and older expansion times were found on the east coast of Madagascar compared with the central highlands. Main conclusions Phylogeographic patterns supported the hypothesis of human‐mediated colonization of R. rattus from source populations in either the native area (India) or anciently colonized regions (the Arabian Peninsula) to islands of the western Indian Ocean. Despite their proximity, each island has a distinct colonization history. Independent colonization events may have occurred simultaneously in Madagascar and Grande Comore, whereas Mayotte would have been colonized from Madagascar. Réunion was colonized independently, presumably from Europe. Malagasy populations may have originated from a single successful colonization event, followed by rapid expansion, first in coastal zones and then in the central highlands. The congruence of the observed phylogeographic pattern with human colonization events and pathways supports the potential relevance of the black rat in tracing human history.     

Effects of starvation on the fighting ability of invasive and autochthonous ants

Ants are widespread in all terrestrial habitats, and competitive interactions between species are common. Although redistribution of food within a colony may buffer the negative effects of temporary resource shortages, colony functionality can be compromised when famine is prolonged. One of the possible effects of famine is impairment of the fighting ability of species, with cascade effects on community. Here, we investigated whether food shortage alters the fighting ability of workers of three dominant species in the Mediterranean area: the invasive alien species, Lasius neglectus and Linepithema humile, and one highly polydomous autochthonous species belonging to the Tapinoma nigerrimum complex. We performed laboratory tests of interspecific one-on-one aggression and pairwise group contests between species, with all possible combinations of artificially satiated and starved groups. Results showed that starvation had a scarce effect on the individual aggressiveness in all three species. Similarly, the outcomes of the group fights were only lightly affected, but with an important exception. The positions of species in the fighting hierarchies were in most cases clear and linear, with L. neglectus at the top. However, we found that L. humile and L. neglectus showed equal mortality when one of the two species was starved and the other satiated. Although we investigated only one aspect of competition, that is, fighting ability, our results provide a piece of the complex jigsaw of competitive interactions of ants, and suggest that food deprivation can be a determinant that alters the relationships between ants and promotes or hampers the coexistence of dominant species.     

Effect of ant predation and abiotic factors on the mortality of medfly larvae,Ceratitis capitata,in the Argan forest of Western Morocco

The Argan Argania spinosa (L.) Skeels (Ericales: Sapotaceae) is an endemic and emblematic tree growing in the centre and the southwest of Morocco. Argan fruits are frequently infested by the Mediterranean fruit fly Ceratitis capitata Wied (Diptera: Tephritidae). To date, no control program has been undertaken in the Argan forest. This research is the first investigation of natural mortality factors of C. capitata larvae in the Argan forest. The study was conducted at two localities near the city of Essaouira with different climatic conditions. Observations were conducted using 260 late instar larvae which were individually deposited on the ground, under fruiting host trees, where they naturally fall to find a suitable pupation site. Overall, 42% of the larvae died before being able to burrow into the ground, irrespectively of the location. Larval survival and their likelihood of burrowing decreased with increasing ground temperature, as 53% of all the larvae deaths occurred at temperatures over 48°C. However, at lower ground temperature, predation by ants and spiders accounted for 47% of all mortality. Four species of ants carried out 94% of the captures. Monomorium subopacum Mayr (Hymenoptera: Formicidae), which frequently forages under Argan trees, was by far the most efficient predator, due to a powerful sting that rapidly immobilizes the prey. Our results suggest that the burrowing performance of larvae is constrained by high thermal conditions and the presence of ant predators at lower temperatures.     

Current distribution and potential extent of the most invasive alien plant species on La Réunion (Indian Ocean,Mascarene islands)

Abstract La Réunion Island has the largest area of intact vegetation of the islands in the Mascarene archipelago. Biological invasions are the primary threat to biodiversity in the intact habitats of the island (those not already transformed by agriculture and urbanization). Our study aimed to identify areas to prioritize in managing invasive alien plants for biodiversity conservation. We used extensive surveys of 238 distinct untransformed areas on La Réunion to define the current distribution patterns of all invasive species. Using expert knowledge, we compiled maps of the current distribution of the 46 most widespread/important invasive plants at the habitat scale (identified according to vegetation structure). Data from 440 botanical relevés for the 20 most threatening invasive alien plant species across the island and climatic envelope models were used to derive climatic suitability surfaces; these were used to map potential distributions for these species. More than 10 species invade 16.7% of the remaining habitat. Five habitat types are invaded by 25 or more species, and eight have fewer than 10 invasive alien plant species. Cluster analysis based on presence/absence of species in the 18 habitat types produced eight groups of species that invade particular habitats. Potential distribution models show that some species have invaded large parts of their potential range (e.g. Fuchsia magellanica, Furcraea foetida, Hiptage benghalensis), whereas others have the potential to increase their range substantially (e.g. Clidemia hirta, Strobilanthes hamiltonianus, Ulex europaeus). Management implications are identified for both groups. Three broad groups of habitats were identified: (i) intact habitats with a low level of invasion (e.g. subalpine shrubland); (ii) moderately invaded habitats with varying levels of intactness (ranging from windward submountain rainforest to the Acacia heterophylla forest); and (iii) habitats with little remaining intact area and high levels of invasion (e.g. lowland rainforest). Different management interventions are appropriate for these three groups.     

千岛湖陆桥岛屿地表蚂蚁群落物种多样性空间格局及其影响因素

为了探讨千岛湖岛屿景观参数对地表蚂蚁群落物种α和β多样性空间格局的影响, 作者分别于2017和2018年的5-8月, 采用陷阱法、凋落物分拣法和手捡法调查了千岛湖33个岛屿上的地表蚂蚁群落, 并依据食性将其划分为捕食性蚂蚁和杂食性蚂蚁。利用回归模型分析了全部蚂蚁、捕食性蚂蚁和杂食性蚂蚁群落α和β多样性与岛屿景观参数的关系。结果表明, 岛屿面积对全部蚂蚁、捕食性蚂蚁和杂食性蚂蚁的物种丰富度均有显著的正向影响, 而隔离度则无显著作用。蚂蚁群落的β多样性由空间周转组分主导。岛屿面积差对全部蚂蚁、捕食性蚂蚁和杂食性蚂蚁群落β多样性的嵌套组分有正向影响, 隔离度差只对杂食性蚂蚁的总体β多样性有正向影响。因此, 岛屿面积是影响千岛湖地表蚂蚁群落物种丰富度的主要因素, 并且岛屿面积通过嵌套组分来影响蚂蚁群落的β多样性, 表现出选择性灭绝过程。此外, 不同食性蚂蚁可能因为扩散能力的差异对岛屿景观参数产生不同的响应。