Ants are highly successful invaders, particularly on islands (Ingram et al. 2006; Cerdá et al. 2012). Their invasion is of concern to conservationists due to their broad range of impacts (e.g., Holway et al. 2002; O’Dowd et al. 2003; Lach and Hooper-Bui 2010) and the difficulties of eradicating them once established (Silverman and Brightwell 2008; Hoffmann et al. 2011). Impacts of invasive ants can include direct effects such as displaced vertebrate (Feare 1999; Holway et al. 2002 and references therein), non-ant invertebrate (Lubin 1984; Hill et al. 2003; Causton et al. 2006), and ant species (Walker 2006; Hoffmann and Saul 2010; Roura-Pascual et al. 2010), and indirect effects on key ecological functions such as frugivory (Davis et al. 2010), pollination and seed dispersal (Hansen and Müller 2009), and seedling recruitment and litter breakdown in a forest ecosystem (O’Dowd et al. 2003). The yellow crazy ant Anoplolepis gracilipes is ranked amongst the top 100 worst global invasive species (Lowe et al. 2000) and is responsible for catastrophic ecological impacts on islands (O’Dowd et al. 1999; O’Dowd et al. 2003).

In the Seychelles, Anoplolepis gracilipes was first recorded in 1962 on the main island of Mahé (Lewis et al. 1976). The species had spread to the neighbouring island of Praslin by 1975 and was eradicated from this island shortly afterwards (Haines and Haines 1978a, b). Since then, despite control measures on Mahé, its range across the Seychelles islands has expanded and, by 2000, the species occurred on nine of the central islands, including Praslin (Hill et al. 2003).

Although the impacts and ecology of Anoplolepis gracilipes have been well documented in degraded habitats in the Seychelles (Haines and Haines 1978a, Hill et al. 2003), little is known about this ant’s invasion potential in endemic palm forest ecosystems. Praslin, the second largest granitic island of the archipelago, is home to Seychelles’ native mature palm forest. This habitat represents one of the last island palm forest ecosystems in the world and hosts many species that are endemic to Praslin or the Seychelles (Beaver and Chong-Seng 1992; Fleischer-Dogley et al. 2011). The forest is dominated by the iconic coco de mer palm Lodoicea maldivica. This palm species, famous for producing the largest seeds in the plant kingdom, has been fundamental in driving the evolution of endemic fauna species, many of which are restricted to Lodoicea maldivica habitat (Noble et al. 2011). Moreover, the most pristine area of palm forest, the Vallée de Mai, renowned globally for its natural beauty, is a major visitor attraction and brings considerable financial benefit to the Seychelles, and to Praslin in particular. Threats to the ecological integrity of this habitat, including those from invasive species, therefore have potential economic as well as conservation implications.

Anoplolepis gracilipes was identified in the Vallée de Mai for the first time in August 2009 (L. Chong-Seng & P. Matyot, pers. comm.). Here, we present research into the distribution of Anoplolepis gracilipes over the subsequent 2.5 year period in the Vallée de Mai palm forest, and its potential impact on a key group of animals in the ecosystem, the arboreal vertebrate and invertebrate palm specialist species. The overall aim of this research was to determine the distribution and spread of Anoplolepis gracilipes in the Vallée de Mai over time and improve understanding of its impact on endemic arboreal species in this unique palm forest. Given the extent of Anoplolepis gracilipes’ impacts elsewhere, we expected that the fauna associated with Lodoicea maldivica would be less abundant where Anoplolepis gracilipes was present. We specifically ask: (1) what are the distribution, spread rate, and activity levels of Anoplolepis gracilipes in the palm forest?; (2) Which environmental variables are associated with Anoplolepis gracilipes distribution?; and (3) Are there differences in the number of species and abundance of endemic arboreal fauna between Anoplolepis gracilipes invaded and uninvaded areas of the Vallée de Mai?

Although Anoplolepis gracilipes has occurred across Praslin for at least the last decade and is present in Praslin National Park, there were no reports of the species occurring inside the Vallée de Mai until 2009. Despite its documented ability to rapidly cover large areas in high densities under optimal conditions (Hill et al. 2003; O’Dowd et al. 2003), the current distribution of Anoplolepis gracilipes in the Vallée de Mai appears to be in a dynamic equilibrium and is restricted to the north-east of the reserve, with highest activity levels close to the firebreak and the visitor centre. This confined and relatively stable distribution invokes several possible explanations.

Firstly, the occurrence of Anoplolepis gracilipes in the Vallée de Mai may be due to localised introduction and disturbance close to the palm forest. Repeated introductions and anthropogenic disruption of ecosystems both increase the likelihood of successful species invasions (Lockwood et al. 2005; Roura-Pascual et al. 2011; Bacon et al. 2014). The current distribution of Anoplolepis gracilipes in the Vallée de Mai includes the visitor centre and entrance area and it is possible that the recent construction of this centre (2007–2009) facilitated Anoplolepis gracilipes establishment and invasion into the palm forest, through both the transport of Anoplolepis gracilipes with construction materials and anthropogenic disturbance at the forest edge. The highest Anoplolepis gracilipes activity and a continuous increase in activity throughout the study period close to the visitor centre suggest that the disturbance has generated favourable conditions, such as high carbohydrate supply, good abiotic conditions and potentially lower biotic resistance, for the establishment and spread of Anoplolepis gracilipes. Similar findings were reported by Drescher et al. (2007) in Borneo where ant densities were highest in anthropogenically disturbed sites. This raises questions about reasons for the unfavourable conditions for colony establishment and growth in non-degraded forest inside the reserve.

Secondly, the lack of population growth in the Vallée de Mai may be due to a shortage of liquid sugary substances, which is an important energy source for workers, especially during invasions (Savage et al. 2011; Shik and Silverman 2013). To secure large quantities of carbohydrates, Anoplolepis gracilipes establishes a mutualism with hemipterans, which provide a reliable source of honeydew as food (Delabie 2001; Abbott and Green 2007; Blüthgen and Feldhaar 2010). In the Vallée de Mai, hemipterans, such as introduced Pulvinaria urbicola and Dysmicoccus sp., which are invasive elsewhere in Seychelles (Gaigher et al. 2011), were rarely observed on palms (CKB, pers. obs.). This may explain why Anoplolepis gracilipes made substantially lower use of trees in the Vallée de Mai (35%), compared to the numbers observed by O’Dowd et al. (1999) who recorded 98.5% of trees being utilised by Anoplolepis gracilipes on Christmas Island. Similarly, Haines and Haines (1978b) reported 59–100% of trunks occupied by Anoplolepis gracilipes at four sites on Mahé, Seychelles. The lack of hemipterans on palms may restrict the spread of Anoplolepis gracilipes to the boundaries of the mixed forest where broadleaved vegetation and associated hemipterans are abundant (Haines and Haines 1978a). Of the trees that are used in the Vallée de Mai by Anoplolepis gracilipes, 20% were the introduced broadleaf Cinnamomum verum, which is disproportionately high compared to its relative abundance (2.9%; SIF, unpubl. data), suggesting an active preference for Cinnamomum verum in the palm forest.

Thirdly, biotic resistance may also explain the observed stable population of Anoplolepis gracilipes in the Vallée de Mai. There is little and ambiguous information on the role of native ant species in conferring biotic resistance to the invasion of exotic ant species. Both Way (1953) and Hoffmann and Saul (2010) concluded that habitat suitability, and not the presence of native ants, is the strongest predictor of invasive Anoplolepis gracilipes distribution. The invasion of other ants, such as the Argentine ant Linepithema humile in Portugal, however, was prevented by dominant native ants in cork oak plantations and pasture habitat (Way et al. 1997). Whether native ant species have the potential to limit Anoplolepis gracilipes invasion in the Vallée de Mai palm forest is unknown, but the effect of native ants may be largest at early stages of invasion when population size of Anoplolepis gracilipes is still relatively small (Menke et al. 2007). According to biotic resistance theory (Elton 1958), native communities should have higher biotic resistance and therefore be less susceptible to invasion than degraded habitat where native ant species numbers have been reduced. At least 10 native ant species occur in the Vallée de Mai. While this is a relatively low number of species compared to ant communities on mainlands it is among the best habitat types for ants on Praslin (B. Fisher, pers. comm.). This richness in native ants may not confer biotic resistance, but it suggests that the habitat is suitable for a variety of ant species, which are likely to compete for resources with the generalist Anoplolepis gracilipes.

A final explanation for the apparent dynamic equilibrium of the Anoplolepis gracilipes population concerns abiotic conditions. Hoffmann and Saul (2010) showed that habitat suitability is the primary determinant of Anoplolepis gracilipes incursions, and climatic variables, especially temperature and humidity, are important factors determining ant distribution (Torres 1984; Porter 1988; Hölldobler and Wilson 1990; Krushelnycky et al. 2005). In the Vallée de Mai, higher humidity and temperature and lower canopy cover were correlated with Anoplolepis gracilipes presence. The interior of the palm forest is characterised by dense canopy cover and lower temperatures. Cold and wet conditions have been shown to affect foraging activity of other invasive ant species such as the Argentine ant in Hawaii (Krushelnycky et al 2005). Nevertheless, the total differences in temperature and humidity between invaded and uninvaded areas in our research are so small (e.g., ground temperature difference was only 0.07 °C) that they are unlikely to affect the behaviour of the invasive Anoplolepis gracilipes (Chong and Lee 2009).

We observed marked variation between the presence of Anoplolepis gracilipes and several arboreal species. The number and abundance of endemic arboreal species was lower on Lodoicea maldivica in invaded areas, and the effect was particularly strong for molluscs, which were abundant throughout the palm forest prior to the arrival of Anoplolepis gracilipes (NB, pers. obs.). To our knowledge this is the first record of Anoplolepis gracilipes invasion coinciding with the disappearance of native molluscs, although cause and effect could not be confirmed. Further spread of Anoplolepis gracilipes through the palm forest could threaten the viability of these species and the relationship between molluscs and palms in this forest. The absence of the slug Vaginula seychellensis, a Lodoicea maldivica specialist, from the invaded area is particularly concerning. Effects of Anoplolepis gracilipes on native species were also recorded on Mahé, where fewer terrestrial reptiles occurred in Anoplolepis gracilipes invaded areas (Haines and Haines 1978a). It is not known whether Anoplolepis gracilipes actively prey upon or aggressively exclude endemic arboreal species in the Vallée de Mai. Our observations suggest that Anoplolepis gracilipes excludes endemic arboreal species from male Lodoicea maldivica trees, which provide a rich pollen food source for many arboreal species including geckos and molluscs. Anoplolepis gracilipes may, however, be attracted to male trees not due to direct pollen resources but to the dead and decaying small invertebrates which accumulate at the base of the catkins (Cuthbert 2010; C. Kaiser-Bunbury pers. obs.).

Island endemics are typically at higher risk of extinction (Gaston 1998) and invasive species can accelerate this process (Wanless et al. 2007; Medina et al. 2011). Attempts to reverse these processes, by controlling the spread and impact of invasive species have been undertaken worldwide (Veitch et al. 2011). Once an invasive species is established it is often difficult to eliminate or prevent further spread (Lewis et al. 1976; Krushelnycky et al. 2004). In the case of Anoplolepis gracilipes in the Vallée de Mai, further research into control or exclusion options, impacts and potential biotic resistance to ant invasion are required in addition to continued monitoring of distribution and abundance. Chemical control poses a considerable risk to non-target species, such as endemic ants. We therefore advise promoting and researching resistance of the palm forest by, for example, creating and maintaining conditions which restrict Anoplolepis gracilipes populations, such as the removal of introduced broadleaf trees which host high numbers of introduced hemipterans, and targeted control of ant nests in the most disturbed habitats. Removal of introduced trees, however, must be carried out with great care to limit canopy gaps and soil disturbance which favour Anoplolepis gracilipes and other invasive plant and animal species. Future research should experimentally assess and quantify parameters that ensure the resistance of the unique Vallée de Mai palm forest and its co-evolved plant and animal species.