ORIGINAL PAPER

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

Chris J. Burwell • Akihiro Nakamura •

Andrew McDougall • V. John Neldner

Received: 10 October 2011 / Accepted: 20 January 2012

� Springer Science+Business Media B.V. 2012

Abstract Infestation of islands by exotic ants is wide-

spread 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 (C 10 ha) P. megacephala exhibited distinct

habitat preferences, occurring in significantly lower abun-

dances within heavily-shaded Pisonia grandis forest in the

centre of islands, compared to more open, fringing wood-

land 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 strate-

gies to manage P. megacephala infestations on these

islands.

Keywords Capricornia Cays � Coccoid scale outbreak �Formicidae � Pulvinaria urbicola

Introduction

Many species of ants have proved highly successful in

establishing persistent exotic populations and causing

ecological degradation throughout the world (McGlynn

1999; Moller 1996). Island ecosystems are particularly

vulnerable and invasive ants frequently devastate their

native biotas (Holway et al. 2002), reducing diversity

through competition and predation and consequently dis-

rupting ecological functions such as plant-herbivore inter-

actions (Gonzalez-Hernandez et al. 1999; Stechmann et al.

1996), seed dispersal (Rowles and O’Dowd 2009), nutrient

cycling and seedling recruitment (O’Dowd et al. 2003). It

has been suggested that propagule pressure (i.e. higher

rates of immigration generally assisted by humans) is a

primary driver of the successful colonisation of island

communities by invasive species (e.g. Blackburn and

Duncan 2001; Lonsdale 1999). This has grave conservation

implications, as escalating human activities will lead to

further introductions of exotic species combined with

increased disturbance of island ecosystems.

C. J. Burwell (&) � A. Nakamura

Queensland Museum, South Brisbane, QLD 4101, Australia

e-mail: chris.burwell@qm.qld.gov.au

C. J. Burwell � A. Nakamura

Environmental Futures Centre and Griffith School of

Environment, Griffith University, Nathan, QLD 4111, Australia

A. McDougall

Department of Environment and Resource Management,

Queensland Parks and Wildlife Service, Rockhampton,

QLD 4701, Australia

V. John Neldner

Department of Environment and Resource Management,

Queensland Herbarium, Toowong, QLD 4066, Australia

123

J Insect Conserv

DOI 10.1007/s10841-012-9463-6

Invasive ants are of great conservation concern for the

coral sand islands of the Capricornia Cays located at the

southern end of the Great Barrier Reef (GBR) tropical coral

system, Australia. In particular, the African big-headed

Ant, Pheidole megacephala (Fabricius) is firmly entren-

ched on a number of islands (Hoffmann and Kay 2009). It

is unknown when the ant was first introduced to islands of

this region, however, Chadwick (1962) reporting on a week

spent on Heron island in June 1951, noted that P. mega-

cephala ‘‘was the commonest insect on the island and was

found in a great variety of habitats’’.

This species is considered one of the world’s worst

invasive ant species (Lowe et al. 2000). It can attain very

high population densities in its introduced range and its

negative impacts on native ants, and to a lesser extent other

invertebrates, have been documented in Australia (Callan

and Majer 2009; Heterick 1997; Hoffmann et al. 1999;

Hoffmann and Parr 2008; Vanderwoude et al. 2000) and

elsewhere around the world (McGlynn 1999; Wetterer

2007).

In addition to direct impacts on ants and other inverte-

brates, P. megacephala may threaten the ecological

integrity of the Capricornia Cays. During the last 20 years,

outbreaks of the introduced scale insect Pulvinaria urbi-

cola Cockerell, with large populations of tending

P. megacephala, have been observed on one of the domi-

nant tree species Pisonia grandis R. Br. on a number of the

islands (Olds 2008). Scale outbreaks cause defoliation, but

Pisonia trees subsequently re-sprout. On Tryon Island

(Fig. 1), however, multiple outbreaks over several years

caused the death of substantial numbers of Pisonia trees

(Kay et al. 2003). Pisonia grandis is a coral cay specialist

restricted to the Indo-Pacific region (Turner et al. 2006;

Walker 1987), with over 70% of the Australian distribution

found on the Capricornia Cays (Queensland Parks and

Wildlife Service 1999; Walker 1991). Pisonia grandis also

provides important nesting sites for large populations of

seabirds (Turner et al. 2006).

Pulvinaria urbicola outbreaks have caused the death of

Pisonia grandis on several islands widely spread across the

Indo-Pacific (summarised in Greenslade 2008). Outbreaks

are always associated with scales being tended by invasive

ants, including P. megacephala, the yellow crazy ant

Anoplolepis gracilipes (Smith, F.) and Tetramorium bica-

rinatum (Nylander). Outbreaks may be caused by tending

ants, but they may be due to other factors such as physical

stress of trees (due to lack of water and nutrient input from

sea bird guano, Greenslade 2008) and natural population

fluctuations of scale insect predators and parasitoids (Olds

2008; Smith and Papacek 2001). Regardless of the factors

that initiate outbreaks, mutualistic interactions between the

scales and invasive ants are likely to exacerbate and pro-

long outbreaks.

Given the potentially serious ecological impacts of

P. megacephala, we surveyed the ant fauna of coral sand

islands in the southern Great Barrier Reef. We investi-

gated factors potentially influencing the distribution

of P. megacephala at large (inter-island) and small

(intra-island) spatial scales. We also examined the impacts

of P. megacephala on other ant species. In the light of our

findings we make recommendations for management of

this invasive species in this group of islands.

Methods

Study area

Between March 2008 and April 2009, we surveyed 14 veg-

etated coral cays, including 13 islands within the Capricorn

and Bunker Groups (CBG) which are situated astride the

Tropic of Capricorn at the southern end of the Great Barrier

Reef Marine Park, approximately 80 km east of the city of

Gladstone (23�110–24�070S, 151�420–152�430E: Fig. 1). The

other island surveyed was Lady Elliot Island (24�060S,

152�420E) located approximately 40 km south-east of the

Bunker Group. Islands varied in size from 2.3 to 119.8 ha,

but most were under 20 ha (Table 1).

Average maximum and minimum temperatures at Heron

Island are 29.8 and 24.2�C, respectively, in mid-summer

(January), and 21.5 and 16.5�C in mid-winter (July).

Average annual rainfall is 1,034 mm at Heron Island with

most (139 mm) falling in February. Prevailing winds are

easterly to south-easterly (Bureau of Meteorology 2011).

Vegetation of the islands generally consists of Pisonia

grandis closed-forest in the interior, with Casuarina

equisetifolia L. open-forest, and mixed shrubland of

Argusia argentea (L.f.) Heine, Pandanus tectorius

Parkinson ex Z and Scaevola taccada (Gaertn.) Roxb.

covering the circumference of islands (Walker 1991).

Patches of open grassland and herbland occur near the

beach or in the interior of some islands.

All islands are fully or partially protected as State or

Commonwealth reserves (Appendix 1), but many are sub-

ject to current human disturbances through recreation

(resorts, camping areas), navigation (lighthouses) and

research activities (research stations). Many islands have

also been variously subjected to past human uses, including

guano mining, turtle soup canneries, resorts and as military

practice targets (Batianoff et al. 2009; Mather and Bennett

1978: Table 1).

Sampling

Relative to the size of the island, three to eight survey sites

were established per island (Table 1). Multiple sites were

J Insect Conserv

123

deployed to sample different habitat types, including

Pisonia forest, other woody vegetation, open grassland and

disturbed areas such those within resort and research

facilities (Appendix 1). On relatively large islands two or

more sites were often located within the same habitat type.

Locations of survey sites on each island are detailed by

Burwell et al. (2010).

Ants were collected using pitfall traps, Malaise traps,

and timed day hand collection. Malaise traps generally

collect flying insects but this method also yielded large

numbers of primarily arboreal ants. At each site, four small

pitfall traps (45 mm diameter, 120 ml cylindrical plastic

vials) were arranged at the ends of an approximately

2 9 2 m cross with a large pitfall (square, 1 litre plastic

container with an attached lid with an approximately

10 9 10 cm square hole) in the centre. Black plastic rain

covers were suspended above traps. Traps were filled with

95% ethanol and left open for 36–48 h. Before analyses,

ants collected by small and large pitfall traps were pooled

at each site. One Townes Malaise trap (Sante Traps, Lex-

ington, KY) with a collecting jar filled with around 250 ml

of 95% ethanol was set in the vicinity of the pitfall traps

and operated for 36–48 h at each site. Hand collecting was

carried out for two person-hours during the day (between

8:00 and 17:30 h), targeting foraging and nesting ants

within a variety of arboreal, epigaeic and subterranean

habitats. Search areas were limited to an approximately

50 m radius of the pitfall trap array; where habitat zones

were small, searching was confined to within the boundary

of that habitat.

Abundance data were based on counts of worker ants

collected by pitfall traps, and incidence (presence/absence)

data on workers collected by the combination of pitfall and

Malaise traps and timed hand collecting. Abundance data

were either natural log-transformed (univariate analyses) or

square-root transformed (multivariate analysis). All ants

were identified to morphospecies or, where possible,

described species by CJB. Voucher specimens are stored at

the Queensland Museum, South Brisbane, Australia.

Environmental and spatial structure variables

Various environmental and spatial variables were gener-

ated to characterise each island (Appendix 2). Island size

and plant data were obtained from vegetation surveys

conducted by the Queensland Herbarium (led by George

Batianoff) between August 2007 and September 2008.

Native and exotic terrestrial vascular plants were censused

and dominant vegetation types mapped for each of the 14

islands. Exotic species were plants introduced by humans

from the mainland and outside Australia, and their richness

was log-transformed to minimise the effects of outliers

(much larger numbers of exotics from Lady Elliot and

Heron Islands). Island size, total vegetation cover and the

area occupied by Pisonia forest (all log-transformed) were

Fig. 1 Map of coral sand islands of the Capricorn and Bunker Group. All of the islands are incorporated within the Capricornia Cays National

Park and National Park (Scientific), with the exception of North Reef and Lady Elliot Islands (Commonwealth Islands)

J Insect Conserv

123

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calculated from vegetation maps based on colour aerial

photographs of each island taken in 2009.

We categorised on-going and/or previous events of

human disturbance on each island (based on information in

Mather and Bennett (1978) and Batianoff et al. (2009, and

references therein), Table 1). Three types of human dis-

turbance were used for analyses (presence/absence of:

present and/or past human disturbance, disturbance asso-

ciated with resort use, and disturbance associated with

frequent human visitation, see Appendix 2).

As sampling was conducted at different times of the year

at each island (Appendix 1), mean maximum and minimum

temperatures and rainfall data for the period before (up to

1 month) and while sampling took place were incorporated

into analyses (see Appendix 2 for more details). As recent

weather data were not available for individual islands (with

the exception of Lady Elliot Island where temperature and

rainfall were recorded almost daily), temperature and

rainfall data were estimated by averaging records from the

two nearest weather stations located on the mainland or

continental islands.

We generated variables representing the ‘connectivity’

among islands and to the mainland (Appendix 2). In

addition, we generated variables associated with the geo-

graphical arrangement of the islands to account for the

spatial autocorrelation of island faunas. Latitudes and

longitudes of the islands were centred and rotated using

PCA ordination in PRIMER 6 (Clarke and Gorley 2006).

Resultant X and Y coordinates were analysed individually

and in combination, expressed as third-order polynomial

functions (Legendre and Legendre 1998).

The effects of P. megacephala were incorporated as its

presence or absence, and level of infestation on islands.

Infestation level was classified as 0 (P. megacephala was

not collected from the island), 1 (collected from at least one

sampling site, but not all sites on the island) or 2 (collected

from all sampling sites on the island).

Data analyses (inter-island distribution)

We first investigated inter-island distribution patterns of

P. megacephala and other ants. All assemblage-level

analyses were conducted using ‘other ants’ (i.e. excluding

Pheidole megacephala, as this species was incorporated as

predictor variables in the analyses), using zero-adjusted

Bray-Curtis similarity measures between sites (Clarke et al.

2006). A zero-adjusted Bray-Curtis index incorporates a

dummy variable (we set a value of 1) added to all samples.

The use of a dummy variable generates ecologically

meaningful Bray-Curtis similarity values when the com-

pared samples are depauperate, consisting of very few or

no species (Clarke and Gorley 2006). Due to varying

sampling intensity per island (3–8 sites), we generated

‘average’ similarity values by calculating the centroid of

the similarity matrix of the sites within each island, in

accordance with Huygens’ theorem (Anderson et al. 2008).

For the same reason, we adjusted the species richness (of

other ants on each island) at N = 3 samples (sites), using

sample-based species rarefaction curves using 999 per-

mutations in PRIMER 6. Abundance data were averaged

across sites within each island.

To visually investigate inter-island variation in assem-

blage composition of ants other than P. megacephala, we

used PRIMER 6 (Clarke 1993) to generate a non-metric

multi-dimensional scaling (NMDS) ordination based on

incidences of ant species. The ordination was generated

using the centroids of the similarity matrix within each island

using the first Kruskal fit scheme and 25 random restarts.

Relationships of environmental and spatial structure

variables with P. megacephala abundance and the adjusted

species richness, average abundance and assemblage com-

position of other ants were analysed using distance-based

linear models (DISTLM routine) in PRIMER 6 and PER-

MANOVA ? add-on software (Anderson et al. 2008).

These are analogous to multiple linear regression models

except that DISTLM fits the predictor variables to both

univariate (e.g. abundance) and multivariate (e.g. assem-

blage) datasets, and provides quantitative measures of how

much variation in the data is explained by various linear

combinations of the predictor variables. The optimum,

parsimonious model was determined by a step-wise selec-

tion procedure using a modified Akaike’s Information

Criterion (AICc: Anderson et al. 2008). To determine

whether the selected model explained a significant amount

of the variation in the data, a randomisation procedure

(4,999 permutations) was used within the DISTLM routine.

The final model therefore consisted of one or more predictor

variables that yielded the lowest AICc value and explained

significant amount of variation in the data. All environ-

mental and spatial structure variables were normalised prior

to analyses (default procedure within PERMANOVA?).

Data analyses (intra-island distribution)

We tested whether local-scale, within-island variation in

habitat condition influenced the abundance of P. mega-

cephala. Two separate PERMANOVA procedures (avail-

able from PRIMER 6 with PERMANOVA? software:

Anderson et al. 2008) were carried out to test the effects of

vegetation type (Pisonia forest versus other woody vege-

tation including habitats dominated by Casuarina, Argusia

and Pandanus trees, see ‘‘Appendix 1’’), and local human

disturbance (sites within or in the immediate vicinity of

human disturbance versus sites relatively far from it), on

the abundance of P. megacephala among sites on infested

islands. Type III sums of squares were used to calculate

J Insect Conserv

123

F statistics, and P values were obtained using 4,999 per-

mutations of residuals under a reduced model. Univariate

analyses were performed using Euclidean distances which

yield Fisher’s traditional univariate F statistic (Anderson

et al. 2008). For the analysis of vegetation type, island size

was incorporated as a fixed factor to compare P. mega-

cephala abundance between islands with small (\10 ha)

and large (C10 ha) vegetated areas. Island was incorpo-

rated as a random factor nested within island size. Dis-

turbed and undisturbed comparisons were made between

sites from the same habitat type within each island. Island

was also incorporated as a random factor in this analysis.

Samples collected from Lady Elliot and Tryon Islands were

excluded from these analyses due to extremely small areas

of Pisonia forest relative to the island size (Lady Elliot), or

because of previous formicide application (Tryon).

Chi-square tests were conducted to investigate whether

the occurrence of pitfall-trapped P. megacephala was inde-

pendent of other introduced ants (and vice versa), using the

two-way contingency table of observed number of sites with

and without these two groups of introduced ants.

Results

We collected workers of a total of 24 ant species from 62

sites across the 14 islands. Of these species, nine were

considered exotic. Every island had at least one exotic

species (Table 1). Individual exotic species were collected

from one to four islands with the exception of Pheidole

megacephala (11 islands), the cryptic, subterranean Hyp-

oponera ‘punctatissima’ (10 islands) and the primarily

arboreal Plagiolepis alluaudi Emery (6 islands). Pheidole

megacephala was the most abundant species, comprising

67,363 individuals of a grand total of 72,494 worker ants

collected by pitfall traps. On heavily infested islands (level

2 infestation), P. megacephala often comprised 95–100%

of pitfall-trapped ants. North Reef Island was most heavily

infested, with 25,526 workers of P. megacephala collected

by pitfall traps from three sites. This was the only island

where we detected no other ant species (Table 1).

Inter-island distribution of P. megacephala

The occurrence of frequent human visitation and, to a

lesser extent, the presence of human disturbance explained

a significant amount of variation in the incidence of

P. megacephala (Table 2). For the abundance of pitfall-

trapped P. megacephala, the occurrence of frequent human

visitation (explaining 28% of the variation) was the only

predictor variable selected in the DISTLM model. How-

ever, the model was not statistically significant at

P = 0.059. Both the incidence and abundance of

P. megacephala were positively related to islands with

frequent human visitation and disturbance.

Intra-island distribution of P. megacephala

Although we found a significant effect of habitat type on

the abundance of P. megacephala, there was also a

Table 2 Summary results of DISTLM procedure, showing predictor

variables included in the best models (according to AICc) that

explained a significant amount of the variation in: (1) the incidence

and abundance of P. megacephala; and (2) assemblage composition

(based on incidence and abundance data), adjusted species richness

and average abundance of other ants among 14 islands surveyed

df P 1st predictor variable selected 2nd predictor variable

selected

(1) Pheidole megacephala

Incidence (three methods combined) 11 0.014 Frequent human visitation

(49%)

P/A of human disturbance

(23%)

Abundancea (pitfall traps only) – – – –

(2) Other ants (without P. megacephala)

Adjusted species richness (three methods combined) 11 0.027 P. mega. infestation level

(66%)

X9Y (12%)

Average abundance (pitfall traps only) 11 0.004 P. mega. infestation level

(70%)

Y3 (18%)

Assemblage composition based on incidence

(three methods combined)

11 0.011 P. mega. infestation level

(33%)

X9Y (14%)

Assemblage composition based on abundance (pitfall traps

only)

11 \0.001 P. mega. infestation level

(26%)

Island size (19%)

Numbers in parentheses are the percentages of variation explained by individual predictor variables. P values and residual degrees of freedom

(df) are also shown

P. mega. Pheidole megacephalaa The selected model was not statistically significant

J Insect Conserv

123

significant interaction between the effects of habitat type

and island size (Table 3). The average abundance of

P. megacephala was similar in Pisonia forest and other

woody vegetation on small islands, whereas substantially

fewer P. megacephala were collected from Pisonia forest

on large islands (Fig. 2). None of the factors were signif-

icant for the second test that investigated the effects of

human disturbance (Table 3).

Inter-island variation in ant assemblage composition

A significant amount of the variation in both the species

richness and abundance of other ants (without P. mega-

cephala) was explained by the level of infestation by

P. megacephala and, to a lesser extent, spatial structure

(X9Y, Y3: Table 2). Both species richness and abundance

of other ants declined with increased levels of infestation

(Fig. 3).

Patterns of ant assemblages (without P. megacephala)

were consistent with those of species richness and abun-

dance. Significant variation in both incidence- and abun-

dance-based assemblage compositions was explained by the

level of infestation by P. megacephala and, to a lesser

extent, spatial structure (X9Y) or island size (Table 2). The

NMDS ordination showed islands heavily infested with

P. megacephala (infestation level 2) clearly separated from

uninfested islands (level 0), and islands with spatially pat-

chy infestations (level 1) were intermediate (Fig. 4).

Heavily infested islands (level 2) clustered marginally more

tightly than uninfested and moderately infested islands

(levels 0 and 1) largely due to their depauperate assem-

blages. Tryon Island was classified as moderately infested

(P. megacephala was collected from only two of four sites)

but clustered with heavily infested islands (Fig. 4).

Table 3 Summary results of PERMANOVA analyses which tested

the effects of (1) habitat type and (2) local human disturbance on

P. megacephala abundances

df pseudo-F P

(1) Habitat type

Habitat type (a) 1 17.74 0.005

Island size (b) 1 1.00 0.295

Island (c) 7 2.17 0.134

Interaction (a9b) 1 10.96 0.015

Interaction (a9c) 7 0.11 0.994

Residual 9

(2) Local human disturbance

Disturbance 1 1.81 0.256

Island 5 0.94 0.938

Interaction 5 0.13 0.977

Residual 4

Degrees of freedom (df), pseudo-F and P values (significant P values

in bold) are shown

Fig. 2 Average abundance (±SE) of pitfall-trapped P. megacephala(natural log-transformed) in Pisonia forest (open bars) and other

woody vegetation (closed bars) within small (\10 ha in vegetated

area) and large (C10 ha) islands

Fig. 3 Adjusted species richness and average abundance (natural log-

transformed) of ants (without P. megacephala) across 14 islands with

different levels of P. megacephala infestation

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123

Associations between P. megacephala and other exotic

ants

Apart from Wreck Island and a number of sites on mod-

erately (level 1) infested islands, each site had at least one

exotic ant species collected by pitfall trapping (Fig. 5).

Pheidole megacephala rarely occurred together with other

exotic ants, with the exception of P. alluaudi which

co-occurred at ten sites regardless of the abundance of

P. megacephala (Fig. 5). Chi-square tests of association

between P. megacephala and other exotic ants were not

statistically significant (v2 = 3.12, P = 0.077). However,

when P. alluaudi was removed from the dataset, the results

became highly significant (v2 = 15.38, P \ 0.001), sug-

gesting that the occurrence of P. megacephala and other

exotic ants (Paratrechina longicornis and Tetramorium

bicarinatum) were not independent of each other.

Discussion

Distributional patterns of Pheidole megacephala

We found P. megacephala very widely distributed across

the coral sand islands of the Capricornia Region. Only

three of 14 islands were apparently uninfested. The inter-

island distribution of P. megacephala strongly suggests

that human activities, particularly the frequency with

which islands are visited, facilitate the establishment of this

species. Propagule pressure, in this case the frequency with

which dispersing female reproductives (queens) reach an

island, is one of the major factors influencing the estab-

lishment of invasive ant species (Suarez et al. 2005).

Colonies of P. megacephala are typically founded by

budding and a queen usually must be accompanied by a

group of workers in order to successfully establish (Chang

1985). Long distance dispersal of P. megacephala queens

across water is highly unlikely and the species is largely

spread by human activities. Consequently, in our system,

propagule pressure more or less equates to the frequency

with which islands are visited by humans.

Although subsidiary, the presence of human distur-

bances (past and present) was also important in explaining

the occurrence of P. megacephala across the region. Dis-

turbed areas may facilitate the establishment and expansion

of colonies following their transport to an island. However,

although regarded a disturbance specialist, P. megacephala

has successfully invaded relatively undisturbed environ-

ments in Australia (Hoffmann 1998).

Significantly fewer P. megacephala in Pisonia forest

compared with other woody vegetation suggests that

Pisonia forest is a less favourable nesting and/or foraging

habitat for this species. However, the difference was

restricted to larger islands. Less extensive stands of Pisonia

forest on small islands are probably susceptible to edge

effects (Saunders et al. 1991), facilitating invasion of

Fig. 4 NMDS ordination of ant species assemblages (without

P. megacephala) based on incidence data collected by three collecting

methods. Each point represents an ‘average’ assemblage composition

(centroid) calculated from three or more sampling sites within each island

(see ‘‘Methods’’ for more details). Levels of infestation by P. megacep-hala are indicated by open (level 0), shaded (1) and closed circles (2)

Fig. 5 Abundance of Pheidolemegacephala (above the centreline, natural log-transformed),

and other exotic ants (below thecentre line, natural log-

transformed) collected by

pitfall-traps across all 62 sites

surveyed. Sites are ordered by

abundance of P. megacephalafrom the left and the combined

abundance of other exotic ants

from the right

J Insect Conserv

123

P. megacephala from the surrounding habitat. Differences

in P. megacephala abundance in Pisonia forest between

small and large islands found in this study are in agreement

with Hoffmann and Kay (2009). They investigated the

spatial extent of the edge effect within Pisonia forest on

three infested islands in the region, and concluded that

P. megacephala was able to penetrate approximately 50 m

into the forest. Examination of vegetation maps of all 11

infested islands within the CBG (see Burwell et al. 2010)

reveals that all Pisonia forest on small islands lies within

50 m of the boundary with woodland or other open vege-

tation (East and West Hoskyn, North Reef, One Tree,

Wilson Islands). In contrast, core areas of Pisonia forest on

larger islands are located well beyond 50 m from the

habitat boundary (Heron, Lady Musgrave, Masthead, North

West, with the exception of Lady Elliot and Tryon Islands

where only small remnant patches of Pisonia trees exist).

Hoffmann and Kay (2009) considered the virtual

absence of P. megacephala from the core of Pisonia stands

unusual. However, throughout islands of the Pacific,

P. megacephala is typically largely absent from intact

natural forests and thrives in open, disturbed habitats,

particularly those with weedy plant species that support

honeydew-producing Hemiptera (Wetterer 2007). This is

the situation in the Capricornia Cays, where we observed

large numbers of aphids and mealybugs (e.g. Planococcus

sp., Pseudococcus longispinus Targioni Tozzetti, see Bur-

well et al. 2010) on low-growing vegetation, particularly

Argusia argentea, in fringing open shrubland and wood-

land. Hoffmann and Kay (2009) argued that temperature

and moisture profiles in the heavily shaded Pisonia forest

are within the physiological tolerances of P. megacephala.

They suggested biotic factors limited its distribution within

Pisonia forest and postulated that the nutritional quality of

honeydew excreted from sap-sucking insects feeding on

Pisonia grandis is poor or unsuitable for P. megacephala.

However, empirical studies are needed to scrutinise their

hypothesis.

Impacts of Pheidole megacephala on other ants

Despite considerable differences in plant species richness,

vegetated area, geographic configuration and the timing of

sampling among the surveyed islands, the infestation level

of P. megacephala best explained the variation in species

richness, abundance and assemblage composition of other

ant species. Clearly the ant fauna of these coral cays is

strongly affected by the severity of infestation by

P. megacephala. We found progressive declines in species

richness and overall abundance of other ants with

increasing levels of P. megacephala infestation (Fig. 3),

which were also reflected in patterns of ant assemblage

composition (Fig. 4). The impacts of P. megacephala were

particularly pronounced on heavily infested, small islands,

where very few other ant species were able to co-exist.

Almost invariably, invasions of P. megacephala in Aus-

tralia and elsewhere are accompanied by substantial losses

of other ant species and dramatic changes in the compo-

sition of ant assemblages, both in insular and continental

systems (Hoffmann 1998; Heterick et al. 2000; Vander-

woude et al. 2000; Callan and Majer 2009). In rare

instances, invasion results in the complete extirpation of

other ants, as seen on North Reef Island and in monsoonal

rainforest patches in northern Australia (Hoffmann 1998;

Hoffmann and Parr 2008).

In the absence of pre-invasion data on ant communities

of these islands, there remains doubt as to whether

P. megacephala is the proximate cause of the depauperate

fauna of other ants on infested islands. However, changes

in the ant fauna of One Tree Island between the 1970s and

our survey support our assertion that P. megacephala

is responsible for the decline of other ants. Heatwole

et al. (1981) recorded eight ant species from One Tree

Island; Pheidole megacephala, Ochetellus glaber (Mayr),

Tapinoma minutum Mayr, Cardiocondyla nuda (Mayr),

Camponotus sp., Iridomyrmex sp. and two unidentified

species (one represented only by a single winged repro-

ductive). They did not provide data on the abundance of

P. megacephala, but it was evidently much less abundant

than now, as Ochetellus glaber was the dominant ant on the

island (Heatwole et al. 1981). Evidently P. megacephala

has dramatically increased in abundance on One Tree

Island and probably eliminated many ant species as no

species of Ochetellus, Tapinoma, Cardiocondyla, Camp-

onotus or Iridomyrmex were recorded in the current survey.

One anomalous finding of the current survey was that the

ant fauna of the moderately infested Tryon Island was more

similar to that of a heavily infested island (see Fig. 4).

However, previously all of Tryon Island was heavily infes-

ted with P. megacephala (Hoffmann and Kay 2009) before a

substantial baiting program to control the ant was initiated in

2006, 2 years prior to the current survey. Although the dis-

tribution of P. megacephala was patchy at the time of our

sampling, the depauperate ant fauna on this island is most

likely the legacy of the species’ previous impacts.

At each site, Pheidole megacephala and other epigaeic

exotic ant species were virtually mutually exclusive

(Fig. 5), which may reflect the competitive superiority of

P. megacephala. An exception to this pattern was P. allu-

audi, which often co-occurred with P. megacephala, even

when the latter was highly abundant. Albeit low in abun-

dance, one other exotic (Hypoponera ‘punctatissima’) and

one putatatively native ant species (Solenopsis sp. A)

frequently co-existed with P. megacephala. Although

P. alluaudi foraged on the ground, it more frequently for-

aged arboreally (see Burwell et al. 2010), while workers of

J Insect Conserv

123

Hypoponera ‘punctatissima’ and Solenopsis sp. A are pri-

marily hypogaeic. The different foraging habitats of these

three species, in addition to their small body size and/or

cryptic nature, may reduce competitive interactions with

P. megacephala. Certainly, some species of cryptic, hypo-

gaeic ants are known to persist in the face of heavy infes-

tations of other invasive ant species (Ward 1987; Human

and Gordon 1997).

Pheidole megacephala and scale insect outbreaks

Although there is debate over what initiates outbreaks of

Pulvinaria urbicola scale insects (Greenslade 2008), they

are invariably associated with invasive ants (P. megacep-

hala in the Capricornia Cays, Olds 2008; T. bicarinatum on

islands of the Coringa-Herald group in the Coral Sea,

Smith and Papacek 2001; P. megacephala and T. bicarin-

atum on Palmyra Atoll in the Line Islands 2,000 km south

of Hawaii, Handler et al. 2007; A. gracilipes on Bird Island

in the Seychelles, Gerlach 2004; Hill et al. 2003). Mutu-

alistic associations between P. megacephala and sap-

sucking Hemiptera are well documented and increases in

populations of mealybugs as a result of tending ants have

been demonstrated (Holway et al. 2002). Given that

P. megacephala is highly abundant and ubiquitous on small

islands, the severity and extent of scale insect outbreaks

may be greater on small compared to large islands. Con-

sequently, small islands may be at greater risk of the loss of

their entire stands of Pisonia grandis. Pisonia forest in the

interior of large islands, where P. megacephala is low in

abundance or absent, may be buffered from scale out-

breaks. However, other introduced ants, e.g. P. alluaudi,

can be highly abundant within the interior of Pisonia forest

of large islands and their associations with scale insects

should be investigated.

Management implications

Given strong evidence that human activities, whether rec-

reational or scientific, have facilitated the introduction and

perhaps establishment of P. megacephala within the CBG,

priority should be given to minimising human visitation

(and disturbance) to uninfested islands. This is particularly

important for Erskine Island, the only uninfested island still

accessible to the public. We also strongly recommend the

eradication of P. megacephala from heavily infested

islands. The small sizes of these islands and their insular

nature increase the chances of successful eradication.

Additionally, these islands have very few other ant species,

thus impacts on non-target ants will be small.

Eradication of small scale infestations of Pheidole

megacephala is likely to be successful. For example, a

baiting program appears to have successfully eradicated

P. megacephala from Kakadu National Park in the

Northern Territory, Australia, where it occupied around

30 ha across more than 20 separate infestations (Hoffmann

and O’Connor 2004). Similarly, preliminary results suggest

that a baiting program initiated in 2006 to eradicate

P. megacephala from Tryon Island has been successful.

Eradication of P. megacephala on large islands is more

complicated because the precise extent of infestation needs

to be mapped and impacts of non-targets are more likely as

they have more diverse ant faunas. Even if eradication is

successful, the chances of reintroduction of P. megacep-

hala are high as all large islands are frequently visited by

humans.

Acknowledgments We thank numerous staff at Queensland Parks

and Wildlife Service (QPWS), Queensland Museum (QM) and

Queensland Herbarium (BRI) and volunteers for their assistance and

company in the field particularly John Olds (QPWS), Federica Turco

and Susan Wright (QM), David Halford (BRI), Mark Hallam and the

late George Batianoff. Thanks to Steve Shattuck and Brian Heterick

for their assistance with ant identifications. Carla Catterall and other

members of the WEDG discussion group provided useful comments.

Financial assistance was provided by QPWS and Great Barrier Reef

Marine Park Authority.

Appendix 1

See Table 4.

Appendix 2

See Table 5.

J Insect Conserv

123

Table 4 Survey dates, land tenure and the number of sampling sites deployed within each habitat type (dominant vegetation) on the 14 islands

surveyed

Island Survey dates Land tenure Number of sampling sites per habitat type

Pisonia Casuarina Argusia Pandanus Open grassland Total

Erskine 6–8 Oct 2008 National Park 1 1 1 3

Fairfax West 12–14 May 2008 National Park (Scientific) 1 1 1 3

Wreck 29 Apr–1 May 2008 National Park (Scientific) 1 1 2 4

Heron 7–10 Nov 2008 National Park 1 3a 4

Hoskyn East 13–15 May 2008 National Park (Scientific) 1 1 1 3

Lady Elliot 28–31 Mar 2008 Commonwealth Island 1 2 2 1 2 8

Masthead 5–8 Oct 2008 National Park 2 3a 2 7

North West 9–11 Oct 2008 National Park 4a 2 1 1a 8

Tryon 20–23 Aug 2008 National Park 1 1 1 1b 4

Hoskyn West 13–15 May 2008 National Park (Scientific) 1 1 1 3

Lady Musgrave 11–13 May 2008 National Park 2a 2 4

North reef 28–30 Apr 2009 Commonwealth Island 1 1 1 3

One tree 23–25 Sept 2008 National Park (Scientific) 1 1 2a 4

Wilson 29 Apr–1 May 2008 National Park 1 1 2a 4

a One site is located in the vicinity of disturbed areasb Located among young (\3 years old) Pisonia grandis plantings

Table 5 Summary of environmental and spatial structure variables used for analyses

Predictor variable Class of variable Type/unit Data transformation

Environmental variables

Island size Island size ha Log

Vegetated area Plant ha Log

Pisonia forest cover Plant ha Log (x ? 0.01)

Native plant richness Plant Count

Exotic plant richness Plant Count Log

Proportion of native to exotic plant species Plant % Arcsine

Current and/or past human disturbance Human disturbance P/A

Presence of resorta Human disturbance P/A

Frequent human visitiation Human disturbance P/A

P/A of Pheidole megacephala Pheidole megacephala P/A

Pheidole megacephala infestation level Pheidole megacephala 0, 1, 2

Mean daily max. temp.b Weatherc �C

Mean daily min. temp.b Weatherc �C

Total rainfall during sampling Weatherc mm

Total rainfall in the past 4 weeks Weatherc mm

‘Weighted’ total rainfall in the past 4 weeksd Weatherc mm

Spatial structure

Distance to continent Connectivity km

Distance to the nearest island Connectivity km

Distance to the nearest islande Connectivity km

No. of islands within a 30 km radius Connectivity Count

No. of islands within a 30 km radiuse Connectivity Count

Mean distance to 5 nearest islandsf Connectivity km

J Insect Conserv

123

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