Using ants to monitor changes within and surrounding the endangered Monsoon Vine Thickets of the tropical Dampier Peninsula, north Western Australia

Highlights

• Ant spp. richness increased from inside (I), edge (E), outside (O) Monsoon Vine Thickets.

• Distinctly different ant assemblages were found inside the Monsoon Vine Thickets.

• Species inside prefer cool dense vegetation, those at the edges prefer open habitat.

• I, E and O differences matched less litter depth and canopy cover, and increased fire.

• Indigenous rangers conducted the study and will use results for management.

Abstract

The 79 naturally fragmented and localised Monsoon Vine Thicket (MVT) patches on the coast of the Dampier Peninsula, Kimberley region, Western Australia, are listed as: a culturally significant ‘Threatened Ecological Community’ (TEC), ‘Endangered’ under the Commonwealth (EPBC Act, 1999) and ‘Vulnerable’ under Western Australian legislation. Fire and introduced species are significant disturbance factors affecting the MVT patches. Focussing on five patches, this study aimed to: assess ant diversity and composition; determine relationships between environmental variables, fire history and invasive ants; determine the value of ants as bioindicators of environmental change; and involve indigenous communities in planning and data collection. Three habitats, inside (I) and edge (E) of five MVT patches (11 ha–56 ha) and immediately outside (O) in the surrounding Pindan woodland (PW), were sampled over two seasons; 7,342 ants from 7 subfamilies and 81 species were collected by pitfall trapping and hand collecting. A gradation of species richness from I (37 spp.), to E (48 spp.), to O (62 spp.) occurred, reflecting trends in environmental variables of decreasing litter volume, litter depth and canopy cover, and increasing fire frequency across habitat types. NMDS ordination identified a separation of ant composition between I, E and O, while CCA revealed a clear separation between I, E and O driven by litter volume and depth, canopy cover and fire history. Distinctly different ant assemblages were found within the MVT, with species inside being those that prefer cool relatively dense vegetation and those at the edges preferring more open habitat. The degree of openness of the edge of the MVT edges is related to fire frequency, while species within the PW were typically arid-adapted species that prefer a more open habitat. Two invasive ant species, namely Paratrechina longicornis and Monomorium destructor, were found to be present in the patches and, in common with many invasive ant species, P. longicornis was characterised by high numbers, comprising 43% of total individuals across the study. As little data previously existed on the ant communities of the dry MVT patches of the Dampier Peninsula, the results add significant new information on the ant fauna of the MVT patches and illustrate how using ants as bioindicators can assist interpreting the impact of fire, invasive organisms and management on the conservation status of MVT patches. This study also serves as a model for collaborating with indigenous people to undertake data gathering, interpretation and management of such areas.

Introduction

Tropical Western Australia sits within the monsoonal zone and experiences a climate with high temperatures, a dry season and rainfall that is concentrated in the summer wet season, when approximately 90% of the annual rainfall is received. The vegetation of this region of Australia is predominantly comprised of savanna termed Pindan woodland, with a sparse canopy of trees overlying a lower layer of grasses. Embedded in this woodland are numerous small and isolated semi-deciduous monsoon vine thicket (tropical forest) patches (MVT) (Bowman et al., 2010), where trees form a closed canopy which contrasts with the open canopy of the surrounding Pindan woodland. The position of these rainforests is determined by the presence of permanent water or a topography that provides shelter from desiccation and wildfire (Bowman et al., 1991, McKenzie, 1991, Russell-Smith, 1991).

The MVT patches found in the Kimberley of north-western Australia are floristically and structurally less diverse than the humid rainforests of Queensland, 2000 km to the east, but still have a closed canopy of broad-leaved tree and vine species and a high tree richness, with many trees having buttressed roots and a complex structure (Andersen and Burbidge, 1991, Andersen and Majer, 1991, Andersen, 1992, Andersen and Reichel, 1994, Black et al., 2010). Boundaries between the rainforest and savanna in tropical north-western Australia are quite abrupt (Black et al., 2010, Fisher and Beames, 2013).

Seventy-nine discrete and naturally fragmented MVT patches occur along the Dampier Peninsula of the north-west Kimberley region of Western Australia (Fig. 1), and are found in swales and on the leeward side of the often extensive Holocene dune systems. These naturally fragmented and restricted MVT patches are considered a ‘Threatened Ecological Community’ (TEC) and ‘Endangered’ under the Commonwealth (EPBC Act, 1999) and as ‘Vulnerable’ by the State of Western Australia. The MVT patches are culturally significant and are the primary source of indigenous bush foods, medicines, tools, ceremonial areas and law grounds of indigenous people. Such MVT patches cover less than 0.001% of the Dampier Peninsula (Bellfield et al., 2012), and have a scattered distribution there from Broome north to One Arm Point and then on the western side of King Sound as far south as Goodenough Bay. Plant composition in these MVT patches varies, and includes semi-deciduous plants. Ninety-four per cent of the patches are smaller than 100 ha, and the mode patch size is 10 ha, with the median patch size being 32 ha (Black et al., 2010). These small aggregations of rainforest function as botanical refugia and contain nearly a quarter of all known Dampier Peninsula plant species, with some plant taxa being restricted to MVT patches or even to an individual patch (Black et al., 2010).

MVT patches are prone to high impact from disturbances such as fire, aridification, temperature variations, and introduced species, particularly at the edges, all of which can result in changes to micro-climates and reduced availability of energy, water and nutrients. This in turn leads to changes in the interactions between species found in these environments, and many plants and animals can be affected (Brodie et al., 2012, Pellissier et al., 2013, Struebig et al., 2011).

A popular method of measuring processes of change in Australia is the analysis of ant richness, diversity, abundance and community composition, as ants have been found to be good indicators of ecosystem change (Hoffmann and Andersen, 2003, Andersen and Majer, 2004). Hoffmann and Andersen (2003) analysed 45 studies utilising quantitative data measuring responses of ant communities to disturbance and found virtually all studies used pitfall traps and counts of ant species as abundance data, utilising transformed data to reduce distortions caused by large numbers of ants falling into traps placed near colony entrances or along foraging trails (Andersen, 1990). Not only are ants good bioindicators due to their species composition reflecting deviations in the biological integrity of ecosystems, they can also be used to track the efficacy of remediation efforts and management of the environment (Andersen and Majer, 2004). Since ants are particularly sensitive to habitat structural change and alteration in land management, this will produce a measurable effect on ant communities, an analysis of which can provide different and complementary data to that obtained from the measurement of vegetation parameters alone (Andersen and Majer, 2004).

In the Dampier Peninsula, both patch type and fire may influence how successfully MVT patches can compete with the surrounding savanna, and, as outlined above, the analysis of ant community response to these two factors is a useful tool in evaluating this success. Andersen and Burbidge (1991) have characterised the ant fauna of one MVT patch in Western Australia. The impact of fire on the ant fauna of monsoonal savanna and rainforest edges has been reviewed by Andersen et al. (2012) and the impact of invasive ants, most notably Pheidole megacephala, has been investigated in rainforests of the Northern Territory (Hoffmann et al., 1999). Hoffmann and Saul (2010) have also reviewed the impact of the introduced ant, Anoplolepis gracilipes, in northeast Arnhem Land. This paper provides further information on ant communities and the impact of fire and invasive ants in the Western Australian MVT patches.

In addition to the above, we outline some lessons concerning the involvement of local indigenous people in the monitoring and management of such patches. Traditional Owners recognise that ants have a role in cleaning up the fruits and seeds and maintaining the health of MVT patches. Traditional Owners and Indigenous Ranger Groups from Bardi Jawi and Nyul Nyul Ranger Groups utilised their cultural knowledge in field site selection both in the MVT patches studied and in the demarcation of the inside and edge of the MVT patch and choice of Pindan woodland sites. Site selection reflected areas where Management Plans have been developed and endorsed by Traditional Owners (Environs Kimberley and Bardi Jawi Rangers, 2012). Thus, the results provide data which can be used in future studies where the implementation of management actions have provided an opportunity to monitor environmental changes, using protocols developed with this aim in mind (Fisher and Beames, 2013).

This paper has four aims:

• To assess the ant richness, diversity, abundance and community composition of five MVT patches of the Dampier Peninsula by sampling sites inside (I) and at the edge (E) of the MVT patch, and outside (O) in the surrounding Pindan woodland.

• To determine if there is a relationship between ant community composition, environmental variables and fire history.

• To investigate whether invasive ant species are present and, if present, whether these have had an impact on the ant community composition.

• To work together with Indigenous rangers and Traditional Owners to undertake monitoring of MVT patches and provide details about the benefits of this initiative.