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- Materials and methods
1. The identification of dispersal mechanisms which facilitate particular biological invasions is paramount for the successful management of invasive species. If the dispersal mechanism promotes high propagule pressure, the probability of successful establishment and spread is enhanced.
2. Invasive species may enter mainland Australia from Papua New Guinea via the Torres Strait islands, and their dispersal through the region may be assisted by wind. The island sugarcane planthopper Eumetopina flavipes is of particular concern to Australian quarantine authorities. Long-distance, wind-assisted immigration from Papua New Guinea may be responsible for the continued presence of E. flavipes in the Torres Strait islands and on the tip of mainland Australia. Simulation was used to predict E. flavipes wind-assisted migration potential from Papua New Guinea into the Torres Strait islands and mainland Australia. Field studies were used to test the predictions.
3. Wind-assisted immigration from Papua New Guinea was predicted to occur widely throughout the Torres Strait islands and the tip of mainland Australia, especially in the presence of tropical depressions and cyclones. Simulation showed potential for a definite, seasonal immigration which reflected variation in the onset, length and cessation of the summer monsoon.
4. In general, simulation predictions did not explain E. flavipes observed infestations. The discrepancy suggests that post-colonization processes such as the temporal and spatial availability of host may be equally or more important than possible wind-assisted immigration in determining population establishment, persistence and viability.
5. Despite the potential for wide-spread, annual immigration throughout the Torres Strait islands and the tip of mainland Australia, E. flavipes control may be possible by managing the cultivation of host plants on an ongoing annual basis to avoid recolonization, especially prior to or during critical immigration periods.
6. Synthesis and applications. Wind may promote significant incursions of E. flavipes from Papua New Guinea into northern Australia. Management strategies should consider the relative importance of both pre- and post-invasion processes in determining establishment success, so that response measures can be implemented at the appropriate stage of invasion. In this way, successful control may be enhanced, serving to reduce the overall cost of invasion.
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- Materials and methods
The likelihood that a species will successfully colonize a new region is dependent upon a variety of pre- and post-invasion ecological processes. Primary amongst the pre-invasion processes is the ability to reach new locations. This ability may be enhanced through the use of particular dispersal mechanisms (Williamson 1996; Ruiz & Carlton 2003). Should the dispersal mechanism promote high propagule pressure, then successful arrival, establishment, persistence and spread is far more likely (Grevstad 1999; Simberloff 2009).
Many studies have focused on post-invasion determinants of establishment success, and not on pre-invasion processes (Kolar & Lodge 2001; Puth & Post 2005). If the relative importance of different dispersal mechanisms used by a particular pest is well understood, there may be a chance to disrupt these mechanisms and so reduce the risk of new invasions or recolonization (Carlton & Ruiz 2005). Such pre-emptive management is always preferable due to the expense involved in post hoc reactive control and eradication (Leung et al. 2002; Hulme 2006).
A number of dispersal mechanisms that may facilitate invasive species movement into Australia have been noted (Stanaway et al. 2001; Pheloung 2003; Lintermans 2004; Floerl & Inglis 2005). One pathway into northern Australia is from Papua New Guinea (PNG) through the Torres Strait islands (TS) (Fig. 1). The Torres Strait encompasses approximately 48, 000 km2 between the southern coast of PNG and the tip of Cape York, Queensland, Australia. There are over 200 islands in the Torres Strait, seventeen of which are permanently inhabited by Torres Strait islanders of Melanesian origin. On the tip, or northern peninsula area (NPA) of Cape York, Australia, a further five communities of Torres Strait islander as well as mainland Aborigines occur. Islands/communities are clustered into groups based loosely upon geography and cultural relationships (Fig. 1; Table 2). In keeping with Melanesian traditions, varying degrees of subsistence agriculture occur in both the TS and NPA. Gardens can contain a mix of plants that may act as hosts for exotic pests and diseases that are not present in commercial production areas on mainland Australia.
Figure 1. Map of southern Papua New Guinea and Torres Strait and northern peninsula area of Queensland, Australia, showing traditional island/community groups.
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Table 2. Torres Strait island and northern peninsula area locations sampled for predicted numbers of Eumetopina flavipes from resulting distribution
|Traditional group||Location||GPS co-ordinates|
|New Mapoon||10052′01.38″S 142023′08.05″E|
|Western||Moa - Kubin||10014′02.02″S 142013′14.27″E|
|Moa - St Pauls||10011′06.68″S 142019′42.79″E|
|Mabuiag|| 9057′25.26″S 142011′13.88″E|
|Top Western||Boigu|| 9013′50.34″S 142013′11.80″E|
|Dauan|| 9025′08.35″S 142032′29.76″E|
|Saibai|| 9022′54.16″S 142036′42.39″E|
|Eastern||Ugar|| 9030′27.72″S 143032′49.06″E|
|Erub|| 9035′08.24″S 143046′14.67″E|
|Mer|| 9054′53.91″S 144002′29.55″E|
|Central||Masig|| 9045′01.82″S 143024′46.84″E|
|Iama|| 9053′54.93″S 142046′06.97″E|
The TS are of major concern to Australian quarantine authorities because of the unique variety of potential dispersal mechanisms (Walker 1972; Kikkawa, Monteith & Ingram 1981; Lindsay 1987). Very little empirical information exists on the specific mode of operation of different mechanisms, their relative importance, and whether successful establishment could result from invasive species using them. Of these, annual, north-westerly monsoonal trade winds may be significant (Farrow & Drake 1978; Farrow et al. 2001). Unlike other mechanisms, wind may provide the perfect opportunity for a ‘continuing rain of propagules’ from PNG into the TS/NPA, thus enhancing the survival of exotic species arriving this way (Thresh et al. 1983; Simberloff 2009).
The island sugarcane planthopper Eumetopina flavipes Muir (Hemiptera: Delphacidae) poses a high-risk quarantine threat to the commercial production of sugarcane in Australia. E. flavipes is the only known vector for Ramu stunt, a debilitating disease of sugarcane that occurs in PNG, but not Australia (Shivas & Schneider 1999). Disease-free populations of E. flavipes are established in the leaf whorls of sugarcane grown in gardens throughout the TS and NPA (Anderson, Sallam & Congdon 2009). Despite the threat posed by incursions of Ramu stunt vectored by these populations, virtually nothing is known of E. flavipes dispersal potential. In general, planthoppers rely on wind for migrations over significant distances (Kisimoto & Rosenberg 1994). Consequently, it has been hypothesized that wind-assisted, long-distance migration from PNG may explain, at least in part, the distribution and extinction/recolonization dynamics of E. flavipes in the TS/NPA (Anderson, Sallam & Congdon 2009). This hypothesis remains untested.
The likelihood and relative magnitude of long-distance, wind-assisted migration can be determined using trajectory analyses that incorporate meteorological data and ecological parameters of the organism of interest (Reynolds et al. 1997). In this study, such analyses were used to determine if wind-assisted migration of E. flavipes from PNG into the TS/NPA and beyond is possible, and to gain an insight into its potential frequency and the likely resulting distribution. Information on mechanisms that contribute to dispersal, and thus impact upon invasion success, are essential for making informed management decisions. The results from this study will contribute directly to the development of management options that may reduce the risk of E. flavipes incursion into commercial Australian sugarcane.
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The simulation results strongly suggest that wind provides multiple opportunities for E. flavipes to migrate from PNG into the TS/NPA. Although based on general planthopper flight behaviour, this result could be true for any organism that migrates with wind assistance. Simulations predict that immigration should begin in late November or December, peak between January and March, and rarely continue past April. This finding is consistent with the frequently observed movement of large numbers of different insect taxa from PNG into the TS during the monsoon season (Farrow & Drake 1978). No immigration was predicted from June through to October during the dry season, when circulation is dominated by south-easterly trade winds (Suppiah 1992). Variability in the onset, length and cessation of the monsoon season, including associated summer monsoon winds, is complex and closely linked to cycles that include the Madden-Julian oscillation, El Niño/Southern oscillation phenomenon and the Quasi-biennial oscillation (Suppiah 1992). The intricate way that these and other cycles interact to cause monsoon onset make it very difficult to develop accurate, predictive models of year–year variation in immigration from PNG. However, analysis of wind direction and strength associated with particular synoptic events may allow risk alerts at appropriate times.
On average, cyclones pass through the TS once every eight or so years (Babbage 1990). Our study spanning five years and including one cyclone is thus fairly characteristic of average extreme weather event occurrence. Cyclones are known to affect monsoon onset (Suppiah 1992), so delayed monsoon cessation in April 2006 resulting in a continuation of the immigration season until May of that year, may have been caused by the presence of TC Monica. As a general observation, a depression or cyclone in the Gulf of Carpentaria or over Cape York Peninsula establishes suitable wind conditions to allow for long-distance, widespread immigration from PNG into northern Australia. Such wind conditions were sufficient to carry mosquitoes from PNG to the NPA for 79% and 57% of the days during December 1997 and January 1998, respectively (Ritchie & Rochester 2001). Winds on one particular night transported mosquitoes a distance of approximately 678 km (Ritchie & Rochester 2001). For planthoppers, seasonal displacements in Asia are known to occur annually on monsoon winds, particularly those associated with frontal depressions and typhoons (Rosenberg & Magor 1987). The continuous air currents allow long-distance transport from several hundreds to thousands of kilometres away from the source population (Kisimoto 1976; Seino et al. 1987). The development of a low pressure system in the Gulf of Carpentaria, at the least, was thought to be essential for insect migration from PNG to Cape York (Farrow & Drake 1978). Our results suggest there is potential for E. flavipes to easily be transported similar distances without the aid of such systems. However, when low pressure systems are present, not only may they extend the immigration season and potentially promote widespread immigration, they may also potentially transport E. flavipes south of the NPA to commercial sugarcane growing regions near Cairns. Of interest is that E. flavipes has not been detected south of the NPA. Many factors could be responsible for this anomaly. Perhaps it is only a matter of time, as was the case with the incursion of sugarcane smut into the Ord River Irrigation Area in Western Australia, which was highly suspected to be wind-borne from Indonesia (Croft & Braithwaite 2006).
Even allowing for minor flight control, it appears that prevailing wind conditions and distance from PNG are ultimately responsible for the resulting distribution of E. flavipes. The Top Western group of islands may have received the greatest number of immigrants because they are close to PNG, and because trajectories over a range of wind directions, from north-west through to south-east, contact islands in the group, particularly Boigu. This finding is consistent with the Top Western islands, of all islands, receiving the greatest numbers of exotic fruit fly species from PNG (Technical Advisory Panel on exotic fruit flies for Plant Health Committee and Primary Industries Standing Committee 2004), and other wind-dispersed organisms like disease-carrying midges and mosquitoes (Johansen et al. 2003). The predicted frequency of immigrants per group dwindles as northerly winds become more frequent and/or with greater distance from PNG. Farrow & Drake (1978) suggest that wind trajectories from the Papuan region would rarely reach Cape York, so that a successful southward crossing of the TS was unlikely. In contrast, our results suggest that E. flavipes, at least, may regularly reach the NPA during the monsoon season, and locations in the Western, Inner and NPA groups, albeit lower than other groups, may still be at risk of annual invasion.
Clearly, uncertainties are an issue in predictive modelling, and error and bias can cause predictions to fail (Regan, Colyvan & Burgman 2002). In this case, the impact of altering some model parameters (for example to reflect natural abundance variation in the source population) may lead only to over or under-estimation of individuals in the resulting distribution. As discussed earlier, the resulting distribution is primarily driven by wind, not arbitrary decisions made during the modelling process. Therefore the predictive power of the model itself is high, and distributional inferences are unlikely to be incorrect (Johnson & Gillingham 2008).
Overall, our results demonstrate a high potential for wide-spread, wind-assisted immigration from PNG into the TS/NPA. There are some locations where wind-assisted immigration alone appears to be a good predictor of observed infestation. It may be that levels of immigration are sufficient at those locations to ensure that establishment is highly successful. In general however, the predicted distribution does not match the observed patterns of infestation throughout the TS/NPA. Importantly, E. flavipes is absent at some locations, despite predicted wind-assisted immigration and abundant host plants. These findings suggest that alternate factors may influence establishment in the TS/NPA. On-island processes and or propagule pressure provided by other immigration pathways may be of equal, or greater relative importance in determining the distribution and abundance of E. flavipes in the TS/NPA.
Of the biotic factors that influence establishment and persistence, especially for phytophagous insects like E. flavipes, the distribution and availability of host is among the most important (Hanski 1998; Loxdale & Lushai 1999). Host abundance and stability varies considerably throughout the TS/NPA due to location specific cultivation practices, and for this reason it has been suggested as a major, if not the most, important determinant of E. flavipes establishment success (Anderson, Sallam & Congdon 2009). The general discrepancy between predicted immigration and observed infestation further supports this hypothesis. However, there are still exceptions to this generality, with a number of locations known to have high host availability that have either no E. flavipes, or populations that ‘blink’ in and out of existence (Anderson, Sallam & Congdon 2009).
Anthropogenic movement of infested sugarcane may also contribute, at least in part, to recolonization and supplementation of existing infestations (Anderson, Sallam & Congdon 2007). The relative importance of human-mediated transport in the TS/NPA is unknown, so from a management perspective the monitoring of such pathways must remain a priority. Allsopp (1991) suggested eradication of E. flavipes in the TS/NPA may be in order, and this may be achievable by pruning all leaf whorls off sugarcane plants at all locations simultaneously (Anderson, Sallam & Congdon 2009). Simulation results suggest such a programme is unlikely to be successful over time because there may be potential for replenishment of populations annually during the monsoon season. Examination of the levels of sugarcane cultivation at locations where host is present but E. flavipes is not may provide the clue as to how to keep populations at bay. Management of the wind vector itself is impossible. However, wind-borne immigration into the TS/NPA from PNG appears predictable during certain months of the year. Ongoing, annual management of host plants either prior to or during critical immigration periods may be used to effectively limit establishment, as well as reducing the size of existing infestations. Such a strategy may also curtail the potential for stepping-stone type movements between islands.
In conclusion, wind may be an important dispersal vector for E. flavipes that could allow significant incursions into and throughout the TS/NPA. E. flavipes is known to recolonize certain TS/NPA locations following local extinction (Anderson, Sallam & Congdon 2009), and results suggest wind-assisted migration may contribute to such recolonization as well as supplementation of existing populations. Despite this, on-island dynamics of host availability may be as, if not more, important than wind-assisted immigration in determining establishment and levels of recurring infestation at specific locations. In a bid to narrow down forces that may affect E. flavipes invasion potential, research on alternate transport pathways and on-island processes is ongoing.