Appendix C1 Appendix C Biosecurity risk assessment - Siam Weed (Chromolaena odorata) Status Chromolaena odorata (Siam Weed) is recognised as one of the world’s worst tropical weeds (NT Department of Land Resource Management 2013). Siam weed is a member of the Asteraceae (daisy family) and is native to the warmer parts of North America, Central America and South America. It is recognised as one of the world’s 100 worst invasive species and is a serious weed in South Africa, India, China, Indonesia, East Timor and the Philippines (Figure 1, Figure 2) where it forms dense stands which prevent establishment of other species, both due to competition and allelopathic effects (Lowe et al 2000). Status: = Present, no further details, = Widespread, = Localised, = Confined and subject to quarantine, = Occasional or few reports, = Evidence of pathogen, = Last reported, = Presence unconfirmed, = See regional map for distribution within the country Figure 1 Siam weed global distribution (CABI 2013) Siam weed is one of 28 weed species on Australia’s National Environmental Alert List because it threatens biodiversity, causes other environmental damage and has the potential to seriously degrade Australia’s ecosystems (Department of the Environment 2013, NT Department of Land Resource Management 2013). Siam weed was first discovered in Australia in 1994 at Bingil Bay in Far North Queensland (Biosecurity Queensland 2012) and is believed to have entered Australia as pasture seed contaminant used on a grazing property in the 1960s and 1970s (NT Department of Land Resource Management 2013). In mainland Australia, Siam weed distribution is confined to north Queensland (Figure 2). There a nationally cost-shared eradication program, led by the Queensland Government, has been in place since its discovery in 1994 however recent assessment indicates that eradication is no longer technically feasible (Biosecurity Queensland 2012). All known outbreaks are under active management with the aim of eradicating the weed from the Australian mainland (NT Department of Land Resource Management 2013).
Chromolaena odorata (Siam Weed) is recognised as one of the world’s worst tropical weeds (NT Department of Land Resource Management 2013). Siam weed is a member of the Asteraceae (daisy family) and is native to the warmer parts of North America, Central America and South America. It is recognised as one of the world’s 100 worst invasive species and is a serious weed in South Africa, India, China, Indonesia, East Timor and the Philippines (Figure 1, Figure 2) where it forms dense stands which prevent establishment of other species, both due to competition and allelopathic effects (Lowe et al 2000).
Status: = Present, no further details, = Widespread, = Localised, = Confined and subject to quarantine, = Occasional or few reports, = Evidence of pathogen, = Last reported, = Presence unconfirmed, = See regional map for distribution within the country
Figure 1 Siam weed global distribution (CABI 2013)
Siam weed is one of 28 weed species on Australia’s National Environmental Alert List because it threatens biodiversity, causes other environmental damage and has the potential to seriously degrade Australia’s ecosystems (Department of the Environment 2013, NT Department of Land Resource Management 2013).
Siam weed was first discovered in Australia in 1994 at Bingil Bay in Far North Queensland (Biosecurity Queensland 2012) and is believed to have entered Australia as pasture seed contaminant used on a grazing property in the 1960s and 1970s (NT Department of Land Resource Management 2013). In mainland Australia, Siam weed distribution is confined to north Queensland (Figure 2). There a nationally cost-shared eradication program, led by the Queensland Government, has been in place since its discovery in 1994 however recent assessment indicates that eradication is no longer technically feasible (Biosecurity Queensland 2012). All known outbreaks are under active management with the aim of eradicating the weed from the Australian mainland (NT Department of Land Resource Management 2013).
Status: = Present, no further details, = Widespread, = Localised, = Confined and subject to quarantine, = Occasional or few reports, = Evidence of pathogen, = Last reported, = Presence unconfirmed
Figure 2 Siam weed Asia-Pacific distribution (CABI 2013)
On the neighbouring Christmas Island, Siam weed is a prohibited import. The weed was detected in 2010 at a single site on the island’s north-east1
Director of National Parks 2010 and was promptly treated
( ). In mid 2013, the site was inspected and two individuals were detected. Ongoing monitoring and control is being undertaken to ensure total eradication of Siam Weed from Christmas Island. It may also be possible that there are other as yet undetected infestations are present on Christmas Island.
Siam weed is well-established on the Cocos (Keeling) Islands (CKIs) and its presence there poses a considerable risk for its establishment on North Keeling Island (Pulu Keeling National Park). It is unclear how the species was introduced to the CKIs but the most likely vector is human transport. The species is classified as a plant that is a quarantinable pest in the Quarantine (Cocos Islands) Proclamation 2004. It currently grows on Home Island, West Island and Horsburgh Island. The Western Australia (WA) Department of Agriculture and Food are undertaking a control program on Home and West Islands (Reeves and Rayner 2010). However, there is no intention to extend the control program to Horsburgh Island at present (B. Rayner 2013, pers. comm., 25 Oct). This risk forms part of the local pathway and is the subject of this assessment.
1 Outbreak located on the North-South Baseline by the road side near the Grants Well turn off.
The current distribution of Siam weed and the extent of the ongoing control program on CKIs can be seen in Figure 3.
Figure 3 Siam weed distribution Cocos (Keeling) Islands southern atoll (Reeves and Rayner 2010)
Biology
The following knowledge of the biology of Siam Weed is available from its role as an internationally significant weed and through control efforts in its naturalised Australian distribution. Siam weed:
• is an upright shrub growing up to 5 metres(m) tall that forms dense spreading thickets and sprawling or scrambling plants growing up to 20 m high if supported by native vegetation
• is a plant with many slender stems that become woody at the base, its side branches are often in pairs
• is identified by its leaves that are arrowhead shaped, 50-120 millimetres (mm) long and 30-70 mm wide with characteristic veins in a “pitchfork” pattern
• as the name “odorata” suggests is pungent when leaves are crushed
• produces numerous seeds which can live in the seed bank for 6 years and effectively germinate
• seed dispersal occurs naturally (non-biotic) and biotically (vector transmission, accidental and intentional introduction) both short and long distances
• seeds germinate successfully in high humidity when temperatures are above 20°C during the wet season but can also germinate with rain after dry periods (NB: has been seen to germinate sporadically on CKIs
• is perennial, with long-lived root stock consisting of a primary root and a very dense branching network that lives for up to 10 years
• after a year’s growth horizontal roots develop into a lignified storage organ which reaches 20 centimetres (cm) diameter and re-sprouts if vegetative structures are damaged
• follows the C3 photosynthesis pathway and has a very efficient resource allocation strategy that allows the plants to grow very rapidly after germination (up to 20 mm/day)
• flowering generally occurs during the dry season and are visited by non-specialist insects, although reproduction is often apomictic
• seventy-five days after flowering fruits are ready for dispersal (up to 1 billion/hectare)
• seedlings success is between 20–46% and appear in densities of >2000 m2 (CABI 2013, NT Department of Land Resource Management 2013).
Ecology
The following aspects of its ecology are particularly relevant in terms of assessing risk of introduction to Pulu Keeling National Park (PKNP) and its potential impact on island biodiversity. Siam weed:
• is well adapted to a wet-dry tropical climate as above ground foliage can die off during the dry season and grow back vigorously from root stock during the wet season
• grows on most soil types from sand to heavy clay, preferring rich well-drained soils
• is adapted to grow in full sun and high productivity is dependent on seedlings having maximum light
• grows best in the tropics and subtropics in areas that receive >1,200 mm of rainfall (1,000 mm with short dry season)
• is limited to altitudes below 2,000 m
• is opportunistic and forms pure stands when established, often in disturbed areas, such as forested edges, grasslands, plantations, road verges and clearings
• is highly competitive and once reproducing its fast growth outcompetes many species because of its massive habit, domination of resources, mechanical pressure and potential allopathic effect (CABI 2013, NT Department of Land Resource Management 2013).
NB: There are natural enemies (biocontrols) that have been translocated to Africa, Ghana, Guam, Malaysia, Mariana Islands, Micronesia, South Africa and Thailand (CABI 2013). If biocontrol was to be considered as a method of control, a risk assessment should be completed that determines the most appropriate species relating to climatic area.
Establishment risk
There is a range of factors relevant to assessment of the risk of establishment of Siam Weed on PKNP that are considered below.
Departure points
The following table outlines the major pathways that Siam weed may use to invade PKNP (Table 1). It shows areas where the species is currently established those where there is the potential for naturalised distribution, the potential pathways and the risk of introduction. Siam Weed’s range does not currently extend to PKNP; however it is foreseeable the island could form part of the future range of this species as the conditions are high suitable as outlined below. The following information has been used to identify where threat prevention work is needed.
As the species is already established in the CKIs the risk of introduction by sea conveyance via the local pathways is high for islands where the species is already naturalised. If effective threat prevention strategies including quarantine protocols were introduced and carried out, the risk of introduction could be lowered.
Currently, there are no direct domestic conveyance pathways, either air or sea, between the mainland and PKNP nor is the species naturalised at any of the domestic origins points. For these reasons the risk via domestic pathways has been identified as low.
Table 1 Siam weed invasive pathways and risk
Origin Est.? Pathway Likelihood of
introduction Air Sea Perth (Australian Mainland) N Y N Very unlikely Broome (Australian Mainland) N Y 1 N Very unlikely Darwin (Australian Mainland) N Y 1 N Very unlikely Fremantle (Australian Mainland) N N Y Very unlikely Christmas Island N2 Y 1 Y Very unlikely Cocos Home Island Y N Y Possible Cocos Horsburgh Island Y N Y Possible Cocos West Island Y N Y Possible Remaining islands within the southern atoll N N 1 Y Unlikely
Indonesia Y 3 N Y Unlikely Sri Lanka native 3 N Y Unlikely 1 Biosecurity Queensland 2011 Potential for naturalised distribution ( ). 2
Director of National Parks 2010 Detected on Christmas Island in 2010 and control commenced immediately after. The only known
infestation is considered to be under control ( ). 3
The risk of introduction by SIEVs via the international sea pathways is less certain due to the lack of ability to control its arrival using threat prevention strategies, thus the risk has been identified as medium.
Suspected Illegal Entry Vessels (SIEVs)
Transportability
Siam weed seed is highly transportable by humans and animals via almost any vector including vehicles, clothing, tents and other equipment. Unintended transport of vegetative material is less likely to occur but there is a possibility that seeds could be transported in uncleaned equipment emanating from areas naturalised areas, such as Horsburgh, West and Home islands. The frequency of trips by Parks staff to Horsburgh Island and the transport of clothing and equipment between the islands is cause for concern and with inadequate cleaning of equipment, the transport of seeds between the two islands is likely.
Climatic suitability
Siam weed is well established in the CKIs which now forms part of its naturalised range. The tropical, high-humidity climate and distinct wet season of PKNP closely match that of Siam weed in its naturalised global range. Hence, the climate of PKNP is considered highly suitable.
Habitat suitability
Siam weed has the potential to become a harmful invasive species in natural forests in the tropics (CABI 2013). Although the species prefers disturbed areas, it is predicted that the weed will still thrive on forested edges slowly encroaching into clearings, the launch campsite, at igloos, in survey transect areas and where there are breaks in the canopy with readily available light. The island’s extensive survey network would likely facilitate its spread island-wide if the species were to establish. The potential behaviour of Siam weed on PKNP
is currently unknown but the islands habitat suitability and the lack of limiting factors are a source of concern. The known habitat preferences and presence of Siam weed in the CKIs, combined with highly fertile soils on PKNP suggest that habitat is highly suitable.
Impact on biodiversity
The principal biodiversity impacts of Siam weed in its naturalised range in Australia stem from its establishment of monocultures which out-compete native vegetation. The current impact on Horsburgh Island is unquantified and there is no control program in place and the impacts to biodiversity cannot be calculated. On Horsburgh the soil type and vegetation structure and types are significantly different to those on PKNP as the island does not have extensive forested areas. One could deduce that Horsburgh would succumb too many of the same impacts as PKNP (listed below) if the distribution of Siam weed became widespread.
Pulu Keeling National Park
There is a high likelihood of the following adverse biodiversity impacts occurring if Siam weed became invasive at PKNP:
• It could significantly change the structure of the Pisonia forest community by out-competing the native vegetation. Cocos nucifera (coconut) is one of the main host plants affected in plantations and is abundant on PKNP (CABI 2013). Its impacts to other species are unconfirmed but could be major or catastrophic
• It would indirectly impact the EPBC listed migratory birds and the endemic subspecies Gallirallus philippensis andrewsi (Cocos Buff-banded rail) that nest on PKNP. The extent of future impacts/consequences are uncertain but could be major2
• The lagoon on PKNP is listed as a Ramsar wetland of international importance and the establishment of Siam weed would be expected to have an adverse impact on the ecological character of the wetlands via disruption to ecological processes. If the species were to establish at the lagoon site the impacts would be catastrophic.
In the event that Siam weed established on PKNP, the benefits provided by its complete eradication would be greater than the off-target impacts of the control program. Any control program would use methods that do not compromise and minimise impacts on the islands natural values including its biodiversity.
The natural values and threatened flora and fauna of PKNP would be placed at threat if the species were introduced as this species would readily establish in and dominate a variety of habitat types. Based on this, you could expect a range of adverse biodiversity impacts if the species naturalised and that consequences could be majorcatastrophic.
Conclusions
Siam weed is highly transportable as seed and Parks, local and international vessel arrivals could carry plant material from areas where it is established. The high frequency of visits by Parks Australia staff between Horsburgh Island and PKNP greatly increases the chances of 2 Horsburgh Island is the location of the translocated population of CBBRs. The interactions between Siam weed and the CBBR is not clear, however, could be detrimental to the survival of the population. Siam weed control is not currently planned for Horsburgh Island.
Siam weed introduction to the park. The introduction of Siam weed to PKNP via the local pathway has been assessed as likely. Therefore, adequate threat prevention measures are needed to prevent incursions.
The prolific biological success of the species in many habitat types is of concern. Additionally, its ecological requirements including climatic conditions correlate with the environment on PKNP. Additionally, the species has established with ease on Horsburgh, Home and West islands in the southern atoll. If introduced the establishment of Siam weed on PKNP is assessed as almost certain.
The consequences of Siam weed introduction and establishment on PKNP have been assessed as major and catastrophic respectively. This is based on the availability of large areas of suitable habitat including forest edges and clearings and the potential impacts to the values of PKNP.
Based on the above assessments, introduction of Siam weed via the local pathway represents a VERY HIGHEXTREME biosecurity risk to PKNP.
The outcomes of this assessment are summarised in matrix below.
SIAM WEED INTRODUCTION AND ESTABLISHMENT RISK MATRIX
Likelihood of Introduction Consequences
Insignificant Minor Moderate Major Catastrophic Almost certain Will probably occur once in 1 month
Low Medium High Very High Extreme
Likely Will probably occur once in 6 months
Low Medium High Very High Very High
Possible Will probably occur once in 2 years
Low Low Medium High High
Unlikely Will probably occur once in 10 years
Minimal Minimal Low Medium High
Very Unlikely May occur only once in 50 years or more
Minimal Minimal Low Low Medium
Likelihood of Establishment
Consequences
Insignificant Minor Moderate Major Catastrophic Almost certain Will probably occur once in 1 month
Low Medium High Very High Extreme
Likely Will probably occur once in 6 months
Low Medium High Very High Very High
Possible Will probably occur once in 2 years
Low Low Medium High High
Unlikely Will probably occur once in 10 years
Minimal Minimal Low Medium High
Very Unlikely May occur only once in 50 years or more
Biosecurity Queensland. (2012). "Siam weed ", from http://www.daff.qld.gov.au/4790_7353.htm.
CABI. (2013). "Chromolaena odorata (Siam weed)." Invasive species compendium Retrieved 1 November 2013, from http://www.cabi.org/isc/?compid=5&dsid=23248&loadmodule=datasheet&page=481&site=144.
Department of the Environment. (2013). "Invasive weeds alert list." Retrieved 1 November 2013, from http://www.environment.gov.au/biodiversity/invasive/weeds/weeds/lists/alert.html.
Director of National Parks (2010). Christmas Island Weed Management Plan 2010-2015. Internal Plan. Christmas Island, Director of National Parks.
NT Department of Land Resource Management. (2013). "Siam weed (Chromolaena odorata)." Retrieved 1 November 2013, from http://www.lrm.nt.gov.au/natural-resource-management/weeds/find/?a=13154.
Reeves, A. and B. Rayner (2010). Siam Weed (Chromolaena odorata) Control on Cocos (Keeling) Islands, Department of Agriculture and Food (WA).
