Appendix C2
Appendix C Biosecurity risk assessment – Rodents
Status
Rodents have successfully colonised almost every continent having spread across the world
with human settlement (Banks and Hughes 2012).Globally, their population numbers and
their distribution continues to expand. During the last Millennium, island ecosystems have
been affected significantly by the introduction of rodents from Europe and the Indian
subcontinent. By 2009, rodent species had variously invaded over 80% of the world’s major
island archipelagos (Russell, Towns et al. 2008, Commonwealth of Australia 2009, Harris,
Gregory et al. 2012). Rodents are widely accepted as a major cause of species extinctions,
extirpation and native species decline in island ecosystems (Groombridge 1992, Towns,
Atkinson et al. 2006, Commonwealth of Australia 2009). Rodents are considered as highly
successful and have the potential to invade and become established on new islands at an
alarming rate (Commonwealth of Australia 2008). In 2009, invasive rodents had been
eradicated from 350 islands in 21 countries (Commonwealth of Australia 2009).
In Australia, the most commonly introduced rodent species are the ship or black rat (Rattus
rattus)1, the Norway rat (R. norvegicus), the Pacific rat (R. exulans) and the house mouse
(Mus musculus)2. The origin and timeframe of when most exotic rodent species invaded is
unclear but it is likely that they arrived on European ships (Banks and Hughes 2012). The
Australian Government listed exotic rodents on islands as a key threatening process under
the Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act) in 2006
(Commonwealth of Australia 2009). In 2009, a threat abatement plan for rodents (rats, mice)
occupying islands less than 100 000 hectares in area was written (Commonwealth of
Australia 2008). It sets the Australian Government priorities and strategic directions for the
management of rodents (priority actions to prevent invasion at Addendum A).
The house mouse and ship rat are by far the most common rodents on Australian islands
(Commonwealth of Australia 2009) and are known to be present on 120 of ~8300 islands in
Australia (Commonwealth of Australia 2008). To date, exotic rodents have been eradicated
from 39 islands, almost all from Western Australia (Commonwealth of Australia 2009).
Ship rats are present on Christmas Island in the Indian Ocean. They transported
trypanosomes, a unicellular parasitic flagellate protozoa, to the island which were
responsible for the rapid extinction of the islands two endemic mammals (Banks and Hughes
2012), the Maclear's rat (Rattus macleari) and the bulldog rat (Rattus nativitatis) (Director of
National Parks 2013). Rat baiting and monitoring control programs have been underway in
the settlement areas of Christmas Island since 2012 and are currently ongoing (Director of
National Parks 2013).
1
The ship rat (black rat, roof rat) is a species-complex of up to five genetically distinct forms. The two most
globally common forms are recorded in Australia: the Oceanic form of Rattus rattus and an Asian form, Rattus
tanezumi. Research is under way at the South Australian Museum to identify where these different types occur in
Australia and their origin (Banks and Hughes 2012).
2 The genus Mus is comprised of four subgenera- Pryomus, Coelomys, Mus and Nannomys - containing ~40
species and an unknown number of subspecies (ISSG 2010).
Both ship rats and house mice have established on the southern atoll of the Cocos (Keeling)
Islands (Home Island, West Island and other Islands in the atoll). Ship rats (combined with
feral cats) caused the extirpation of the Cocos Buff-banded Rail (CBBR) from the southern
atoll (Commonwealth of Australia 2009). Rodent control methods used in the CKIs include:

Direction Island-Rattus spp. trial and eradication program, 2012 (current rodent status is
unknown)

West and Home Islands - domestic baiting products used to control rodents around their
occupant residences

West and Home Islands - Cocos (Keeling) Islands Shire conduct intermittent control
programs using DX verminator (P. Chase, 2013, pers. comm., 10 Oct).
Rodent populations in the southern atoll pose a considerable threat to the values of North
Keeling Island (Pulu Keeling National Park), which is currently free of rodents. In 2012 three
Suspected Illegal Entry Vessels (SIEVs) landed at Pulu Keeling National Park (PKNP). A
monitoring program carried out between June 2012 and February 2013 concluded that
rodents were not present on PKNP but monitoring is continuing. The risk of their introduction
and establishment on PKNP provides the purpose for this biosecurity risk assessment.
Biology
There has been an extensive amount of literature written on rodent biology. Exotic rodent
morphological characteristics are in Addendum B. For the below assessment, ship rats and
house mice will be used as proxy species for all exotic rodents and their biology is listed
below.
Ship rat (Rattus spp.)
The following features are broadly characteristic of Rattus spp.:

colour variation can be charcoal grey to black or light brown above and cream or white
below with a sleek smooth coat

a pair of chisel shaped incisors with hard yellow enamel on the front surfaces

a long pointed head (can be more rounded in juvenile)

large thin ears (20mm+) that reach middle of eye when bent forward

a scaly tail that is much longer than the head and body combined

a size range between: body 165-205 millimetres (mm), tail 185-255 mm and weight 95–
340 grams (g)

reproduction is placental, sexual and the cycle is endogenous likely modulated by
nutrition and, possibly, population density. Small numbers of rodents have been known
to colonise islands in less than two years

breeding age from 3 months, producing 10 litters of 3–10 (av. 5–8) per year, gestation
20–22 days (Russell, Towns et al. 2008, Invasive Species Specialist Group 2011,
Australian Museum 2013).
House mouse
The following features are broadly characteristic of Mus spp.:

colour variation from light brown to dark grey coat

a body up to 65-95 mm length, hairless long 60-105 mm tail and weight up to 30 g

large prominent black eyes, round ears, a pointed muzzle with long whiskers and feet
small in relation to body size

prominent incisor teeth that grow continuously, length controlled by gnawing

require between 3–5 g food (vegetable matter- fleshy roots, leaves, stems, insects) daily

can live and breed without water but require 15% water content in food to survive

generally nocturnal but can be active in daytime hours

reproduction is placental, sexual and the cycle is endogenous likely modulated by
nutrition and, possibly, population density

life span 1–2 years, breeding age from 5 weeks, producing 10 to 12 litters of up to 10 per
year, gestation between 19–21 days

breed throughout the year and mostly commensal in wild populations (Invasive Species
Specialist Group 2010, NSW Government 2011, NSW Government 2013).
Ecology
The following aspects of rodent ecology are particularly relevant in terms of assessing the
risk of introduction to Pulu Keeling National Park (PKNP) and their potential impact on island
biodiversity. Ship rats and house mice will be used as proxy species for all exotic rodents.

they thrive in most natural habitats including agricultural areas, coastland, natural
forests, planted forests, range/grasslands, riparian zones, ruderal/disturbed,
scrub/shrublands, urban areas

have adapted to a range of environmental conditions and reach high densities in habitat
with dense groundcover

requirements include adequate food and water, stable environmental temperatures, soil
moist enough to burrow, nesting conditions favourable, and diseases, parasites and
predation are low (Mitchell and Balogh 2007, Invasive Species Specialist Group 2010,
Invasive Species Specialist Group 2011).
Establishment risk
There are a range of factors relevant to the assessment of the risk of introduction and the
potential establishment of rodents on PKNP. These are considered below.
Departure points
The following table outlines the major pathways that rodents could use to invade PKNP
(Table 1). It shows the areas of current establishment, those where there is potential for
naturalised distribution, the potential pathways and the likelihood of introduction. There are
no exotic (or native) rodents on PKNP; however there have been biosecurity breaches in the
past. It is foreseeable the island could form part of the future range of rodent species. The
following information has been used to identify where threat prevention work is needed.
Table 1 Rodent species invasive pathways and risk
Origin
Perth (Australian Mainland)
Broome (Australian Mainland)
Darwin (Australian Mainland)
Fremantle (Australian Mainland)
Christmas Island2
Cocos Home Island
Cocos Horsburgh Island
Cocos West Island
Remaining islands within the
southern atoll
Indonesia3
Sri Lanka3
Est.?
Y
Y
Y
Y
Y
Y
Y, N1
Y
Pathway
Air
Sea
Y
N
Y
N
Y
N
N
Y
Y
Y
N
Y
N
Y
N
Y
Likelihood of
introduction
Very unlikely
Very unlikely
Very unlikely
Very unlikely
Very unlikely
Possible
Possible
Possible
Y
N
Y
Possible
Y
Y
N
N
Y
Y
Possible
Possible
1
House mouse present, potential for naturalised distribution of the ship rat.
Control programs underway in 2012–13.
3 Suspected Illegal Entry Vessels (SIEVs).
2
The risk of introduction by sea conveyance via the local pathway from CKIs is high as
rodents have naturalised on the southern atoll. If effective threat prevention strategies
including quarantine protocols were introduced and carried out, the risk of introduction could
be lowered. However, illegal entry/landing onto PKNP poses a significant threat because
people who enter illegally are unlikely to adopted recommended quarantine measures.
Currently, there are no direct domestic conveyance pathways, either air or sea, between the
mainland and PKNP, however, rodents are naturalised at all domestic origin points. For
these reasons the risk via domestic pathways has been identified as low.
The risk of introduction by SIEVs via the international sea pathways is high to the lack of
ability to control the arrival of rodents using threat prevention strategies.
Transportability
Rodents can only be transported large distances by human means. Historically, rodents
transported by sea conveyance have been known to travel hundreds to thousands of
kilometres (km) and survive (Russell, Towns et al. 2008). Ports can be high activity areas for
rodents that arrive in shipping containers or local rodents that are attracted to the site
(Russell, Towns et al. 2008). If boats are not actively checked for rodents before leaving for
PKNP, this could significantly increase the risk of a rodent being transported by vessel and
making it ashore.
Rodents, specifically rats can invade islands by self dispersal. Swimming in warm waters
some species can travel between 300 metres (m) and 2 km when conditions are favourable
(Russell, Towns et al. 2008, Harris, Gregory et al. 2012). They can also travel to islands in
cargo and equipment that is brought ashore or swim from a boat moored to the shoreline
(Russell, Towns et al. 2008).
Vessels of concern for potential to transport rodents to PKNP include:

those used to transport Parks staff, which are moored at the Home and West Island
piers for sporadic periods of time during and between uses

local vessels and yachts (with or without permits) that travel to PKNP to fish in its marine
waters and/or anchor close proximity to the islands beaches. These are within easy
swimming distance of some rodents

foreign fishing vessels illegally operating in Australia’s waters

SIEVs travelling from Sri Lanka (less frequently Indonesia) that land or shipwreck on
PKNP
Although the frequency of trips from the southern atoll to PKNP is low the transport of
rodents is still a cause for concern. If responsible persons do not perform adequate checks
of their vessels and rodent/s are on board, the transport of one or a few rodents from the
southern atoll is possible. Predicting the potential for transport by SIEV is more difficult but
due to the unpredictable nature of SIEV arrivals and their potential cargo, rodent introduction
is possible.
The extent of the damage rodents would be able to do is limited if only one rodent is
transported and the individual is not pregnant. The likelihood that a pregnant female or that a
male and female would be transported simultaneously and breed is unlikely. However, if
this did occur the consequences would be greatly amplified.
Climatic suitability
Rodents have naturalised across many continents and in varying climates. They are well
adapted to the tropical climate of the CKIs which now forms part of their naturalised range.
Hence, the climate of PKNP is considered highly suitable.
Habitat suitability
Rodents are habitat generalists. The natural values and threatened flora and fauna of PKNP
would be placed at threat if rodents were introduced because they would quickly reproduce
and exploit all habitat types. The known habitat preferences and presence of rodents in the
CKIs, combined with lack of reproduction limiting factors and a suitable predator suggest that
habitat is highly suitable.
Impact on biodiversity
The ability for rodents to invade, occupy and negatively impact islands is well documented
(Capizzi, Baccetti et al. 2010, Banks and Hughes 2012, Harris, Gregory et al. 2012). The
severity of rodent impact on island ecosystems and the diversity of species affected vary
depending on the species that was introduced. Impacts to ecosystem biodiversity from
rodent establishment are listed below (compiled from literature written on the ship rat and
house mouse but could be extrapolated to many rodent species, with varying levels of
impact).

predation on a variety of different kinds of native wildlife, with varying size up to prey as
large as their own body

competition and disease could have a role in species decline, extirpation and extinction

opportunistic feeders on food sources on PKNP could include vegetation, seeds, fungi,
small vertebrates and the young or eggs of larger invertebrates, primarily seabirds and
the endangered CBBR

rat and mouse direct predation on seabirds and CBBR could lead to extinction and
endangerment: any burrow nesting species and ground dwelling species could be
greatly impacted. Smaller species would be more vulnerable than larger species (prey
size range 27 g–2855 g)

disruption of recruitment of the palm trees via fruit, root and seedling predation (Auld,
Hutton et al. 2010). This could result greater weed species success

as vectors of disease they could introduce helminths, bacteria, viruses and protozoa that
could impact on the seabirds (Banks and Hughes 2012)

contribute to native invertebrate population suppression and local extirpation (Banks and
Hughes 2012)

support hyper-predation processes by alien species, such as Feral Cats (if introduced)
(Banks and Hughes 2012)

other indirect impacts to ecosystem services are possible (Wanless, Angel et al. 2007,
Commonwealth of Australia 2008, Holly P. Jones, Tershy et al. 2008, Commonwealth of
Australia 2009, Ratcliffe, Mitchell et al. 2009, Meek, Hawksby et al. 2011, Banks and
Hughes 2012)

when mice are the only introduced species on an island their behaviour becomes similar
to that of rats with consequently larger impacts on native species and ecosystems
(Invasive Species Specialist Group 2010).
The contribution of PKNP to Australia’s biodiversity assets is out of proportion to its area as
it represents a less-disturbed example of a typical Indian Ocean coral atoll ecosystem. It
provides refuge for threatened and migratory species that would be impacted by rodent
introduction. If any of the above adverse biodiversity impacts occurred, there would be
significant changes to the islands ecosystems. Based on this, if the species naturalised the
consequences could be catastrophic.
The EPBC listed endemic subspecies the Cocos Buff-banded rail (Gallirallus philippensis
andrewsi) has been translocated to Horsburgh Island. If the translocation project were
compromised by rodents predating on rail chicks or the introduction of pathogens or disease
that adversely impacted the survivorship of the population the long-term consequences
could be major.
The principles of Harris and Gregory et al. (2012) suggest that if an incursion occurred and
was subsequently eradicated the reinvasion risk is low. The long-term benefits of eradication
would well outweigh the initial financial costs and logistic difficulties (Ratcliffe, Mitchell et al.
2009). Threat prevention and surveillance strategies should be introduced to encourage
successful quarantine and detection/ interception methods. In the event that rodents
established on PKNP a complete eradication program should be launched. Eradication
methods have been well-tested in Western Australia and across the world (Commonwealth
of Australia 2009) and any control program must use methods and baits that do not
compromise the islands’ natural values. Additionally, potential non-target species impacts
should be assessed.
Conclusions
It is possible that rodents could be introduced to PKNP via the local pathway by human
transport from the southern atoll. However, it is unlikely that a male and female would be
introduced at the same time and an individual cannot successfully reproduce unless already
pregnant. However, if rodents reproduced on PKNP, the likelihood of establishment would
be almost certain.
Rodents have naturalised across many continents and in varying climates, which is a cause
for concern. The climate and habitat of PKNP is considered highly suitable.
The consequences of rodent introduction and establishment on PKNP have been assessed
as catastrophic respectively. This is based on the diversity of biodiversity impacts that
would occur, which would reduce the natural values of PKNP.
Based on the above assessments, introduction of rodents via the local pathway represents a
VERY HIGH biosecurity risk to PKNP.
The outcomes of this assessment are summarised in matrix below.
RODENT INTRODUCTION AND ESTABLISHMENT RISK MATRIX
Consequences
Likelihood of Introduction
Almost certain
Will probably occur once in 1
month
Insignificant
Minor
Moderate
Major
Catastrophic
Low
Medium
High
Very
High
Extreme
Low
Medium
High
Very
High
Very High
Low
Low
Medium
High
High
Minimal
Minimal
Low
Medium
High
Minimal
Minimal
Low
Low
Medium
Likely
Will probably occur once in 6
months
Possible
Will probably occur once in 2
years
Unlikely
Will probably occur once in 10
years
Very Unlikely
May occur only once in 50
years or more
Likelihood of
Establishment
Almost certain
Will probably occur once in 1
month
Consequences
Insignificant
Minor
Moderate
Major
Catastrophic
Low
Medium
High
Very
High
Extreme
Low
Medium
High
Very
High
Very High
Low
Low
Medium
High
High
Minimal
Minimal
Low
Medium
High
Minimal
Minimal
Low
Low
Medium
Likely
Will probably occur once in 6
months
Possible
Will probably occur once in 2
years
Unlikely
Will probably occur once in 10
years
Very Unlikely
May occur only once in 50
years or more
Addendum A
Actions to prevent invasion or reinvasion of rodent species on islands less than
100 000 ha (Commonwealth of Australia 2009)
Actions to prevent invasion or re-invasion
Priority and timeframe
4.1 Develop generic contingency plans for reaction to any
new rodent invasions
4.2 Apply quarantine systems on rodent-free islands and
where eradication is achieved
4.3 Develop island-specific contingency capabilities for
islands at high risk of invasion
4.4 Reduce risk of rodents gaining access to key vessels at
key ports
4.5 Identify and reduce the frequency of rodent infestation
on key Australian vessels, i.e. those regularly berthing on
priority islands
4.6 Survey rodent species and prevalence on foreign boats
that present risks to Australian islands
4.7 Develop and test on-island prophylactic (e.g. permanent
bait stations at high-risk sites) and reactive (e.g. surveillance
and prompt control after any detection of rodents) strategies
to detect and deal with incursions
4.8 Develop fast response capabilities to react to shipwrecks
on priority islands
4.9 Actively involve island residents and ship owners in the
management of incursion risks
High priority, short term
High priority, ongoing
High priority, short term
Medium priority, medium term
Medium priority, medium term
Medium priority, short term
High priority, short term and ongoing
High priority, short term
High priority, long term
Addendum B
Australian exotic rodent morphological characteristics (Commonwealth of Australia
2009)
Morphological
characteristic
Rattus rattus
Species
R. norvegicus
R. exulans
Mus musculus
Body weight (g)
95-340
200-400
30-100
10-25
Ears
Large, and cover
eyes when pulled
forwards
Do not cover
eyes when pulled
forwards
Smaller than for
R. rattus but also
cover eyes when
pulled forward
Smaller than for
the rats
Belly fur
(characteristic
for R. exulans)
Tail
N/A
N/A
White-tipped with
grey underneath
N/A
Much longer than
head/body length
(HBL);uniformly
dark
Clearly shorter
than HBL; thick
with pale
underside
About the same as
HBL; thin and
uniformly dark
About the same
as HBL;
uniformly grey
brown
Tail length (mm)
185-245
150-215
125-135
75-95
No. of nipples
10-12 (usually 10)
12
8
10
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