Provision of Services to Develop a Risk Assessment Protocol for Alien Species and
Conduct Risk Assessment and Advise on Management of Alien Herpetofauna in
Hong Kong
Working Paper No. 3 (Final)
30 January 2020
Environmental Strategy, Conservation and Management Co. Ltd. Unit 1701, 17/F, Prosperity Millennia Plaza 663 King’s Road, North Point, Hong Kong T: (852) 2811 2278 | E: [email protected]
Working Paper No. 3
Table of Content
1. Introduction ..................................................................................................................................... 1
a) Background .................................................................................................................................. 1
b) Objectives of the Working Paper No. 3 ....................................................................................... 2
2. Study Approach ............................................................................................................................... 3
a) Evaluations of overseas examples of eradication/ management options for herpetofauna ...... 3
b) Rationales for species prioritisation ............................................................................................ 3
c) Best eradication/ management options ...................................................................................... 4
3. Results ............................................................................................................................................. 4
a) Evaluations of overseas examples of eradication/ management options for herpetofauna ...... 4
b) Rationales for species prioritization .......................................................................................... 10
c) Best eradication/ management options .................................................................................... 13
4. Discussion .................................................................................................................................... 177
5. References ................................................................................................................................... 199
List of Tables
Table 1 Matrix table for prioritisation of the four herpetofauna species with overall risk level of “medium”
or above.
Environmental Strategy, Conservation and Management Co. Ltd. i
Working Paper No. 3
Environmental Strategy, Conservation and Management Co. Ltd. 1
1. Introduction
a) Background
Invasive alien species (IAS) can cause serious impacts on native biodiversity (Rands et al., 2010) and there
are many documented cases of species extinction and population declines due to them (e.g. Clavero and
Garcia-Berthou, 2005; Molnar et al. 2008). Moreover, IAS impose enormous costs on agriculture, forestry,
fisheries and human health (Wittenberg & Cook, 2001). According to Article 8(h) of the Convention on
Biological Diversity (CBD), contracting parties are encouraged to prevent the introduction of alien species,
and to control or eradicate those alien species which threaten ecosystems, habitats or species as far as
possible and as appropriate. The Aichi Biodiversity Target 9 of CBD relates to the identification and
prioritization of IAS and invasion pathways, and their control and management. The Sustainable
Development Goal 15.8 also states that “by 2020, introduce measures to prevent the introduction and
significantly reduce the impact of invasive alien species on land and water ecosystems and control or
eradicate the priority species”.
Trade is one of the main means through which IAS can be introduced either intentionally or
unintentionally. Measures to restrict the introduction of IAS may be very trade restrictive and can have
devastative consequence on trade (Standards and Trade Development Facility, 2013). For instance, the
Asian longhorn beetle arrived in North America and Europe most likely through the infested wood
packaging material and killed many hardwood trees in forests. This led to new US rules requiring wooden
packing materials from China to be chemically or heat treated to prevent further infestations on local
trees. It was estimated that between $12 and $16 billion, or 17 to 22 percent of total imports from China
to US were affected (Meyer, 1998).
These impacts have led to various response measures in many countries and regions. For example, in
2010 it was found that 55% of the countries signatory to the CBD have adopted legislative tools to
regulate IAS (McGeoch et al., 2010). Some countries, such as Great Britain, have developed IAS strategies.
In the European Union, Regulation (EU) 1143/2014 on invasive alien species (the IAS Regulation) entered
into force on 1 January 2015, fulfilling Action 16 of Target 5 of the EU 2020 Biodiversity Strategy.
In Hong Kong, there are a number of established IAS in the terrestrial, freshwater and marine
environment and control measures are in place for some of them. Since Hong Kong is a free port, the
flow of goods, ships, vehicles, flights and people from around the world is immense and the risk of both
intentional and unintentional introduction of alien species is very high. Many live animals and plants are
imported for food, pets, agriculture, aquaculture, landscaping and ornamental purposes. Some of these
may escape or are intentionally released to the wild. To better understand the risks posed by the arrived
or potential alien species and to prioritize the species for management/preventative measures, a
standardized risk assessment tailored to the Hong Kong situation should be developed.
Working Paper No. 3
Environmental Strategy, Conservation and Management Co. Ltd. 2
The purpose of this study “Provision of Services to Develop a Risk Assessment Protocol for Alien Species
and Conduct Risk Assessment and Advise on Management of Alien Herpetofauna in Hong Kong” (the
Services hereafter) is, in response to Action 7 of the Hong Kong Biodiversity Strategy and Action Plan
2016-2021, to develop a protocol for conducting risk assessment of IAS, making reference to
international best practices. The protocol should be applicable to a wide range of taxonomic groups.
Specific objectives stated in the service specifications are as follows:
To review international best practices in managing IAS
To develop a generic risk assessment protocol for alien species
To assess the risks of potential and arrived alien herpetofauna using the protocol developed
To recommend management options for priority herpetofauna species
In addition to above, the Contractor will also attend meetings to present preliminary findings and
respond to views expressed by participants of the meetings.
b) Objectives of the Working Paper No. 3
Environmental Strategy, Conservation and Management Co. Ltd. (ESCM, the Contractor) was awarded by
Agriculture, Fisheries and Conservation Department (the Proponent) on 19 November 2018 to carry out
the Services. The Services will be implemented within a period of 12 months. Following the submission of
Working Paper No. 1 - Review of international IAS Strategies and recommendations to the formulation of
an IAS strategy for Hong Kong, Working Paper No. 2 - Protocol for Risk Assessment and Risk Assessment
for Herpetofauna, was prepared according to the tender document and submitted in Month 8 of the
contract period. The paper included a generic risk assessment protocol for Hong Kong, explained in
details the information requirements and outlines the procedure of risk assessment. It also documented
the assessment results of the arrived and potential herpetofauna and several alien plants and animals
that were used to test the assessment protocol. An expert workshop was also arranged on 29 October
2019 to discuss the herpetofauna assessment, and the protocol and assessment were revised based on
the comments received during the iterative process. The overall risk of four IAS herpetofauna species
ranked as “Moderate” (no species ranked as high) during the assessment exercise would be subject to
recommendations of management options in Working Paper No. 3. These are Rhinella marina,
Eleutherodactylus planirostris, Lithobates catesbeiana and Trachemys scripta elegans.
The objectives of this Working Paper No. 3 - Recommendations of Eradication/ Management Options are
to evaluate overseas examples of eradication/ management options for herpetofauna, identify the best
eradication/ management options and explain the rationales for species prioritisation. Due to the time
required to address comments and revise both the protocol and the Working Paper No. 2 after the expert
meeting, it was agreed that Working Paper No. 3 would be delivered in Month 12 instead of original
programme (Month 11) of the contract period.
Working Paper No. 3
Environmental Strategy, Conservation and Management Co. Ltd. 3
2. Study Approach
a) Evaluations of overseas examples of eradication/ management options for
herpetofauna
Overseas examples of eradication and management options for herpetofauna in general, including
preventing their introductions (Kraus, 2009) and those that are specific to the four selected species are
reviewed (D’Amore, 2012; Ficetola et. al., 2012; Shine, 2012; CABI, 2019; Global Invasive Species
Database, 2019). No management or eradication programme targeting Eleutherodactylus planirostris can
be found in the literature (Kraus, 2009; CABI, 2019; Global Invasive Species Database, 2019).
Eleutherodactylus coqui (Sin and Radford, 2007; Beard and Pitt, 2012), a closely related species with
rather similar biology and invasion pathway, was used as a surrogate to gain useful information on the
management measures that can be applied to E. planirostris. Particular attentions were paid to the
appropriateness of the management/ eradication options in the Hong Kong context.
b) Rationales for species prioritisation
The assessed alien herpetofauna species with overall risk level of ‘Medium’ or above were selected for
consideration of the best eradication/ management options. Four species were short-listed (see above).
Management/eradication measures are resource-demanding and often require sustained efforts to be
effective. Therefore, prioritisation is needed to ensure the limited resources are committed to actions
that are likely to be most cost-effective. The ranking of alien species based on (1) their relative
environmental and socio-economic impacts and (2) the relative priority of actions to effectively and
efficiently prevent or mitigate the impacts (McGeoch et. al., 2016) was used as a prioritizing tool.
For the four selected species, they are in different stages of the invasion process. Rhinella marina is a
potential IAS that has not yet arrived in Hong Kong. Lithobates catesbeiana has been in the food/pet
trade and individuals have been recorded in the wild but not yet established. While Eleutherodactylus
planirostris and Trachemys scripta elegans have already established and widespread locally. The priority
management options should be targeted to effectively and efficiently deal with the three stages of the
invasion process. That is (1) preventing introductions of R. marina, (2) early-detection and rapid-response
to eradicate incipient populations of L. catesbeiana, and (3) eradicating/controlling well-established E.
planirostris and T. scripta elegans to reduce the negative impacts.
For the relative environmental and socio-economic impacts, the Impact Scores in the risk assessment of
the four selected species were compared. The effectiveness and efficiency of the management options
for each species were based on the resources required, the duration of actions, the likelihood of success
and public support. Overseas experiences of the management measures for the target IAS or the
surrogate (covered in 3a) were used to provide the information. A matrix capturing all these sub-
categories were created for the management actions for each species. These together with the impact
Working Paper No. 3
Environmental Strategy, Conservation and Management Co. Ltd. 4
scores were used to prioritise species for eradication/management. Detailed eradication/management
actions best suited for each priority species will be elaborated in the following section.
c) Best eradication/ management options
Based on the overseas examples of eradication and management measures (including preventing
introductions) relevant to the selected herpetofauna species (in 3a) and the species prioritization (in 3b),
recommendations would be made on the best eradication/ management options against these IAS in
Hong Kong.
As mentioned in 2(b), the four selected species are in different stages of the invasion process. Therefore,
the best eradication / management options recommended for each of them target the respective
invasion stage they are currently at.
3. Results
a) Evaluations of overseas examples of eradication/ management options for
herpetofauna
General Approach
Similar to other IAS, the most effective way to reduce impacts from alien amphibians and reptiles are
having a prevention programme. However, as shown in the review in Working Paper 1, very few places
have a strict biosecurity policy and legislation restricting the importation of alien species. Herpetofauna
attracts even less attention than plants and insect pests in this regard. Only New Zealand and Australia
ban the importation of amphibians and reptiles. Hawaii allows the importation of dozens of species that
are approved for commercial sale, permits the importation of several dozen additional species by
scientific and educational institutions and ban the remaining (Kraus, 2009). Other places do not have any
import control or just have restriction on a handful of herpetofaunal species known as pests, such as the
prohibition of import of Brown Tree Snake, three tortoise species, turtles under four inches and large
constrictors in USA (Kraus, 2009) and the ban on importation of Lithobates catesbeiana to E.U. in 1990’s
(D’Amore, 2012). Very different results were obtained from the different approaches. New Zealand has
not had a naturalized alien amphibian or reptile since the 1960’s while USA, Europe and Asia have seen
many amphibian and reptile introductions (Kraus, 2009).
There are not many eradication and/or management programme for alien herpetofauna and such control
operations have met with relatively little success when compared with plants or mammals (Kraus, 2009).
Eradication and or management methods can be broadly divided into (1) physical removal by using
different traps, nets, hand, shooting, electrofishing and sometimes with the assistance of trained
Working Paper No. 3
Environmental Strategy, Conservation and Management Co. Ltd. 5
detector dogs, (2) chemical treatment using lethal chemicals or hot water, (3) biological control using
parasites, and, (4) habitat alteration to kill the alien herpetofauna present and/or to make the habitats
unavailable or unsuitable. Quite often, an integrated approach employing a combination of the methods
is used to eradicate or control invasive herpetofauna.
(1) Physical Removal
Physical removal is often used in the eradication or long-term control of alien herpetofauna but they are
resource demanding and volunteers/communities are often involved to provide the necessary
manpower. This is because invasive alien herpetofauna often have cryptic habits, with high reproductive
rates and high densities (Kraus, 2009). It would be a huge challenge to catch all the individuals in a big
area and successful eradication have only been achieved for incipient populations over relatively small
areas. Some successful examples, other than the selected species, listed in Kraus (2009) are:
- Eradication of Limnodynastes dumerilii in northern New Zealand that involved surveys in all the
catchments of the mountain range within two months of its discovery and all tadpoles and
metamorphs found were destroyed.
- Eradication of Bufo gargaizans in northern Okinawa by removal of adults, eggs and tadpoles from
late 1980’s through to mid-1990’s.
- Eradication of an isolated population of Xenopus laevis in USA by trapping that may have been helped
by severe winter freezing.
More often, physical removal is used in combination with other methods to control the population
density or to prevent the IAS from spreading into sensitive sites/habitats. The Guam programme in
controlling the Brown Tree Snake, Boiga irregularis, is a good example. Much of the efforts have been
directed to reduce snake densities surrounding airports and seaports, and, to search hitch-hiking snakes
in out-bound cargo and vehicles in order to prevent this invasive snake from spreading to other Pacific
Islands. Dense array of custom-designed, flap-door traps with live mouse as bait is used to capture snakes
in order to reduce its population and is supplemented by nighttime searches of fences which are often
used by this arboreal species (Engeman and Vice, 2002). Cargo searches are largely carried out by the dog
team with trained Jack Russell Terriers and snake detection rate can reach 50-80% (Engeman and Vice,
2002). Reduction of rat populations through the use of bait stations and the use of cage traps to reduce
exotic bird numbers help to lower prey populations around ports and increase the efficacy of snake traps
(Engeman and Vice, 2002). In addition, other methods, such as deploying barriers, devising attractants,
repellents, toxicants and toxicant-delivery systems are also being used or developed to control Brown
Tree Snakes on Guam (Engeman and Vice, 2002; Kraus, 2009).
Amphibians and reptiles are highly diverse groups in terms of habitats, food habits, reproductive biology
and activity pattern. Effective physical removal methods have been documented for only a very small
Working Paper No. 3
Environmental Strategy, Conservation and Management Co. Ltd. 6
number of them (see Kraus, 2009) and these methods cannot be copied and expected to work similarly
well on other species. In general, trapping is often more effective than hand-collecting (e.g. Muller and
Schwarzkopf, 2018) and the type of trap used depends on the biology and behavior of the species. For
instance, the use of basking trap seems to be more effective in catching T. scripta elegans than funnel
traps (Ficetola et al., 2012) but this may not work on many Asian turtles that seldom bask. Effective
means of locating and removing targeted alien amphibians and reptiles have to be investigated. Some
notable measures are covered in the species-specific examples.
(2) Chemical and Hot Water Treatment
Chemical treatment may involve applying lethal chemicals or hot water to kill amphibians. Over 90
chemical agents have been tested by US Department of Agriculture as potential frog toxicants and
caffeine, citric acid and hydrated lime were found to be effective and approved for frog control (Beard
and Pitt, 2012). Caffeine was later retracted for use as control because of human health effects. Citric acid
is the minimum risk chemical and is used legally in Hawaii to control E. coqui. It has been used
successfully in killing Eleutherodactylus coqui frogs and eggs both in nursey plants cargo and also in the
wild (Beard and Pitt, 2012). However, it has phytotoxic effects on plants and can leave white to yellow
dots on leaves (Pitt and Sin, 2004). Hot water spray or vapour (45oC for three minutes) is also effective in
eliminating frogs and their eggs from potted plants during quarantine but may have effects on some
plants (Beard and Pitt, 2012).
Toxicants have also been tested and used to control Brown Tree Snake on Guam. A large variety of
chemicals have been examined and Rotenone, propoxur, natural pyrethrins, allethrin, resmethrin,
diphacinone, warfarin and aspirin were found to be orally toxic to brown tree snakes (Brooks et al. 1998).
Other compounds have been tested and acetaminophen is highly effective and delivered through frozen
pinky mice (Savarie et al., 2001). Due to the much depleted bird fauna on Guam as a result of Brown Tree
Snake predation, the only concern of poisoning of non-target native wildlife include the Mariana Crow,
Coconut Crab and land hermit crabs. Monitoring showed that there were no primary or secondary
hazards and this type of toxic baits have been used on Guam (Engeman and Vice, 2002) and large-scale
aerial delivery has been tested and found to be effective (Clark et al., 2018).
Toxic fumigants had also been looked at to treat outbound cargo to kill stowaway Brown Tree Snake.
Several pyrethrin/pyrethroid insecticide foggers were found to be not effective while methyl bromide is
effective, it is highly toxic, expensive and time consuming to apply (Engeman and Vice, 2002). Hence
cargo fumigation to control Brown Snake Tree has not been commercially adopted (Clark et al., 2018).
Chemical treatment has the potential to be an effective control method because it can have a widespread
effect on a habitat (e.g. the poison of a pond in USA to eradicate the aquatic Xenophrys laevis cited in
Kraus, 2009) or a population. However, chemicals tend not to have a species-specific effect and the
Working Paper No. 3
Environmental Strategy, Conservation and Management Co. Ltd. 7
impact on non-target species have to be carefully considered. In the control of Eleutherodactylus coqui in
Hawaii and Brown Tree Snake on Guam outlined above, chemicals have been used both in the wild and
also to treat cargo/nursery materials. These two places are somewhat unique in that Hawaii does not
have any native herpetofauna and Guam does not have any native snakes, thus limiting the risks of toxic
chemicals to non-target species. Choosing a good attractant (e.g. bait) for the target species may also
help to reduce unintended impacts on native wildlife.
The application of chemicals/hot water to kill alien species can be controversial. In Hawaii where E. coqui
is a well-known invasive causing much ecological and economic damages, there are a lot of support to
control this alien but there are also individuals who opposing it (see Beard and Pitt, 2012). Opposition to
killing alien herpetofauna may arise from people that generally oppose to killing of any vertebrates
(Kraus, 2009). Such sentiment is to be expected among a certain proportion of the general public.
Hopefully through good communication and engagement, understanding of the issue can be improved
and results in less opposition.
(3) Biological Control
Using living organisms to control pest species has been most common in insects and plants. Successful
applications to vertebrates has been limited and there is no report of successful case of biological control
on invasive alien herpetofauna. Introducing new predators as a control would be highly risky and this is
the pathway through which a number of notorious IAS (e.g. Javan Mongoose and Cane Toad) were
introduced across the world.
Using parasites as biological control against Cane Toad, Eleutherodactylus coqui and Brown Tree Snake
has been considered but not progressed very far (Kraus, 2009; Beard and Pitt, 2012; Shine 2012). There
have also been suggestions on using chytrid fungus to control E. coqui in Hawaii (Beard and Pitt, 2012)
and using Paramyxovirus to control Brown Tree Snake on Guam (Engeman and Vice, 2002) but the risks of
the pathogen spreading to other areas and species are high.
The efficacy of a biological agent depends on its ability to reproduce, disperse, find and have a significant
impact on the target species preferably without subsequent human assistance (Howarth, 1999). The
difficulty of guaranteeing host specificity, vector availability, and their significance in regulating host
population make biological control highly risky, success uncertain, development costs high and difficult to
justify the funding (Kraus, 2009). Biological control remains the least applied method in dealing with alien
herpetofauna and its potential is limited when compared to other methods.
(4) Habitat Manipulation
There are a number of successful eradication of alien herpetofauna by habitat manipulation. For
instance, Xenophrys laevis infested ponds in USA were drained in autumn so that all individuals froze over
Working Paper No. 3
Environmental Strategy, Conservation and Management Co. Ltd. 8
in winter (Kraus, 2009). Other examples of its use in combination with other methods in the eradication
of isolated populations of Rhinella marinus, Lithobates catesbeiana, Eleutherodactylus coqui and
Trachemys scripta elegans are covered in the species-specific section below.
Ponds as habitats and/or breeding sites for target alien herpetofauna allow control operations at well-
defined space. Barriers around the ponds may be put up to prevent the access of the target alien species
and those individuals present inside can be removed by trapping, netting, hand-capture, shooting,
electrofishing etc. Ponds may even be drained or filled to eradicate the aliens and, in the latter case, to
eliminate the habitat altogether. This approach is particularly suitable for the management of amphibians
and reptiles that live or breed in small, lentic habitats.
Terrestrial habitats can also be modified to reduce its attractiveness to the alien herpetofauna. For
example, clearing of vegetation was used to reduce the habitat of E. coqui but this method can have
lasting impact on native wildlife if applied in natural habitat as a long-term control method.
Species-specific Approach
(1) Rhinella marina
Rhinella marina were successfully eradicated from Nonsuch Island in Bermuda by installation of a high
density polyethylene barrier fence around the only freshwater pond on the island to prevent them from
breeding and toads were removed both outside the fence and in the pond (Wingate, 2011). In Viwa
Island in Fiji, some ponds were fenced to exclude toads from breeding and rehydrating while other ponds
were filled. The measures were quite effective but faced funding uncertainty (Kraus, 2009).
In Australia, this species was first introduced to control invertebrate pests in Queensland and has been
spreading southwards to New South Wales and westwards to Northern Territory and Western Australia. It
would appear impossible to eradicate this species from Australia and control efforts focus on research
into effective means of long-term control, preventing the establishment of new populations along the
invasion fronts and public education (Kraus, 2009). Research includes searching for a viral control but
failure to find toad-specific pathogen within the toad’s native range or to create an effective and
environmentally acceptable genetically modified viral control resulted in termination (Shine, 2012).
Genetic manipulation so that only males are produced or release of sterile males have been suggested
but these face technical difficulties and have not been investigated in details (Shine, 2012). The high
profile of this invasive toad resulted in active participation of the local communities in removal such as
hand-collecting, trapping using artificial light to attract insects as bait, fencing of ponds and even spraying
household disinfectant onto newly metamorphosed toadlets gather around water edge (Shine, 2012).
However, native frogs were mistaken and also killed and collateral damage to native wildlife from
spraying poison and fencing of ponds are likely to happen (Shine, 2012).
Working Paper No. 3
Environmental Strategy, Conservation and Management Co. Ltd. 9
Other novel approaches include making spawning sites unsuitable by planting dense vegetation around
ponds, introducing nematode lungworm to the toad invasion front, attracting predatory ants to toad
metamorphosing sites and application of alarm pheromone to spawning sites (Shine, 2012). The
feasibility of these are not clear yet. For these to reduce the ecological impact of Cane Toad, they would
need to have a negative impact on toad abundance and the toad impact is density-dependent but this
may not be true (Shine, 2012).
(2) Lithobates catesbeiana
There was a case of successful eradication of Lithobates catesbeiana in East Sussex, U.K. Surveys
identified seven ponds infested by American Bullfrog and these were fenced and frogs were captured by
aquatic traps, pit-fall traps, hand, shooting and electro-fishing in late 1990’s. At the end of 1999, the
ponds were drained to exterminate remaining tadpoles, allow the removal of surviving adults and have
the pond silt excavated and buried (Kraus, 2009). Similar approaches were adopted to eradicate several
populations in Germany and in Netherlands (Kraus, 2009).
Similar methods have been used in an attempt to eradicate/control this IAS in areas where it is already
widespread but the long-term effectiveness is not known because of its very high reproductive potential
and recolonization from adjacent ponds (Kraus, 2009). The poor success may also be explainable because
population growth rate in American Bullfrog is most influenced by tadpole development rate and early
postmetamorphic survival rate (Govindarajulu et al., 2005) while control mostly target adult removal
which may increase metamorphs survival by reducing cannibalism. Modeling suggests removal of
metamorphs in autumn together with removal of egg masses in spring may be more effective in
controlling this IAS (Govindarajulu et al., 2005).
(3) Eleutherodactylus coqui (similar to E. planirostris in biology and invasion pathway)
Hawaii launched a major campaign to control the Coqui Frog around 2005 and it was successfully
eradicated from Oahu by a combination of frequent citric acid spraying and hand-capture (Beard and Pitt,
2012). On Kauai, thick vegetation was cleared to reduce suitable habitat and citric acid was sprayed to
reduce the population to a very small area (Beard and Pitt, 2012). On Molokai, infestation was avoided by
immediate response to frogs arriving in nursery materials (Kraus, 2009). Long-term surveillance against
further incursions from established nursery populations on other Hawaii islands will be necessary. This is
helped by regulatory requirement that all exported shipments from known infested nurseries be treated
with citric acid or hot water spray to kill hitch-hiking frogs (Kraus, 2009).
Beard and Pitt (2012) provided a detailed account of the control measures on Big Island where at least
25,000 ha has been infested. Yearly ground and aerial operations of citric acid, hydrated lime and
mechanical techniques to eradicate populations from isolated areas such as greenhouses have been
Working Paper No. 3
Environmental Strategy, Conservation and Management Co. Ltd. 10
used. Traps are effective where natural retreats are rare such as in resort areas. However, the area
treated each year has been declining due to reductions in fund. On the other hand, a considerable
amount of control has been undertaken by community groups who raise funds and invest lots of
volunteer time to monitor and control local populations.
(4) Trachemys scripta elegans
O’Keeffe (2009) documented the cooperative efforts between government agencies, landowners and
community groups in the eradication of two populations of Red-eared Sliders in Queensland. A
combination of techniques including draining water bodies, intensive trapping and netting, and using
trained dogs to detect nests (O’Keeffe, 2009) have been used. Infested ponds have also been filled in and
compacted, or drained, desilted and fenced to prevent re-colonisation. The largest population was
successfully eradicated while the second small population has been reduced and required additional
effort to complete the work. Reliable detection of this turtle is the greatest hurdle and the use of eDNA
technology may solve this problem.
Ficetola et al. (2012) outlined the control approach for Red-eared Sliders. Floating basking traps and
funnel hoop traps are the most frequently used method and basking traps seem particularly effective.
However, small juveniles are more difficult to capture especially when using hoop traps. Other methods
include the use of nets, draining of wetlands followed by removal of sliders, and the use of sniffer dogs to
detect sliders and their eggs. However, if sliders are still being released by people, this will hamper the
removal efforts.
b) Rationales for species prioritization
The four species with overall risk level of ‘Medium’ or above selected for consideration of the best
eradication/ management options are: Rhinella marinus, Lithobates catesbeiana, Eleutherodactylus
planirostris and Trachemys scripta elegans (Table 1). These four species are in different stages of the
invasion process. The management option considered is targeted towards dealing with their specific
stage of the invasion process: (1) preventing introductions, (2) early-detection and rapid-response to
eradicate incipient populations, and (3) eradication/control of well-established IAS to reduce the negative
impacts.
Prioritisation is based on (1) their environmental and socio-economic impacts and (2) the effectiveness
and efficiency of management options measured by the resources required, the duration of actions, the
likelihood of success and public support. Below is the matrix table capturing these two aspects and the
sub-categories. Species with a comparatively higher impact score and a good degree of effectiveness and
efficiency for the targeted management option will be accorded a higher priority. The important factors
to consider for effectiveness/efficiency of the management options are “Resources Required” and
Working Paper No. 3
Environmental Strategy, Conservation and Management Co. Ltd. 11
“Success Likelihood”. Those required relatively lower resources and higher likelihood of success would be
deemed of higher effectiveness and efficiency. As shown from many overseas cases of managing IAS and
also in the matrix table, the duration of management actions are often long-term and this factor is mainly
for planning the management option, rather than determining the prioritisation. Public support is
important when animals need to be removed from the wild. Public view can be steered with good
communication and early engagement with stakeholders. This factor is captured in the matrix table to
flag those species or management options that will likely generate opposition from the public so that
communication and stakeholder engagement will be included as part of the management package.
Table 1 Matrix table for prioritisation of the four herpetofauna species with overall risk level of
“medium” or above.
Species Impact
Score
Management Options Specific to IAS Invasion Stage Priority for
Management
Option Resources
Required*
Duration Success
Likelihood*
Public
Support
Other Factors to
Consider
Rhinella marina 12 Prevention +
Early
detection &
Rapid
response
Low; become
Medium
when
incursions
occur
Long-term High Needed,
possibly
positive
Incursions may repeat,
sustained surveillance
needed.
Modification of
breeding sites likely
affect native
biodiversity.
High
Lithobates
catesbeiana
11 Early
detection &
rapid
response
Low; become
Medium
when
incursions
occur
Long-term High Needed,
possibly
positive
Incursions may repeat,
sustained surveillance
needed.
Modification of
breeding sites likely
affect native
biodiversity.
High
Working Paper No. 3
Environmental Strategy, Conservation and Management Co. Ltd. 12
Species Impact
Score
Management Options Specific to IAS Invasion Stage Priority for
Management
Option Resources
Required*
Duration Success
Likelihood*
Public
Support
Other Factors to
Consider
Eleutherodactylus
planirostris
8 Eradication/
Control
Very High for
complete
eradication;
Medium to
High for
prevention in
not yet
infested areas
Long-term Very Low for
complete
eradication;
Medium to
High for
prevention in
not yet
infested areas
Needed,
possibly
positive
Reinvasion highly likely.
Eradication by citizen
scientists may affect
Liuixalus romeri due to
confusion.
Medium
Trachemys scripta
elegans
8 Eradication/
Control
Very High for
complete
eradication;
High for
control in
targeted areas
Long-term Very Low for
complete
eradication;
Medium for
control in
targeted areas
Needed,
may be
negative
Reinvasion highly likely.
Habitat modification
may affect native
aquatic biodiversity
Medium
* important factors used to assess the effectiveness and efficiency of management options
Based on the matrix above, Rhinella marina and Lithobates catesbeiana should be accorded high priority
for management. These two IAS have relatively higher Impact Score and their respective management
options have high effectiveness and efficiency.
For Eleutherodactylus planirostris and Trachemys scripta elegans, the priority would be medium because
of lower Impact Score, higher resource requirement and lower likelihood for success especially for
complete eradication. However, it has to be stressed that the impacts of these last two established and
widespread species on Hong Kong biodiversity and the ecological function have not been scientifically
studied. Their actual ecological impact may be higher than that reflected in the current low Impact Score
and there is a need for research, as captured in their risk assessment. Once relevant research is
completed regarding their ecological impacts, there may be strong support to adjust the impact score
upward and the priority to manage them will be raised.
Working Paper No. 3
Environmental Strategy, Conservation and Management Co. Ltd. 13
c) Best eradication/ management options
The best management options for the four short-listed alien herpetofauna according to their current
invasion stage are covered below. Due to the paucity of information available on the ecological impacts of
Eleutherodactylus planirostris and Trachemys scripta elegans, research is also considered as part of the
management package.
(1) Rhinella marina
Since Rhinella marina has not yet arrived and there is no current pathway for this species to Hong Kong,
the best management option is to prevent the introduction of this potential IAS. As mentioned in
Working Paper 1, prevention is regarded as the most effective measures in dealing with IAS and this also
applies to alien herpetofauna (Kraus, 2009) but the import of amphibians into Hong Kong is currently not
covered by existing legislations.
As shown in the species risk assessment, there are three likely pathways through which the species can
arrive in Hong Kong. One of them is the intentional release and the others are the unintentional transport
of this species as contaminant nursery material or as stowaway in container/bulk. To minimize the
intentional import of this species into Hong Kong and its subsequent release, its import should be
discouraged. If a new legislation with a list of serious IAS under import control (i.e. a Black List) would be
enacted in the future, R. marina should be included in the list.
To counter the unintentional transport of this species into Hong Kong, heightened vigilance in ports and
airport against this species in nursery materials and containers originated from source and infested
countries, such as the Caribbean Islands, Australia and Pacific Islands, would help.
Despite these preventive measures, it is probable that some individuals may eventually arrive and reach
the wild. Hence, a rapid detection and early response programme is needed to prevent its establishment
and spread in Hong Kong. This programme should include collaboration with field biologists, NGOs and
citizen scientists such as those using iNaturalist to report and even remove individuals found in the wild.
Active communication, together with capacity building in detection, species identification, handling etc.
when this species has arrived in HK, to these groups is essential for the programme to be effective.
As mentioned in section 3(a) above, isolated breeding populations can be successfully eradicated. If a
breeding population is located in Hong Kong, effective eradication measures from successful case study
elsewhere should be implemented in a timely manner. This will include putting up a Cane Toad-specific
barrier fence around the breeding site and active removal of adults, juveniles, tadpoles and eggs inside
the fence. Night-time patrol along the fence should also be conducted to capture adults trying to enter
the breeding site. If infestation is serious, draining, removal of silt and/or filling of the breeding site can
be considered. However, collateral damage to native biodiversity must be assessed before such drastic
Working Paper No. 3
Environmental Strategy, Conservation and Management Co. Ltd. 14
habitat modification technique is undertaken. Monitoring is needed to assess the effectiveness of the
eradication measures and the status of the alien toad. This species probably matures in one to two years
in invaded areas (Shine, 2012) and if no more individual is detected two years in a row, the eradication
programme can be a called a success and the management measures can be scaled back to keeping a
vigilance of its presence in the wild.
If the recommended measures will be adopted, a detailed plan should be formulated so that all the
parties and stakeholders involved will fully understand the objective and actions required for this long-
term programme.
(2) Lithobates catesbeiana
Lithobates catesbeiana can sometimes be found in the pet and/or food trade in Hong Kong but seems to
be not popular in recent years. Individuals had also been encountered in the wild before. Preventing its
entry to Hong Kong is not possible under the existing legislations (see Working Paper 1). If a new
legislation with a list of serious IAS under import control (i.e. a Black List) would be enacted in the future,
L. catesbeiana should be included in the list because of the potential major impact on native aquatic
biodiversity and as a vector of chytrid fungus.
If import and trade of live individuals of this species into Hong Kong resumes to a substantial level, it is
possible that some individuals will reach the wild and this alien frog may eventually become established
if given the opportunity. Therefore, a rapid detection and early response programme should be in place
to prevent this. An import/market monitoring is helpful to provide an early warning when considerable
number of this species is imported into Hong Kong. When that happens, an early detection and rapid
response programme similar to the one on Rhinella marina should be launched in collaboration with field
biologists, NGOs and iNaturalist citizen scientists to report and remove this species. Communication and
capacity building activities can be combined for both species if the timing overlaps.
If a breeding population is located, successful eradication measures based on overseas experience should
be implemented in a timely manner. The breeding ponds/wetlands will be fenced and frogs, tadpoles and
eggs removed by traps, hand, nets, and even shooting and/or electro-fishing if deemed appropriate.
Draining and de-silting of the ponds/wetlands can be considered to exterminate remaining tadpoles and
to capture surviving frogs if the breeding site is small in size and collateral damage to native biodiversity
is acceptable. Monitoring is needed to assess the effectiveness of the eradication measures and the
status of the alien frog. This species can reach sexual maturity in two years after metamorphosis (CABI,
2018) and in warmer climate tadpoles can metamorphose in the same year (D’Amore, 2012). If no more
individual is detected in three consecutive years, the eradication programme can be a called a success
and the management measures can be scaled back to keeping a vigilance of its presence in the wild.
Working Paper No. 3
Environmental Strategy, Conservation and Management Co. Ltd. 15
If the recommended measures will be adopted for this alien, a detailed plan should be formulated so that
all the parties and stakeholders involved will fully understand the objective and actions required for this
long-term programme.
(3) Eleutherodactylus planirostris
This species is already widespread in disturbed areas and forest edges in Hong Kong and quite well-
established. It undergoes direct development, i.e. does not have an aquatic tadpole stage and any control
measures have to target the terrestrial frogs.
There is no attempted eradication and/or management measures reported in the literature. For the
surrogate E. coqui, intensive eradication efforts targeting small population can be successful as explained
in 3(b). However, the two most commonly used control measures against E. coqui, i.e. citric spray and
hand capture would deem not appropriate or ineffective to use against E. planirostris in Hong Kong. The
use of harmful chemical spray (such as citric acid or hydrated lime) or hot water spray would have
negative impacts on native amphibians and other wildlife. E. coqui is much larger in size and more
conspicuous because of its very loud call and lives in undergrowth vegetation, while E. planirostris is tiny
in size, produces a very soft call and lives among leaf litter and debris on the ground, making it much
more difficult to detect. Hand-capture of E. planirostris would not be an effective control measure as it is
difficult to find and capture all the individuals. Since this species is already widespread locally,
management measures should focus on reducing its negative impacts.
As shown in the risk assessment, the biggest potential impact of E. planirostris is on the similarly-sized
endemic Liuixalus romeri. The Greenhouse Frog already occurs in New Territories, Hong Kong Island,
Lantau Island and Lamma Island where there are native or translocated populations of L. romeri. The two
species have been found to co-exist in some sites but there have been no scientific reports on the
impacts of this alien frog. What is most sensitive is probably Po Toi Island where the native L. romeri
population is smallest and genetically most distinct (Lau, 1998) and the alien seems not to have reached
yet. Po Toi Island is comparatively drier than the other places that support L. romeri but E. planirostris is
adapted to live in both wet and dry habitats (CABI, 2019). If this alien frog reaches Po Toi, it may exert a
negative impact L. romeri that is not experienced by other populations on more favourable environment.
As a precautionary principle, control measures should be put in place to prevent this alien from arriving
to Po Toi by targeting its major pathway of contaminant on plants for landscaping projects. Planting
materials to be used on Po Toi should be isolated and pre-treated with citric acid or hot water spray just
before their transport to the island. Field monitoring should also be carried out on the island for early
detection. If E. planirostris reaches Po Toi, intensive search and capture should be mobilized quickly with
an aim to eradicate this alien before it can establish on the island. The window of opportunity for
eradication is narrow and timely action is the essence of success. Such effort is labour intensive but there
are likely to be interested volunteers and NGOs who would like to take part in conservation work. Prior
Working Paper No. 3
Environmental Strategy, Conservation and Management Co. Ltd. 16
communication and engagement with these parties would be needed to quickly mobilize this human
resource into action. Support, training and coordination are also integral to such volunteer participation.
Monitoring the distribution and spread of E. planirostris are also needed to determine if there are any
important L. romeri (e.g. the largest population of this species on Ngong Ping) or invertebrate
populations in not yet infested areas but will potentially be threatened in the future. In such cases,
control measures similar to those used for Po Toi outlined above should be adopted to reduce the
potential impacts.
Research is needed to look into the negative impacts, if any, of this alien frog on L. romeri and leaf litter
fauna (particularly threatened ants) in good forests. If negative impacts are detected, then the respective
impact score and the priority for management should be adjusted accordingly. Targeted management
measures outlined above for L. romeri should be implemented whenever appropriate. For important leaf
litter invertebrates, specific management measures should be designed to reduce and/or mitigate the
impacts from this alien frog.
There is a Species Action Plan for Liuixalus romeri and it will soon be reviewed. The potential threat of
Eleutherodactylus planirostris should be covered in the plan. The recommended management measures
outlined above should be carefully considered and, if adopted, should be included the plan.
(4) Trachemys scripta elegans
Successful eradication has been documented for small and isolated populations of Red-eared Slider in
Queensland. However, it is deemed not possible to eradicate this alien species in Hong Kong because it is
already established and occurs in many wetlands throughout the territory. Moreover, it remains to be the
most popular pet turtle species and many individuals are subsequently released after being purchased.
Research is needed to look into the negative impacts, if any, of this alien on native lowland turtles,
particularly Mauremys reevesii and possibly Pelodiscus sinensis. If negative impacts are detected, then
management measures including intensive trapping (basking traps and funnel hoop traps) and draining
water bodies to remove individuals should be carried out to reduce the population (or even eradicate this
alien in small isolated sites) and its impacts in sites that support good populations of native turtles. Using
trained dogs to detect nests should also be explored and if effective, can be used both to reduce the
nesting success and to monitor the breeding of this alien. Efforts should also be made to prevent the
release of sliders into these important sites.
Possible candidate sites are Long Valley and Deep Bay wetlands where both native turtle species occur.
However, not much information is available regarding their population size and there is a need for field
research to identify the best habitats and sites for them. As both Mauremys reevesii and Pelodiscus
sinensis are globally threatened, it will be very useful if these knowledge gaps can be filled in the near
Working Paper No. 3
Environmental Strategy, Conservation and Management Co. Ltd. 17
future. The recommended management measures against Trachemys scripta elegans outlined above
should be carefully considered and adopted if this IAS is found to have significant negative impacts on
these native turtles.
Monitoring is required to assess the effectiveness of the control measures and also to determine if the
impacts on the native turtles have been reduced or even reversed. The intensive trapping can yield such
valuable information and it should be designed in such a way that scientific data can be obtained. The use
of eDNA technology will probably help in detecting this alien and native turtles and should be tested for
different habitats in the field.
One note about the management of Red-eared Sliders is that this turtle is a popular pet species. In many
places, the removal of this turtle has faced resistance and how to handle the captured animals can be
controversial (Ficetola et al., 2012). Hence, a well-planned stakeholders’ engagement, in particular with
conservation NGOs, animal welfare groups and academics should be carried out before any actions are
taken. A communication strategy should also be designed and implemented if large-scale removal will be
attempted.
4. Discussion
This Working Paper has reviewed overseas practices in the management and eradication of alien
herpetofauna, prioritized the risky alien amphibians and reptiles and gave recommendations on the best
control measures for them.
There are not many cases of eradication and management of alien amphibians and reptiles and they met
with relatively little success when compared with mammals and plants (Kraus, 2009). The control
methods covered in Section 3 are based on overseas cases. Development and refinement of management
measures for alien species in Hong Kong, not just herpetofauna but also other taxa groups, would be
highly useful to formulate effective control methods suitable for the local context.
Before considering the available risk management options, a judgement on the acceptability of the risk
identified in the risk assessment is required (EPPO, 2011). The potential impacts of the four prioritized
aliens on Hong Kong biodiversity have not been properly studied and research into this area will help in
making that judgement. Moreover, prioritization for actions should simultaneously consider many species
from different taxa groups in order to ensure the best use of limited resources to generate the optimum
results in reducing the IAS impacts. Therefore, before costly, long-term eradication/management
measures are implemented, prioritization of multiple alien species that pose significant potential risk
based on risk assessment should be undertaken. The matrix used in this Working Paper to prioritise alien
herpetofauna can be a useful reference for this exercise.
Working Paper No. 3
Environmental Strategy, Conservation and Management Co. Ltd. 18
During the review of overseas examples, there are cases in which control efforts were compromised by
inability to guarantee long-term investment in funding and personnel. Political will in treating the risks
posed by IAS seriously is needed. Risk communication is crucial not only to get policy and resources
support from within the Government, but also to gather public support for the eradication/management
actions which can be controversial. Risk communication is outside the scope of this consultancy study but
its importance must be stressed.
Kraus (2009) summarized neatly that effective IAS control would require positive research particularly on
eradication and control methods; comprehensive planning; sufficient funds and dedicated personnel
trained in wildlife control. These enabling conditions also apply to all the IAS management work in Hong
Kong.
Working Paper No. 3
Environmental Strategy, Conservation and Management Co. Ltd. 19
5. References
Beard, K.H. and W.C. Pitt (2012) Eleutherodactylus coqui Thomas (Caribbean tree frog), in R.A. Francis
(ed.) A Handbook of Global Freshwater Invasive Species. Earthscan, London, pp 311-319.
Brooks, J.E., P.J. Savarie, and J.J. Johnston, J.J. (1998) The oral and dermal toxicity of selected chemicals
to brown tree snakes. Wildlife Research 25, 427–35.
CAB International (2018) Invasive Species Compendium, Rana catesbeiana (American Bullfrog).
(https://www.cabi.org/isc/datasheet/66618)
CAB International (2019) Invasive Species Compendium, Eleutherodactylus planirostris (Greenhouse
Frog). (https://www.cabi.org/isc/datasheet/84737)
Clark, L., C.S. Clark and S. Siers (2018) Brown Tree Snakes Methods and Approaches for Control. USDA
National Wildlife Research Center - Staff Publications. 2032.
https://digitalcommons.unl.edu/icwdm_usdanwrc/2032
D’Amore, A. (2012) Rana [Lithobates] catesbeiana Shaw (American bullfrog), in R.A. Francis (ed.) A
Handbook of Global Freshwater Invasive Species. Earthscan, London, pp 321-330.
Engeman, R.M. and D.S. Vice (2002) Objectives and integrated approaches for the control of brown tree
snakes. Integrated Pest Management Reviews 6: 59-76.
EPPO (2011) Guidelines on Pest Risk Analysis. European and Mediterranean Plant Protection
Organization, Paris.
Ficetola, G.F., D. Rodder and E. Padoa-Schioppa (2012) Trachemys scripta (slider terrapin), in R.A. Francis
(ed.) A Handbook of Global Freshwater Invasive Species. Earthscan, London, pp 331-339.
Govindarajulu, P., R. Altwegg and B.R. Anholt (2005) Matrix model investigations of invasive species
control: bullfrogs on Vancouver Island. Ecological Applications 15: 2161-2170.
Kraus, F. (2009) Alien Reptiles and Amphibians, a Scientific Compedium and Analysis. Springer.
McGeoch, M.A., P. Genovesi, P.J. Bellingham, M.J. Costello, C. McGrannachan and A. Sheppard (2016)
Prioritizing species, pathways, and sites to achieve conservation targets for biological invasion. Biological
Invasions 18: 299-314.
Howarth, F.G. (1999) Environmental risks of biological control of vertebrates. In G. Rodda, Y. Sawai, D.
Chiszar and H. Tanaka (eds.) Problem Snake Management: Habu and Brown Treesnake. Cornell University
Press, New York, pp. 399–410.
Lau, M.W.N. (1998) Habitat use by Hong Kong amphibians with special reference to the ecology and
conservation of Philautus romeri. Ph.D. thesis, University of Hong Kong, Hong Kong.
Muller, B.J. and L. Schwarzkopf (2018) Relative effectiveness of trapping and hand-capture for controlling
invasive cane toads (Rhinella marina), International Journal of Pest Management 64: 185-192.
O’Keeffe, S. (2009) The Practicalities of Eradicating Red-eared Slider Turtles (Trachemys scripta elegans).
Aliens: The Invasive Species Bulletin 28: 19-25.
Pitt, W.C. and H. Sin (2004) Testing citric acid use on plants. Landscape Hawaii, July/August: 5-12.
Working Paper No. 3
Environmental Strategy, Conservation and Management Co. Ltd. 20
Shine, R. (2012) Rhinella marina L. (cane toad), in R.A. Francis (ed.) A Handbook of Global Freshwater
Invasive Species. Earthscan, London, pp 299-309.
Savarie, P.J., J.A. Shivik, G.C. White, and L. Clark (2001) Use of acetaminophen for large scale control of
brown treesnakes. Journal of Wildlife Management 65: 356–65.
Sin, H. and A, Radford (2007) Coqui Frog Research and Management Efforts in Hawaii, in G.W. Witmer,
W.C. Pitt, K.A. Fagerstone (eds) Managing Vertebrate Invasive Species: Proceedings of an International
Symposium. USDA/APHIS/WS, National Wildlife Research Center, Fort Collins.