USING PATHWAY ANALYSIS TO INFORM PREVENTION STRATEGIES FOR ALIENREPTILES AND AMPHIBIANS
FRED KRAUS, Bishop Museum, Honolulu, Hawaii, USA
Abstract: Alien reptiles and amphibians are deserving of greater attention that has hitherto been bestowed
upon them by managers and researchers. Eradication or control of established taxa will generally be
infeasible, leaving prevention of introductions as the primary management tool for controlling herpetologicalinvasions. I analyzed >5,700 introductions of alien reptiles and amphibians worldwide to obtain the pathway
information necessary for design of informed prevention programs. Six pathways account for the large
majority of introductions: accidental introductions via cargo and the nursery plant trade and intentional
introductions for biocontrol, food use, the pet trade, and aesthetic purposes. Pathway importance varies
taxonomically, temporally, and geographically. Unlike other taxa for which introductions have been
dominated by either accidental pathways alone or intentional pathways alone, reptile and amphibian
introductions involve a mix of both. Consequently, prevention programs must involve a two-pronged
approach for these taxa: risk assessment of pathways for taxa introduced accidentally and risk assessment ofspecies for taxa introduced intentionally. Because of variation in pathway importance, information on how
taxonomic, temporal, and geographic variables co-vary with economic and social data may allow forpredictive assessment of pathway risk for accidental introductions. In contrast, some predictive assessment oftaxon risk was achieved using variables that measure climate-matching between native and introduced ranges,
phylogenetic risk, and prior history of successful taxon establishment.
Key Words: alien species, amphibians, importation, invasive species, nursery trade, pet trade, reptiles, risk
assessment.
Managing Vertebrate Invasive Species: Proceedings ofan International Symposium (G. W. Witmer, W. C. Pitt,
K. A. Fagerstone, Eds). USDA/APHIS/WS, National
Wildlife Research Center, Fort Collins, CO. 2007.

(Bufo marinus) in Australia, sustained efforts toassess the ecological impacts of alien populationsof amphibians or reptiles are lacking. However,
impacts have been reported for a number of otheralien herp populations (Kraus, in prep.) anddescriptive autecological data are available for amodest number of additional populations. This is aremarkable degree of inattention given that 1,034
naturalizations of 315 species of alien herps havesuccessfully established in 287 politicaljurisdictions worldwide. The reasons for thisnegligence are likely twofold. First, most alien
reptiles and amphibians do not directly affecthumans, their domestic stock, or crops. Indeed,
their effects are likely to be nuanced ecologicalimpacts on other wildlife (e.g., insects, other herps)
whose ecological importance is readily discounted
by most humans. Hence, they are easily perceived– including by most scientists – as of little concern.
Second, most species, even large ones, are cryptic.
This makes them difficult for the average human tonotice and difficult for any scientist to study. Theirstudy is made more difficult by the compounding

INTRODUCTION
In the past decade, invasive alien species havebegun to receive (outside the Austral Englishspeaking countries, which have long been
responsive to the problem) the increased scientificand managerial attention they merit by virtue of
their ecological and economic destructiveness. For
terrestrial invasive species, this attention has amarked taxonomic bias, with considerable study
and management directed toward mammals, plants,
and economically important insects, and less sotoward other taxa. This is reasonable inasmuch as
those priority taxa cause tremendous damage that isfrequently apparent even to the casual observer.
The concern, though, is that silence about theignored taxa not be mistaken for a tacit admissionof ecological inconsequentialness.
Among those alien taxa largely overlooked as
scientific or management subjects are reptiles andamphibians, collectively referred to as “herps”.
With the exceptions of brown treesnakes (Boiga
irregularis) in Guam, bullfrogs (Rana catesbeiana)
in the western United States (US), and cane toads
94

fact of their nuanced ecological effects, which often
takes careful experimental research to determine.
Consequently, alien herps rarely give theimpression of occurring at plague proportions, as
do more visible taxa.
In this paper, I briefly review evidence
suggesting that this frequent oversight of alien
herps is neither warranted nor advisable. Thisevidence is presented in greater detail in aforthcoming book (Kraus, in prep.), and I will give
only the most cursory summary of that evidence
here. I then examine what mitigation measures
might be feasible for alien reptiles and amphibians
and how information derived from analysis of thepathways by which these animals are transported
can potentially assist in the design of effective
measures to prevent future invasions. That work is
ongoing, so this paper largely represents a broadconceptual overview of how alien herp invasions
might best be mitigated and a short status report ofefforts to date to meet that goal.

include predation or poisoning of native species,
secondary trophic effects, competition, andvectoring of parasites; evolutionary effects include
genetic contamination via hybridization, as well aschanges to morphology, physiology, and behavior(Table 1). A number of these impacts directly
affect endangered native species, with perhaps themost dramatic example affecting Raffone’s wall
lizard (Podarcis raffonei), which is almost extinct
due to competition (and perhaps genetic swamping)from introduced Italian wall lizard (P. sicula;
Capula et al. 2002). Direct effects on humans
include economic losses to agriculture, power
supplies, and property values; health impacts via
envenomation, water contamination, and disease
vectoring; and loss of scientific knowledge inbiogeography, taxonomy, and ecology (Table 1).
In short, alien reptiles and amphibians are capableof causing many of the negative impacts widely
appreciated in better-known taxa. Yet, despite thisbroad array of documented effects, ecologicalimpacts have been little studied in alien herps,
being noted for only 23 of 315 species having
established extra-limital populations. And nonepistemological effects on humans are reported for
only six of these populations (Table 1). Loss ofscientific knowledge has been a fairly common
result and only some of the more obvious examples
are provided in Table 1, although additional
examples from Mediterranean, Caribbean, andSoutheast Asian islands likely exist. It is to be
expected that damaging impacts are far morewidespread than currently evident and thatexamples will multiply once a broader array of taxa
and populations is sampled.
Establishment of jurisdictional populations ofalien reptiles and amphibians has been growing
exponentially since 1850 (Figure 1), with adoubling time of 30.4 years (Kraus, in prep.).
Thus, introductions are not abating, and the pool ofinvasive herp species is likely to grow in theforeseeable future. The wide array of impacts
already documented despite sparse study, coupledwith the rapidly increasing pace of alien herp
naturalization, lead me to conclude that negligenceof alien reptiles and amphibians is not warranted oradvisable from either an ecological or purely
anthropocentric viewpoint. Instead, I suggest that
greater scientific and managerial attention to thesetaxa, their impacts, and potential tools for their
management is overdue.

MATERIALS AND METHODS
I reviewed the literature up through the end of2005 to construct a database of alien reptile andamphibian introductions worldwide. Data
collected, when available, included jurisdiction towhich introduced, date(s) of introduction,
pathway(s) of introduction, minimum number of
introductions involved, and whether the
introduction led to a currently establishedpopulation. Jurisdictions were typically countries
but distinct island groups were tracked separatelyfrom the remainder of the country (e.g., Galapagos
separate from Ecuador, Ryukyu and Ogasawara
archipelagos separate from Japan) and species weretracked by state and province within the US andCanada. From these data, analyses were conducted
to assess the pathways by which herp species werebeing transported and to determine how thosepathways have varied. The same literature wasreviewed to assess what ecological and economic
impacts have been reported for alien herp
introductions and what control measures have been
attempted to date.

DO ALIEN REPTILES AND
AMPHIBIANS WARRANT BEING
IGNORED?
A variety of ecological and social impacts fromalien reptiles and amphibians has been documented,
even though only a small pool of species has beenstudied rigorously. Documented ecological impacts
95

Table 1. Impacts reported for alien reptiles and amphibians. Only ecological impacts on natives arenoted; species reported to affect other aliens are omitted. Only a few of many possible examples are
given of the scientific impacts. Data taken from Kraus (in prep.), which details references for all
instances.
ImpactEcological
Predation
Poisoning
Secondary trophic
effects
Competition

Disease vector
Evolutionary
Hybridization

Changed morphology
Changed physiology
Changed behavior
Economic
Agriculture
Power supplies
Property values
Human Health
Envenomation
Water contamination
Disease vector
Airstrike hazard
Scientific
Biogeography

Taxonomy
EcologySpecies
Anolis carolinensis, Anolis sagrei, Boiga irregularis, Bufo marinus, Natrix
maura, Osteopilus septentrionalis, Rana catesbeiana, Xenopus laevis
Bufo marinus
Anolis carolinensis, Boiga irregularis, Bufo marinus, Carlia ailanpilai,
Hemidactylus frenatus, Rana perezi
Anolis carolinensis, Anolis sagrei, Bufo marinus, Carlia ailanpilai,
Eleutherodactylus johnstonei, Hemidactylus frenatus, Osteopilus
septentrionalis, Podarcis sicula, Rana catesbeiana, Trachemys scripta
Ambystom tigrinum, Rana catesbeiana, Xenopus laevis
Ambystom tigrinum, Anolis distichus, Anolis sagrei, Cuora
flavomarginata, Emys orbicularis, Iguana iguana, Podarcis sicula,
Protobothrops elegans, Rana esculenta, Rana lessonae, Rana ridibunda,
Sauromalus spp., Trachemys scripta, Triturus carnifex
Bufo marinus
Bufo marinus
Bufo marinus, Natrix maura, Rana catesbeiana
Boiga irregularis, Bufo marinus, Varanus indicus
Boiga irregularis
Eleutherodactylus coqui
Boiga irregularis
Bufo marinus
Bufo marinus, Eleutherodactylus johnstonei
Iguana iguana
Cryptoblepharus poecilopleurus, Eleutherodactylus johnstonei,
Eleutherodactylus martinicensis, Emoia cyanura, Emoia impar, Gehyra
mutilata, Gehyra oceanica, Geochelone carbonaria, Hemidactylus
garnotii, Hemiphyllodactylus typus, Iguana iguana, Lepidodactylus
lugubris, Lipinia noctua, Nactus pelagicus, Rana ridibunda, Trachemys
decussata
Anolis distichus, Trachemys spp.
Boiga irregularis

96Despite this evidence and the damage that could
reasonably be predicted to follow widespread
establishment, State officials resisted efforts tointervene in this invasion for years, until well after
the narrow window had passed during whichstatewide eradication was possible (Figure 2). The
reasons for this failure resulted from theintersection of the species’ high intrinsic rate ofincrease with the social constraints that (1) officials
did not believe that a tiny frog could constitute a
pest problem, and (2) effective control methods
were lacking and took years to identify (Kraus andCampbell 2002). The biological parameterenforced a narrow window of control opportunity
before populations were irredeemably entrenched;
the social limitations emasculated response options
until after that window had passed.
A somewhat similar dynamic pitting highintrinsic growth rates of an alien herp against lackof human imagination characterized the recent
invasion of Burmese pythons (Python molurus) in
southern Florida. Despite having scores of capturesand reliable sightings of pythons in Everglades
National Park by 2002 (including 27 in that year
alone), biologists consulted to assess whether apopulation might be established advised that all
represented released pets and posed no cause forconcern (R. Snow, Everglades National Park,
personal communication). It was clear to everyone
even two years later (and clear in 2002 to anyone
with invasive reptile experience) that pythons werewidespread within the park and reproducing. They
are now known to occur in high population
densities and to range over an extensive area,
including well beyond park boundaries (Snow et al.
2007). Again, the narrow window of opportunity
for effective human response passed before it waseven recognized.
Both these examples illustrate the common
reaction from managers and many scientists toinvasive herps: disbelief that a problem exists. And
both species were allowed to expand beyond thepoint of effective control despite the fact thatcertain control advantages were present in these
instances that would not obtain for most alien
reptiles and amphibians. In particular, the problem
of crypsis, which seriously hampers detecting mostherp species, did not apply in the case of the coquibecause the males’ loud calls make them readilytargetable for control. And the extremely large sizeof the pythons (>7 m), which certainly makes themmore conspicuous than most alien herps, provided
no advantage for early recognition of theirnaturalization. I suggest that both of these

Figure 1. Cumulative growth in reptile and amphibian
introductions from 1850–2005. Curve from 1850-1999(before artifactitious decline resulting from reportingtime lag) is approximated by the exponential growthequation y = 46.014e0.2271x, with R2 = 0.9993 and adoubling time of 30.4 years.

HOW MIGHT ALIEN HERP INVASIONSBE FEASIBLY MANAGED?
Mitigation of impacts from invasive pests
logically involves either preventing introductions in
the first place, detecting and quickly eradicating
new incursions, or managing entrenched
populations so as to minimize their long-term
impacts. I argue that the crypsis, high reproductive
output, and high population densities thatcharacterize many invasive herps make the latter
two mitigation measures impossible to achieve inmost circumstances. Very few examples of
eradication or control have been attempted for alien
herps. I will briefly review two examples that, I
believe, are illustrative of the biological and social
limitations that will frequently hinder suchattempts.
Coqui frogs (Eleutherodactylus coqui) wereintroduced into Hawaii in the late 1980s or early
1990s in nursery plants imported from Puerto Rico.
Immediately upon discovering this species in
February, 1997, I repeatedly urged State officials
having the responsible authorities to eradicate thespecies before it was irredeemably beyond control.
This advice was motivated by knowledge of thehigh population densities the frogs achieve,
recognition that their establishment would insert anovel trophic level into Hawaiian forest
communities, and appreciation that their loud calls
make the males readily detectable and targetable(as well as obnoxious), a necessary condition forany eradication campaign (Kraus et al. 1999).
97

Figure 2. Contrast in growth in numbers of Hawaiian populations of coqui (Eleutherodactylus coqui) with milestones ofState government response. Populations became too numerous to track after mid-2001 but presumably continued to
grow exponentially, at least for a time. Despite that presumption, I merely outline the 2001 figures for all remaining
years; estimated infestation by 2006 on Hawaii Island alone was >5000 acres (M. Wilkinson, HI Dept. Land & Nat. Res.,
personal communication). 1 = Alarm first raised by DLNR and USDA employees regarding threat posed by coqui,
March 1997; 2 = first significant media coverage of coqui threat, November 1998; 3 = first State support to identify
means of chemical control for coqui, June 2000; 4 = first Legislative action to facilitate control of coqui, indemnifying
landowners to allow access for control operations, May 2003; 5 = first quarantine measures requiring treatment of interisland shipments from infested nurseries, pending as of February 2006; Legislature declares coqui a pest, May 2006.

examples were unusual only in that biologicalweaknesses existed that could potentially have been
exploited for effective control. However, theseadvantages were squandered by blinkered human
responses that did not recognize the evidence of or
admit the potential seriousness of the situation.
On the other hand, the coqui and python
examples well exemplify the high reproductive
rates (Kraus et al. 1999, Kraus and Cravalho 2001,
Snow et al. 2007) that typify many other alien herp
species. These high intrinsic population growth
rates create very narrow windows of opportunity
for implementing successful control. It is true thatearly detection and eradication of alien herps will
sometimes be possible (cf., Whitaker and
Bejakovich 2000 for Eastern banjo frog
[Limnodynastes dumerilii] in New Zealand, andFisher and Garner 2007 for bullfrogs in Britain).
However, I suggest that most alien herp species
will be even more difficult to control than coquis
and pythons because (1) we will typically lack thebenefit of having a readily identified biologicalfulcrum against which to act, (2) other species arelikely to be more cryptic, and (3) psychologicalunresponsiveness to alien herps is likely to persist
for the foreseeable future. From these
considerations, I conclude that meaningfulmanagement of alien reptiles and amphibians mustlargely rely on prevention of their introduction.
But design and implementation of effective
prevention programs requires knowledge of how
and why these animals are being transported. In
short, one must understand the pathways by whichalien reptiles and amphibians are moved and how
those pathways vary.

HOW ARE ALIEN REPTILES AND
AMPHIBIANS BEING TRANSPORTED?
Only one study (Kraus 2003) has quantified how
alien herps are being transported by humans andthat study was preliminary to the expanded
database analyzed in Kraus (in prep.) and briefly
summarized here. Kraus (2003) showed, based ona dataset of 577 introduction records, that six
pathways have accounted for most herp

98

Figure 3. Numbers of introductions of alien reptiles and amphibians by pathway, 1850-2005. Open bars indicate
intentional pathways; closed bars accidental pathways. “Intentional” refers to deliberate private introductions foraesthetic self-indulgence or other purpose.

number of pathways too, but at lower levels, andsalamanders and crocodilians have been transported
relatively rarely (Figure 4). Only the pet trade andpersonal aesthetic pathways have involved all
higher-level taxa, whereas the other four pathways
have been more taxonomically limited in scope(Figure 4). Relative success in leading tonaturalization varies among pathways, withtransport via the nursery, biocontrol, and food
pathways being far more likely to lead to successful
establishment than transport via the other three(Kraus 2003, in prep.).
Pathway importance has varied temporally, withthe cargo pathway being of greatest importance forthe second half of the 19th Century, intentional
aesthetic introductions of greatest importance forthe first part of the 20th Century, and the pet tradeof greatest predominance since the 1970s (Figure5). The biocontrol pathway has been largely
quiescent for the past several decades, the food-use
pathway has grown relatively little during that same

introductions and that pathway importance variedtaxonomically, temporally, and geographically.
These major conclusions are upheld by the far more
complete sampling (>5,700 introduction records)
now available. Unlike most higher-level taxa, bothintentional and accidental pathways have been
important in the introduction of reptiles andamphibians (Figure 3). Intentional pathways
include introductions for biocontrol, food use, thepet trade, and personal release of animals foraesthetic self-indulgence. Accidental pathways
include transport as aquacultural contaminants,
cargo stowaways, and hitch-hikers in nursery plantmaterials, the last a special subcategory of cargo
stowaways deserving of separate tracking becauseof its increasing importance.
Frogs and lizards have been the most commonlytransported herps and are the only major taxa tohave utilized all six major pathways (Figure 4).
Snakes and turtles have been introduced via a99

FrogsSalamanders

Lizards

SnakesT urtlesCrocodilians%Contribution to Pathway

100%

80%

60%

40%

20%

0%
Biocontrol

Cargo

Food

Intentional

NurseryPet

Pathway
Figure 4. Relative taxonomic distribution among the six primary introduction pathways, 1850-2005. “Intentional”
refers to deliberate private introductions for aesthetic self-indulgence.

period, but the nursery-trade pathway has increased
considerably in importance since the 1970s (Figure5). Introduction rates from 1980-2005 have been
considerably higher for the pet-trade, nursery-trade,and cargo pathways than for the period 1850-1979(Kraus, in prep.). Rates of introduction forbiocontrol have been drastically less during thatsame period, while rates for food use andintentional aesthetic introductions have remained
unchanged (Kraus, in prep.).
The large majority of documented introductions
have involved Europe and North America (Kraus,
in prep.). Kraus (2003) showed pathway
importance varies among the US, Caribbean, andPacific regions, and this pattern holds true for otherregions as well. Generally speaking, the pet tradeis the most important pathway for introductions
into North America, Europe, and Asia, whereas
cargo has predominated in the Pacific, and cargo
and the nursery plant trade are co-dominant in the
Caribbean (Figure 6). The upshot is that eachmajor

geographical region has its own unique signature ofpathway representation and relative importance(Kraus, in prep.).

HOW MIGHT EFFECTIVE
PREVENTION PROGRAMS BE
DESIGNED FOR ALIEN HERPS?
The information above has important
ramifications for designing and implementing
effective prevention strategies for invasive reptiles
and amphibians. First, strategies to abate
intentional versus accidental introductions are quite
different, but both will need to be included to limit
further alien herp incursions. Intentional
introductions may be addressed by implementing
some form of taxon-based risk-assessment system
incorporating features that appear diagnostic foreither naturalization or invasiveness, as is done forweeds (Pheloung et al. 1999, Walton et al. 1999,
Williams et al. 2002) and some vertebrates(Bomford and Hart 1998, Kolar and Lodge 2002,
Bomford 2003, Bomford and Glover 2004). Such amethod requires identification of ecological orother predictors of naturalization/invasiveness for100

Figure 5. Changes in global pathway importance for the six major pathways, 1850-2005. Solid line, biocontrol; open
circles, cargo; open triangles, food use; X’s, nursery trade; filled squares, pet trade; filled diamonds, intentional aesthetic
releases.
0.7

Percentage of Introductions

0.6

0.5

0.4

0.3

0.2

0.1

0
Biocontrol

Cargo

Food

Intentional

Nursery

Pet

Other

Pathway
Figure 6. Variation in pathway importance by geographic region for all documented introductions, 1850-2005. Diagonallines, North America; stippling, Europe; horizontal lines, Pacific region; and solid bars, Caribbean.

101the targeted taxa. Higher numbers of predictive
variables should be expected to lead to more robust
results, as typifies the weed-risk-assessment
systems (Pheloung et al. 1999, Daehler et al. 2004).
Bomford et al. (2005) tested the utility of four
variables for their usefulness in predicting
naturalization among alien reptiles and amphibians
introduced to Great Britain, California, and Florida.
They found that degree of climate match between
native and introduced ranges, a taxonomic-risk
variable, and whether a species had previously
established elsewhere were useful in segregating
among species that successfully established in thesethree jurisdictions versus those that did not. In
contrast, they found that size of native range(thought to reflect breadth of ecological tolerance)
had no predictive value. This study is currently
being refined, and a larger number of variables
needs to be included, but their results suggest thatsuch an approach may be promising for predicting
naturalization ability among reptiles andamphibians in particular regions, thereby providing
a tool for screening intentionally introduced
species. Efforts will also soon be made to identify
additional ecological variables that may refine thisapproach.
Approaches toward preventing reptile andamphibian introductions via accidental pathways(primarily cargo and nursery plant trade in mystudy, but also including lesser numbers introducedon vehicles and as aquacultural contaminants) are
more tentative. Recall that each geographicalregion exhibited its own characteristic signature ofpathway representation and importance. It may
prove possible to correlate these patterns withsimple economic or legal variables for any country
within each region and use those indicators to
predict which pathways merit greatest attention
within any particular jurisdiction. Should thatprove true, one could then prioritize limited
resources to those pathways so identified, thereby
maximizing prevention efficiency. For example,
should magnitude of nursery-trade introductions
correlate with income levels, assessing themagnitude of the latter could be used to predict therisk of the former and inform decisions as to howmuch effort to devote to searching or treating thatpathway. Assessing whether economic indicators
correlate with pathway intensity is the subject ofongoing investigation by the author along with B.
Kaiser and K. Burnett.
Should such economic predictors be identified,
it would still leave unresolved how animals in theaccidental pathways are best intercepted. All such

pathways must involve some form of port-of-entry
inspection program, relying on the border
quarantine programs widely in place around theworld. But such programs necessarily sample only
a very small portion of arriving goods and must
rely on repeated sampling schemes to identify
highest-risk pathways and provide quality
assurance. This is where prioritization of
inspection effort informed by economic
predictiveness of pathway importance may prove
useful. However, treatment methods to remove
hitch-hiking herps, other than hand-capture, havebeen little investigated and we require developmentof methods of scale that can effectively treat entire
cargo shipments. As one example of what isneeded in this regard, a short hot-water drench is
sufficient to kill hitch-hiking frogs in potted
nursery plants (F. Kraus unpublished data), apathway of increasing importance, but is justbeginning to be scaled up to a level commensuratewith industry volume in Hawaii (D. Cravalho,
Hawaii Dept. Agriculture, personalcommunication). Similar scaled-up methods that
go beyond visual inspection and hand-capture areneeded to more effectively address reptile transport
via this same pathway and to address all species
transported in other accidental pathways.
Note that both prevention options discussed
above – taxonomic risk-assessment systems forintentional introductions and identification of
economic indicators to assess risk of accidental
pathways – serve (ideally) only to best identify
those taxa or pathways of highest risk for
inadvertent importation or establishment. In many
countries, having that information does notnecessarily mean it will be used for interdiction,
due to a variety of political, legal, and procedural
hurdles discussed in detail by Simberloff (2005) forthe case of the US. For intentional introductions,use is required of a comprehensive white list/black
list system that explicitly adopts the precautionary
principle and forbids importation until a species has
been assessed as posing little risk. Such methods
may have their statistical limitations (cf. Smith etal. 1999, Caley et al. 2006) but are vast
improvements over the minimal, reactive programs
that typify the majority of countries. In the case of
the US specifically, this reactive stance is embodied
in the Lacey Act, part of which includes aminiscule list of “injurious wildlife” banned fromimport because they have proven harmfulelsewhere. A problem with that reactive approach,of course, is that only a tiny fraction of proveninvasive species – those that have garnered some102

DAEHLER, C. C., J. S. DENSLOW, S. ANSARI, AND H. C.
KUO. 2004. A risk-assessment system for screeningout invasive pest plants from Hawaii and other
Pacific islands. Conservation Biology 18:360-368.
FISHER, M. C., AND T. W. J. GARNER. 2007. The
relationship between the emergence of
Batrachochytrium dendrobatidis, the international
trade in amphibians and introduced amphibian
species. Fungal Biology Reviews 21:2-9.
KOLAR, C. S., AND D. M. LODGE. 2002. Ecologicalpredictions and risk assessment for alien fishes inNorth America. Science 298:1233-1236.
KRAUS, F. 2003. Invasion pathways for terrestrial
vertebrates. Pages 68-92 in J. Carlton, G. Ruiz, and
R. Mack, editors. Invasive species: vectors andmanagement strategies. Island Press, Washington,
D.C., USA.
KRAUS, F., AND E. CAMPBELL. 2002. Human-mediatedescalation of a formerly eradicable problem: theinvasion of Caribbean frogs in the Hawaiian Islands.
Biological Invasions 4:327-332.
KRAUS, F., AND D. CRAVALHO. 2001. The risk to Hawaii
from snakes. Pacific Science 55:409-417.
KRAUS, F., E. W. CAMPBELL, A. ALLISON, AND T.
PRATT. 1999. Eleutherodactylus frog introductions
to Hawaii. Herpetological Review 30:21-25.
PHELOUNG, P. C., P. A. WILLIAMS, AND S. R. HALLOY.
1999. A weed risk assessment model for use as abiosecurity tool evaluating plant introductions.
Journal of Environmental Management 57:239-251.
SIMBERLOFF, D. 2005. The politics of assessing risk forbiological invasions: the USA as a case study.
Trends in Ecology and Evolution 20:216-222.
SMITH, C. S., W. M. LONSDALE, AND J. FORTUNE. 1999.
When to ignore advice: invasion predictions anddecision theory. Biological Invasions 1:89-96.
SNOW, R. W., K. L. KRYSKO, K. M. ENGE, L.
OBERHOFER, A. WARREN-BRADLEY, AND L.
WILKINS. 2007. Introduced populations of Boa
constrictor (Boidae) and Python molurus bivittatus(Pythonidae) in southern Florida. Pages 416-438 in
R. W. Henderson and R. Powell, editors. Biology ofthe boas and pythons. Eagle Mountain Publishing,
Eagle Mountain, Utah, USA.
WALTON, C., N. ELLIS, AND P. PHELOUNG. 1999. A
manual for using the weed risk assessment system(WRA) to assess new plants. Australian Quarantine
and Inspection Service, Canberra, Australia.
WHITAKER, T., AND D. BEJAKOVICH. 2000. Exotic frog
incursion. Surveillance 27:12-14.
WILLIAMS, P. A., A. WILTON, AND N. SPENCER. 2002. A
proposed conservation weed risk assessment system
for the New Zealand border. Science forConservation 208:1- 47.

level of political concern – are included on that list
and high-risk species that happen not to have been
imported anywhere yet are ignored entirely.
Similar problems attend assessments of risk forparticular pathways, which typically assess only alimited pool of likely risks and impacts (Simberloff
2005). It is worth noting that continued pursuit ofsuch minimalist policies for preventing invasions isinconsistent with the risk-analyses outlined here,
which are intended to identify high-risk taxa andpathways so that exclusionary measures might
actually be implemented. As far as I am aware, acomprehensive approach based on the
precautionary principle seems to have been adopted
to date only by Australia and New Zealand.

ACKNOWLEDGMENTS
This work was supported by the US Fish andWildlife Service and Hawaii Invasive Species
Council. I especially thank Earl Campbell for his
continued interest in and support for this project
and for commenting on an earlier version of themanuscript.

LITERATURE CITED
BOMFORD, M. 2003. Risk assessment for the import andkeeping of exotic vertebrates in Australia. Bureau of
Rural Sciences, Canberra, Australia.
BOMFORD, M., AND J. GLOVER. 2004. Risk assessment
model for the import and keeping of exotic
freshwater and estuarine finfish. Bureau of Rural
Sciences, Canberra, Australia.
BOMFORD, M., AND Q. HART. 1998. Risk assessment forimporting and keeping exotic vertebrates. Pages
406-410 in R. O. Baker and A. C. Crabb, editors.
Proceedings of the 18th Vertebrate Pest Conference,
University of California, Davis, California, USA.
BOMFORD, M., F. KRAUS, M. BRAYSHER, L. WALTER,
AND L. BROWN. 2005. Risk assessment model forthe import and keeping of exotic reptiles andamphibians. Final report to Australian Departmentof Environment and Heritage.
CALEY, P., W. M. LONSDALE, AND P. C. PHELOUNG.
2006. Quantifying uncertainty in predictions ofinvasiveness, with emphasis on weed risk
assessment. Biological Invasions 8:1595-1604.
CAPULA, M., L. LUISELLI, M. A. BOLOGNA, AND A.
CECCARELLI. 2002. The decline of the Aeolian wall
lizard, Podarcis raffonei: causes and conservation
proposals. Oryx 36:66-72.

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