Diversity and Distributions (2003) 9, 99– 110
SPECIAL ISSUE: AMPHIBIAN DECLINES
Alien predators and amphibian declines: review of two
decades of science and the transition to conservation
Blackwell Publishing, Ltd
LEE B. KATS* and RYAN P. FERRER Natural Science Division, Pepperdine University, 24255
Pacific Coast Highway, Malibu, CA 90263, U.S.A. E-mail: lee.kats@pepperdine.edu
Abstract. Over the last two decades, numerous
studies have shown that alien predators contributed to amphibian population declines. Both
experimental studies and correlative field surveys
implicated alien species of fish, bullfrogs and
crayfish as major contributors to amphibian
population decline, and in some instances local
extinction. Additional studies have demonstrated that alien predators also caused long-term
changes in aquatic communities. Recent studies
have examined the feasibility of removing alien
predators, and provide some evidence that
amphibian populations can recover. Applying
INTRODUCTION
While many suggested causes of global amphibian declines are controversial, others have
become widely accepted as contributors. Debates
continue as to the role of disease and climate
change as contributors to amphibian population
declines, while other potential causes of population declines such as habitat loss and the spread
of alien species have become generally accepted
as detrimental to amphibians (Kiesecker, 2003).
Fifteen years ago, the negative impact of alien
predators on amphibians was still considered a
theory in need of testing (Hayes & Jennings,
1986). Since then, numerous studies have implicated alien species in amphibian population
declines (Tables 1 and 2). Aliens harm amphibians by competing with native species (Kiesecker,
2003), carrying disease (Kiesecker et al., 2001a;
* Corresponding author
information gained from past studies to the
recovery of amphibian populations will be the
challenge of future studies. International, national
and local policies that regulate alien predators
should be based largely on the body of scientific
evidence already in the literature. Scientists
need to be more involved with policy-makers to
most effectively change laws that regulate alien
predators.
Key words. Amphibian decline, biological invasions, conservation, declining amphibian populations, alien predators.
Blaustein & Kiesecker, 2002), hybridizing (Riley
et al., in press) or preying on amphibians. This
review examines recent studies that implicated
aliens as predators on amphibians, and explores
the implications of the continued spread of aliens
in the decline of amphibians.
SOURCES OF ALIEN PREDATORS
Alien species introduced from one continent to
another (e.g. Stein et al., 2000), often have negative
impacts on amphibians (Tables 1 and 2). Translocation of species within a continent generally
has similar ecological implications. In both situations, native species lack evolutionary history
with aliens, and lack of adaptations leads to
population declines. Many amphibian species
apparently lack prior evolutionary experience
even with species functionally similar to most
alien predators (Diamond & Case, 1986); consequently, amphibians are particularly vulnerable
to alien predators.
© 2003 Blackwell Publishing Ltd. http://www.blackwellpublishing.com/journals/ddi
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L. B. Kats and R. P. Ferrer
Table 1 Examples of field surveys showing negative correlations between alien predators and native
amphibian populations
Native amphibian
Alien predator
Reference
Hyla regilla
Rana muscosa
Oncorhynchus sp.
Salmo sp.
Oncorhynchus sp.
Oncorhynchus sp.
Oncorhynchus sp.
Oncorhynchus sp.
Salvelinus fontinalis
Salmo trutta
Rana catesbeiana
Rana catesbeiana
Rana catesbeiana
Rana catesbeiana
Rana catesbeiana
Rana catesbeiana
Oncorhynchus mykiss
Salmo trutta
Oncorhynchus mykiss
Salmo trutta
Oncorhynchus sp.
Oncorhynchus sp.
Salvelinus fontinalis
Gambusia affinis
Procambarus clarkii
Procambarus clarkii
Matthews et al. (2001)
Bradford (1989)
Knapp & Matthews (2000)
Knapp et al. (2001)
Bradford et al. (1993)
Bradford et al. (1998)
Rana aurora
Rana boylii
Rana blairi
Rana pipiens
Litoria spenceri
Litoria phyllochroa
Ambystoma macrodactylum
Taricha torosa
Alien predators have affected almost exclusively amphibians with complex life cycles (adult
and larval stages). Amphibian eggs and aquatic
larvae are particularly vulnerable to alien aquatic
predators (although alien rats reportedly caused
the extinction of species of New Zealand frogs,
Towns & Daugherty, 1994). Alien predators are
spread to new locations both intentionally and
accidentally. Fish are the most widespread alien
predator on amphibians (Stebbins & Cohen,
1995) and in many cases have been placed into
habitats to provide game for sport fishermen
(Cory, 1963; Knapp, 1996; Stein et al., 2000). For
example, salmonids have been introduced
throughout the world, including North America,
Australia, New Zealand, Europe and Central
America (Bradford et al., 1993; Brönmark &
Edenhamn, 1994; Townsend, 1996; Pough et al.,
1998). Mosquitofish (Gambusia) are one of the
most widespread genera of vertebrates because
of their effectiveness in controlling mosquito
Moyle (1973)
Jennings and Hayes (1985)
Adams (1999)
Moyle (1973)
Hammerson (1982)
Hammerson (1982)
Gillespie (2001)
Gillespie (2001)
Tyler et al. (1998a)
Funk & Dunlap (1999)
Gamradt & Kats (1996)
Gamradt et al. (1997)
populations. Unfortunately, they eat more than
mosquitoes (Miura et al., 1979; Bence, 1988) and
their diet includes amphibian larvae (Webb &
Joss, 1997; Goodsell & Kats, 1999). Every fish
introduction for biological control that has been
studied thoroughly has had negative effects on
non-target species (Simberloff & Stiling, 1996).
Bullfrogs (Rana catesbeiana) also have been
intentionally distributed to new habitats as a
human food item (Moyle, 1973; Jennings &
Hayes, 1985). While bullfrogs are thought frequently to be competitors with other amphibians (Kupferberg, 1997; Kiesecker et al., 2001b),
adults are generalist predators and also feed on
native amphibians (Zweifel, 1955; Beringer &
Johnson, 1995; Kiesecker & Blaustein, 1997).
Once bullfrogs colonize a habitat they are difficult to remove, and their effects on aquatic systems are long-lasting (Bury & Luckenbach, 1976;
Todd, 2001; New South Wales National Parks
and Wildlife Service, 2002).
© 2003 Blackwell Publishing Ltd, Diversity and Distributions, 9, 99– 110
Native amphibian
Alien Predator
Effect on native amphibian
Reference
Rana aurora
Rana catesbeiana
Reduced activity/Increased refuge use/
Decreased survivorship
Reduced metamorph size and rate/Survivorship/
Habitat use alteration
Reduced metamorph size and rate/Survivorship/
Habitat use alteration
Reduced feeding activity
Tail injury/Reduced metamorph size/
Decreased activity
Decreased survivorship/Reduced metamorph size
Reduced metamorphosis rate
Tail injury
Reduced metamorph size and rate
Reduced metamorph size and rate/
Decreased survivorship
Decreased larval survivorship
Decreased larval survivorship
Decreased larval survivorship
Decreased larval survivorship
Decreased larval survivorship
Decreased larval survivorship
Decreased survivorship
Decreased larval survivorship/
Habitat use alteration/Decrease in size
Decreased larval survivorship/
Habitat use alteration/Decrease in size and weight
Decreased egg and larval survivorship
Decreased larval survivorship
Tail injury/Decreased breeding activity
Kiesecker & Blaustein (1997)
Rana catesbeiana
R. catesbeiana and M. dolomieui
R. catesbeiana
Gambusia affinis
Rana temporaria
Hyla regilla
Litoria spenceri
Litoria phyllochroa
Bufo sp.
Ambystoma macrodactylum
Rana catesbeiana
G. affinis and R. catesbeiana
Pacifastacus leniusculus
Oncorhynchus mykiss
P. leniusculus and O. mykiss
Gambusia affinis
Oncorhynchus mykiss
Salmo trutta
Oncorhynchus mykiss
Salmo trutta
Pacifastacus leniusculus
Carassius auratus
Oncorhynchus sp.
Ambystoma gracile
Oncorhynchus sp.
Taricha torosa
Procambarus clarkii
Gambusia affinis
Procambarus clarkii
Kiesecker & Blaustein (1998)
Kiesecker et al. (2001)
Lawler et al. (1999)
Nyström et al. (2001)
Goodsell & Kats (1999)
Gillespie (2001)
Gillespie (2001)
Axelsson et al. (1997)
Monello & Wright (2001)
Tyler et al. (1998b)
Tyler et al. (1998b)
Gamradt & Kats (1996)
Gamradt et al. (1997)
Impact of alien predators on amphibians
© 2003 Blackwell Publishing Ltd, Diversity and Distributions, 9, 99– 110
Table 2 Examples of the effects of alien predators on native amphibans in experimental studies
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L. B. Kats and R. P. Ferrer
Populations of alien predators also establish
through accidental introductions or when people
release unwanted animals into the wild. Accidental introductions would include primarily the
unintended dispersal of organisms that had been
stocked as game or as biocontrol agents (Kolar
& Lodge, 2000). Game fish intended for lakes or
ponds are presumably spread easily to other
habitats during floods (e.g. Bradford, 1989). Crayfish, widespread alien predators on amphibians,
are found frequently in lakes and ponds either
because they are stocked intentionally as fish
food or accompany stocked fish (Lodge et al.,
2000). In the Santa Monica Mountains of southern California, crayfish (Procambarus clarkii) are
found in relatively isolated streams where the
source of crayfish colonization is often not clear
(Gamradt & Kats, 1996). However, in some
streams it seems likely that crayfish washed out
of nearby ponds during floods. Their original
source is probably an angler’s bait bucket. The
dumping of bait buckets is the most important
vector for the introduction of alien crayfish
(Ludwig & Leitch, 1996; Lodge et al., 2000).
Mosquitofish also spread via similar mechanisms
after being introduced for biocontrol purposes
(Gamradt & Kats, 1996). While these small fish
are placed typically into ephemeral pools to control mosquito larvae, floods probably sweep these
fish into nearby streams, rivers or lakes. Some
alien predator populations become established
when organisms are released into the wild
because they are no longer wanted as pets or are
no longer needed in captivity. For example,
goldfish (Carassius auratus) have become established in some habitats and while generally not
thought of as predatory, at least one study has
indicated that goldfish will eat amphibians
(Monello & Wright, 2001). Clawed frogs (Xenopus spp.) are used widely in biological experiments. Populations of these animals have
probably been established when people released
clawed frogs that were no longer needed for their
intended purpose.
TYPES OF STUDIES
Two types of studies have demonstrated negative
effects of alien predators on amphibians. Many
studies report a negative correlation in the field
between the presence of alien predators and the
absence of amphibians (Table 1). Experimental
studies demonstrate that either amphibians do
poorly (slowed growth, smaller size) in the presence of aliens or are eliminated in short-term
studies because of high mortality (Table 2). The
large number of convincing studies in both categories has contributed to the widely accepted
knowledge that alien predators are contributing
to amphibian decline.
Correlative studies are based typically on field
surveys, where habitats that currently have alien
predators are compared to nearby, similar habitats that do not contain alien predators. These
comparisons can offer great insights into our
ecological understanding of invasions and subsequent impacts on amphibians (Diamond &
Case, 1986). In these studies, amphibians are then
scored as being present or absent and in some
cases actual densities of amphibians are estimated. Studies of this type are even more useful
if museum records or historical field notes indicate
that amphibians were once present in habitats
where they are now absent. Valuable information
can be obtained from historical records even if
amphibian presence/absence data only are
known. Fisher & Shaffer (1996) note that museum
records, even if there are only a few specimens
from a site, help to establish that amphibians
were once present in a habitat where they might
now be absent. Gamradt & Kats (1996), for
instance, in studying the impact of alien crayfish
and mosquitofish (Gambusia affinis) on amphibians in the streams of southern California, relied
heavily on field surveys that had been conducted
roughly 20 years prior to their own (De Lisle
et al., 1986). The early surveys did not estimate
amphibian densities, yet they established clearly
the presence of amphibian species before the
introduction of alien predators.
Field surveys for amphibians are heavily
dependent upon sampling techniques. Today the
natural history of many amphibians has been
well described for species that tend to inhabit
areas where aliens are a problem. Presence and
absence of amphibians is relatively straightforward to document by directly counting adults,
eggs or larvae. Survey of vocalizing adult frogs
can also be used. If surveys occur during or
immediately after breeding, most amphibian species can be detected with relative certainty
(McDiarmid, 1994).
© 2003 Blackwell Publishing Ltd, Diversity and Distributions, 9, 99– 110
Impact of alien predators on amphibians
Experimental studies complement correlative
studies. Correlative studies report the final outcome; whether amphibians can co-exist with
alien predators or not. Experimental studies
frequently provide the mechanisms by which
alien predators eliminate amphibians (Table 2). In
these studies, alien predators are placed with
amphibians in either field or laboratory experiments. Proper controls then compare the success
of amphibians without alien predators to
amphibians with alien predators. While there has
been, in general, minimal criticism of experimental studies examining alien predator impact on
amphibians, a potential weakness of some experimental work was conducting experiments in
environments that were overly simple (Lawler
et al., 1999). They suggested that experiments
that place only alien predators and amphibians
together might promote the targeting of amphibians as prey by alien predators if no other organisms are part of the system. Mosquitofish, once
thought to be mosquito specialists and potentially too small to feed on amphibians were not
immediately accepted as alien predators on
amphibians. Goodsell & Kats (1999) provided
high densities of mosquito larvae in addition to
frog tadpoles and demonstrated that even when
mosquito larvae are provided, tadpoles are still
decimated by mosquitofish within a few hours.
Further, wild-caught mosquitofish had tadpoles
in their stomachs, providing definitive evidence
that even in the complexity of natural habitats,
alien mosquitofish still eat amphibians.
EFFECTS OF ALIEN PREDATORS
ON AMPHIBIANS
Many correlative studies have reported that alien
predators have eliminated amphibians or have
reduced their numbers (Table 1). As suggested
previously, this occurs because native amphibians
have little or no evolutionary history with alien
predators (Diamond & Case, 1986; Gillespie, 2001).
While many experimental studies report increased
mortality of amphibians with alien predators,
other studies report a more indirect impact of
alien predators on amphibians (Table 2). Studies
have found that amphibian larvae grow less or
metamorphose at smaller sizes when they are
raised with alien predators than when they are
raised without them. Mechanisms for mediating
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these changes in growth and metamorphosis are
probably the result of standard responses to predators; that is, reduced movement and reduced
feeding on the part of amphibians in the presence
of predators (Kats & Dill, 1998).
It is now generally accepted that alien predators can drive local populations of amphibians to
extinction (Bradford, 1991; Bradford et al., 1994;
Gamradt & Kats, 1996; Matthews et al., 2001).
Amphibian populations are now frequently
absent in habitats where alien predators have
become established. Surveys where alien predators and amphibians co-exist may reflect in part
a more recent colonization by the aliens. Surveys
in those instances might capture a ‘snapshot’ of
a longer process that might lead ultimately to
complete elimination of amphibians by alien
predators. Other amphibian populations can be
reduced to such low numbers by alien predators
that they will probably become isolated from
other populations, and may ultimately disappear
(Bradford et al., 1993; see also Fig. 1).
Recent studies have addressed more complicated interactions that involve more than one
alien predator and affects on amphibians. In a
study looking at bullfrog and mosquitofish
impacts on red-legged frog tadpoles (Lawler
et al., 1999), the effects of bullfrogs were so great
that they dominated the smaller effects of the
mosquitofish. Only bullfrog tadpoles were used
and it is not clear if the impact of the bullfrogs
came from competitive effects or predation. In
another study, Kiesecker & Blaustein (1998)
found that the combined effects of bullfrogs
(both adults and tadpoles) and smallmouth bass
(Micropterus dolomieui) on red-legged frog (Rana
aurora) tadpoles was far greater than when each
factor was considered separately. Bullfrogs
continue to spread to areas outside of North
America (see for example, Stumpel, 1992), and
are predicted to impact amphibians negatively in
these new regions as they have in their expanded
range in North America.
Patterns have emerged from the studies that
have implicated alien predators in causing
amphibian declines. Because many of the most
damaging aliens (e.g. fish, crayfish, bullfrogs) are
dependent on permanent or near permanent water
for their survival, amphibians that typically
inhabit permanent water are frequently documented as those most impacted by the aliens. For
© 2003 Blackwell Publishing Ltd, Diversity and Distributions, 9, 99– 110
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L. B. Kats and R. P. Ferrer
Fig. 1 The possible outcomes when alien predators are introduced to native amphibian populations.
example, in southern California, three species of
stream-breeding amphibians inhabit the streams
of the Santa Monica Mountains. California
newts (Taricha torosa) and California treefrogs
(Hyla cadaverina) breed only in permanent
streams. Pacific treefrogs (Hyla regilla) are habitat generalists and breed in both permanent
streams and temporary pools and ponds. Pacific
treefrogs remain common, while both California
treefrogs and California newts are becoming
increasingly rare (Jennings & Hayes, 1994), in
part because of the spread of alien predators in
permanent streams (Stebbins, 1985; Gamradt &
Kats, 1996). Pacific treefrogs are widespread
along the west coast of North America and their
continued success may be due in part to their
ability to breed in ephemeral habitats and avoid
alien predators.
Some species of amphibians co-exist naturally
with many of the same predators that end up distributed as aliens (Petranka, 1983). These species
have evolved defence mechanisms that allow
them to co-exist with predators (Kruse & Francis,
1977). Some species, while not currently coexisting with predators may have come from a
phylogeny that includes defence mechanisms to
many predators. Bufo tadpoles, for instance, are
well known to be distasteful to predators and
Bufo populations that are not found currently
with predators may be preadapted to deal with
alien predators if they were to colonize the
habitat (Diamond & Case, 1986). In other cases,
defence mechanisms that are effective against native predators are apparently unsuccessful against
alien predators. Taricha contain a powerful neurotoxin that makes them unpalatable to almost
all predators (Petranka, 1998), yet alien crayfish
will attack even adult newts, apparently undeterred
by the toxin (Gamradt et al., 1997).
THE BIGGER PICTURE: ALIENS
IMPACTING MORE THAN
AMPHIBIANS
While numerous studies have examined interactions of alien predators and amphibians, more
recent studies have begun to look at the more
widespread impacts of alien predators on aquatic
communities. For example, a recent study extends
the negative correlation between trout and frogs
by looking at a third species, an aquatic garter
snake that feeds primarily on frogs (Matthews
et al., 2002). They found that whenever frog
populations were greatly reduced or absent so
were aquatic garter snakes (Thamnophis elegans).
When habitats did not contain introduced trout,
amphibians still existed in lakes and garter
snakes were also present (Matthews et al., 2002).
This confirmed an earlier observation that suggested that garter snakes might survive the disappearance of some species of amphibians, but
would probably not survive if all amphibians
were impacted by alien predators (Jennings et al.,
1992). In addition, Bradford et al. (1998) noted
© 2003 Blackwell Publishing Ltd, Diversity and Distributions, 9, 99– 110
Impact of alien predators on amphibians
that alien trout not only reduced tadpoles, but
they also reduced or eliminated microcrustaceans
and many macroinvertebrates.
Nyström et al. (2001) examined the effects of
multiple-introduced predators on a littoral pond
community. They found that alien crayfish and
rainbow trout (Oncorhynchus mykiss) negatively
impacted native common frog (Rana temporaria)
tadpoles and had both direct and indirect
impacts on multi-trophic levels in the community.
For instance, both snail biomass and macrophyte
coverage decreased with the alien predators. They
found that aliens can have widespread impacts on
food webs in pond communities.
Ricciardi & Rasmussen (1999) developed a model
that predicts that extinction rates for North
American freshwater fauna will soon be five
times higher than that for terrestrial fauna. There
is little doubt that the spread of alien predators
throughout North America has contributed to
the extinction rates of freshwater fauna including amphibians (Table 1). Our understanding of
amphibian population declines will help us to
understand the larger picture that includes
numerous freshwater taxa. Alien predators will
impact freshwater fauna wherever they colonize
and unfortunately, the Ricciardi & Rasmussen
(1999) predictions will probably hold for habitats
outside of North America as well.
ALIEN PREDATOR
INTRODUCTIONS: POSSIBLE
ECOLOGICAL OUTCOMES
The ecological outcome of new introductions of
alien predators is not predictable (Fig. 1). A
review of the literature provides the possible
paths that systems may follow after alien introductions. Failed incidences of alien predator
introductions will not be well-documented in the
literature, particularly if their persistence was so
short that the impact on native species were
minimal. Knapp et al. (2001) were able to study
alpine lakes that fell into three categories: lakes
with introduced trout, lakes where trout were
unable to persist and lakes that never contained
introduced trout. Native mountain yellow-legged
frogs (Rana muscosa), crustaceans and macroinvertebrates were greatly reduced in lakes containing fish. In lakes where fish had disappeared, the
frogs and both groups of invertebrates began to
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increase 5–10 years after fish disappeared and
converged on fishless lakes 11–20 years after fish
were absent. This study points out that recovery
of lakes after the removal of alien species could
be a slow process and might also depend on the
length of time the alien predators had persisted
in the habitat before they disappeared. Funk &
Dunlap (1999) studied a similar situation where
stocked trout disappeared from certain high elevation lakes. Long-toed salamanders (Ambystoma
macrodactylum) had been eliminated from lakes
with fish; however, salamanders recolonized lakes
where trout had gone extinct within 20 years of
fish disappearance despite the fact that dispersal
in this amphibian was thought to be minimal.
While amphibians have been known to recolonize or increase in numbers after alien predators
either go extinct or are removed (see above), not
all aquatic species recover quickly after fish
removal (Drake & Naiman, 2000). Diatom
assemblages in lakes (Mt Rainier National Park,
Washington, U.S.A.) where trout were removed
did not return to predisturbance assemblages in
the 20–30 years since fish removal. Diatoms are
sensitive indicators of ecological conditions and
this study suggests that a more thorough recovery in these aquatic communities is complex and
that recovery times are often unpredictable
(Drake & Naiman, 2000). Similarly, McNaught
et al. (1999) found that the invertebrate community in alpine lakes recovered slowly (> 15 years)
after the disappearance of stocked salmonids.
Just as lack of evolutionary experience of amphibians with aliens can lead to the demise of
amphibians, lack of evolutionary experience between alien predators and new habitats might
deter their long-term persistence. For instance,
African clawed frogs (Xenopus laevis) have colonized habitats in California, yet conditions that
included prolonged drought wiped out many
populations of these frogs (McCoid et al., 1993).
Clawed frogs are permanently aquatic and have
few adaptations for dealing with drying habitats.
In our own study area, crayfish have been introduced into California as fish bait. These crayfish
are native to the slow-moving streams and rivers
of the south-eastern United States. In 1997, rainfall during an El Niño winter was twice the normal rainfall in southern California (Los Angeles
Flood Control District) and crayfish were swept
out of our study streams. Amphibians were able
© 2003 Blackwell Publishing Ltd, Diversity and Distributions, 9, 99– 110
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L. B. Kats and R. P. Ferrer
to breed successfully in those streams during the
following spring (Kats, unpublished data). The
following year, amphibian breeding was virtually
absent because the few remaining crayfish reproduced quickly and returned to pre-El Niño densities. Waiting for natural events to remove alien
predators may prove to be too late for many local
amphibian populations that are currently
decreasing as a result recently introduced predators, and some scientists have proposed removing
alien predators aggressively as a way of restoring
aquatic habitats for amphibians (see, for example,
Knapp & Matthews, 1998).
CONSERVATION BIOLOGY: ALIEN
PREDATORS AND AMPHIBIANS
Like many problems in conservation biology,
solutions for dealing with alien predators will be
complex. Some biologists have suggested that
eliminating alien predators from habitats in
which they are established is not necessarily a
good idea (Hayes & Jennings, 1986). The concern
is that alien predators, in particular bullfrogs,
may have become enough of an important game
animal that economic costs of removing them
should be considered. In addition, many recreational anglers will also oppose removing alien
game fish or halting the stocking of game fish.
Removal of alien predators will not be met with
universal support and acceptance.
In some instances the removal of aliens might
prove effective, while in other situations removal
might be impossible. In streams in the Santa
Monica Mountains, alien crayfish predators
remain the primary predator on native streambreeding amphibians. These predators have many,
if not most, of the general requirements that
describe a successful invader (Meffe & Carroll,
1997). For example, they are habitat generalists,
have broad dietary requirements and have a high
reproductive potential. In addition, they are very
aggressive toward other species (Gamradt et al.,
1997), a behavioural component that makes them
even more successful as an alien (Holway &
Suarez, 1999). These predators will probably not
disappear naturally and removal of these crayfish
will be difficult. As mentioned previously, local
populations of amphibians have disappeared
since their arrival (Gamradt & Kats, 1996). Recommendations for amphibian recovery plans
often call for restoring habitats to predisturbed
conditions (Semlitsch, 2002); however, as has
already been discussed, this might be virtually
impossible in the case of some alien predators,
particularly those that prove difficult to remove.
In streams where crayfish invasions are recent
enough that adult amphibians are still attempting
to breed, we have proposed ‘restoring’ sections of
streams by removing crayfish from these stream
sections long enough for amphibians to breed
and for larvae to metamorphose. While this is
not the ideal solution, partial and temporary
removal of crayfish combined with occasional
natural conditions that lower crayfish numbers
might provide enough relief from predation to
sustain amphibian populations.
Clearly, the best solution would be to increase
and preserve the number of aquatic habitats that
are free of alien predators. This goal would be
accomplished most effectively by two approaches
that include more careful regulation of the
spread of alien species and land management
decisions that include purchasing and preserving
habitats free of alien predators. Land management and preservation should consider clearly
the current and potential impact of alien species
(Meffe & Carroll, 1997). Because aliens are usually found in permanent water, some have suggested more emphasis on protecting ephemeral
habitats for amphibians (Adams, 1999). This
strategy would probably help amphibians that
breed in ephemeral habitats; it does not, however,
help amphibian species that, like aliens, are also
dependent on more permanent water. Decisions
to purchase and protect land should be influenced by current distributions of aliens and the
probabilities that aliens will invade (Byers et al.,
2002). Unfortunately, models do not currently
exist that would help land managers predict
invasion of aliens in the future. We are currently
collaborating with the U.S. National Park Service
and the United States Geological Survey to
develop models in the Santa Monica Mountains
that may allow land managers to predict future
alien invasions based on current and past patterns of alien species distributions.
The impact of introduced species is one of the
leading causes of biodiversity loss (Meffe &
Carroll, 1997), and introduced species are probably the most important anthropogenic impact
on freshwater ecosystems (Olsen et al., 1991;
© 2003 Blackwell Publishing Ltd, Diversity and Distributions, 9, 99– 110
Impact of alien predators on amphibians
Kolar & Lodge, 2000). The direct effects of alien
predators on amphibians are now well documented. We have only begun to explore the more
subtle indirect impacts of alien predators on
amphibians. For example, recent studies have
demonstrated that multiple stressors, when combined, can impact amphibians severely (Relyea &
Mills, 2001; Blaustein & Kiesecker, 2002). The
role of alien predators as stressors on amphibians in conjunction with other factors has yet to
be studied in detail.
Clearly, more laws and policies are necessary
to prevent and manage the spread of alien species
(McNeely, 2000; Scoccianti, 2001). Federal laws
that attempt to regulate alien species in the
United States date back to 1931 and the Animal
Damage Control Act. The most recent significant
federal laws that attempt to control the spread of
alien species are the Non-indigenous Aquatic
Nuisance Prevention and Control Act (NANPCA),
1990, the National Invasive Species Act 1996
and Executive Order 13112 1999 (for a complete list
of federal laws regulating alien species see
www.invasivespecies.gov/laws). The NANPCA
involves five U.S. federal agencies and was
reauthorized and amended in 1996 as the
National Invasive Species Act. It was expanded
to include more than species spread from ship
ballasts and authorized funding for research on
aquatic nuisance species prevention and control.
Executive Order 13112 calls for a National
Invasive Species Council to produce National
Management Plans for Invasive Species every two
years. States were also encouraged to develop
nuisance species management plans by NANPCA. These statewide plans led to significant
changes in law. For example, because of the
ecological damage caused by the spread of the
rusty crayfish (Oronectes rusticus), it is now
illegal to possess live crayfish while fishing in
Wisconsin. Illinois now prohibits the possession
and sale of rusty crayfish, and Minnesota and
Michigan have similar regulations.
While some states have passed laws that will
protect amphibians from invasive predators,
other states seem to lag behind in making policy.
For example, despite several studies that have
documented the negative impact of alien mosquitofish and crayfish on native California
amphibians, both aliens are still legally obtained
in the state. Gambusia are still being used for
107
mosquito control and alien crayfish are still sold
as fish bait throughout the state. While economic
analyses have prompted effective policies against
some aliens (e.g. Mediterranean fruitfly, cabbage
butterfly; see Diamond, 1996), the economic
costs of losing native amphibians do not appear
to be prohibitive enough to always warrant policy change. Economic analyses will rarely make
the case to save native amphibians, and costs
(both economic and biological) associated with
ecosystems damaged by the loss of amphibians
and other aquatic species may arrive too late to
generate effective policies. Current laws and
subsequent enforcement will have to occur at all
levels of government to succeed. International
agreements will also be necessary to prevent
further spread of species from continent to continent (McNeely, 2000). The effectiveness of these
laws and policies will be dependent on public
education programmes that inform people about
the negative impacts of alien species.
CONCLUSIONS
1. Sceptical policy makers will be most convinced of the negative impact of alien predators
on amphibians when studies include both
field correlations and experimental work demonstrating how aliens affect amphibians.
2. Scientists should be prepared to interact with
others who do not immediately agree that aliens
impact ecosystems negatively and who might
argue that alien predators should not be removed.
3. In order to predict long-term success of a
newly introduced alien predator, two factors
need to be considered: (a) the invasion history
of the alien; and (b) the level of ‘preadaptation’ of the alien for the new habitat. Shortterm success of the invader can still be
reversed years later by rare selection pressures
(e.g. extreme weather events, prolonged drought,
floods) in the new habitat.
4. Community recovery after aliens are removed
can be slow. Decades after eradication communities may still not reflect preinvasion species composition.
5. When complete eradication of aliens is not
possible alternatives should be considered,
including management strategies that attempt
to reduce aliens to low numbers to facilitate
the recovery of native species.
© 2003 Blackwell Publishing Ltd, Diversity and Distributions, 9, 99– 110
108
L. B. Kats and R. P. Ferrer
6. Models should be developed to predict how
sensitive habitats are to invasions by aquatic
aliens. These models would be helpful to land
managers when decisions are made to purchase land for preservation.
7. Since 1990, the United States federal government has increased focus on managing alien
species. Federal policies have required states
to develop nuisance species management
plans and several states have recently changed
laws to limit the spread of aliens. The most
effective way that scientists can impact policy
on aliens is at the state level. Scientists
should work with state policy-makers to protect habitats free of aliens, restore habitats
impacted by aliens and create laws and regulations that would further control the spread
of aliens.
ACKNOWLEDGMENTS
We are grateful to C. Vos Strache, D. Swartzendruber,
D. Baird, R. Buckskin, N. Watts and A. Ferrer
for logistical support. This study was supported
by the Seaver College Dean’s Office and the
Frank R. Seaver endowment. The Santa Monica
Bay Restoration Project also provided funds
to support this study. We are also grateful to
J. Kitz of the Mountains Restoration Trust,
R. Sauvajot and S. Riley of the National Park
Service and R. Fisher of the U.S.G.S.
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