Senior
Seminar 2002
Introduced Species in Hawaii
Eleuthrodactylus
coqui (Coqui) Dendrobates
auratus (Green
and Black Dart-Poison Frog)
Rana
catesbeiana (Bullfrog) Bufo
marinus (Giant
Toad)
Introduction
Researchers
have been studying amphibians for over a hundred years, and a multitude
of articles have been published. However, very little research has been
done to explore the impacts that introduced amphibians have outside of
their home range. Even less research has been done regarding the impacts
of introduced amphibians in Hawaii. Though the field is growing as scientists
and governments become more aware of the economic
difficulties that invasives species may cause in their new
found environement.
Frogs in Hawaii??
While
Amphibians are found throughout the world, they are not native to Hawaii
(Bryan 1931). The earliest account
of frog introductions to Hawaii is from 1857, and most introductions to
the natural Hawaiian ecosystem occurred between 1857 and 1935. Newspaper
accounts of a booming frog industry from 1905-1908, tell stories of Hawaiian
frog farmers not only supplying the Hawaiian islands but also sending
their frogs to San Francisco where they received $4-6 per dozen (Bryan
1931). This was very profitable, especially considering that in 1931 the
price per dozen frog legs was $2.40 (Bryan 1931). However the frog farming
industry seemed to disappear for a while after 1908 until it reappeared
again in 1930 when people began to reconsider the benifits of a frog farming
industry in Hawaii (Bryan 1931). The University of Hawaii was especially
optimistic, and promoted it as a job that taro and rice planters could
have on the side for extra income (Bryan 1931). Frog clubs also began
to form. In 1930, in Kohala, Hawaii the first 4-H frog club in Hawaii
was organized. One frog club, and perhaps others, received shipments of
frogs to raise and presumably sell to whom ever would buy the frogs (Bryan
1931). Many frogs were intentionally introduced to Hawaii for the purpose
of insect control, while others may have escaped from frog farms or were
pets that were released, still others may have arrived as stowaways on
shipments of plants from around the world (Bryan 1931; Svihla 1936; Krause
1999; USGS 2002). Interestingly in a paper written by Svihla (1936), five
years after Bryan (1931), a reference is made to frogs that may have arrived
as stowaways before a stringent quarantine went into effect. I am not
sure if this implies that Hawaii had closed its doors to introduced frogs
by that time or not. It would be interesting to do more research to find
out.
The information
presented in this website outlines the life history of some of the introduced
amphibians found in Hawaii. (For
a complete list of introduced amphibians in Hawaii refer to the USGS website).
Where research has been done regarding the impacts of non-native amphibians
in an introduced environment, that information is presented. Where research
was absent or scarce, I hypothesized ways these amphibians may or may
not become a problem in Hawaii, based on their life history and how well
they fit the profile of a successful invasive.
While I have
reported on some of the successful introductions of frogs to Hawaii, not
all frog introductions have been successful. The first documented account
of frogs being brought to Hawaii reported that the frogs ceased calling
within a few months (Bryan 1931). Another failed introduction was in 1929
when 25 golden tree frogs (Hyla aurea) were distributed throughout the
Manoa Valley (Bryan 1931). After their release the frogs were not seen
or heard again (Bryan 1931).
All
amphibians have the potential to create the same kind of impact on the
native Hawaiian ecosystem. Below I have applied a model presented by Kraus
et al. (1999) from their exploration of the impacts of the Eleutherodactylus
frogs in Hawaii. This model outlines what I see to be the largest problem
amphibians may be causing in Hawaii.
As you can see from the figure above, Hawaii's avifauna suffers the most detriment from introduced amphibians. Hopefully, through continued education and research, Hawaii will be able to successfully manage the introduced populations of amphibians.
To learn more about introduced amphibians in Hawaii visit
this website:
USGS Nonindigenous
Amphibians
By clicking on Data Queries and Species Lists, you can learn more about
each of the invasive species presented in this website, plus many more.
http://nas.er.usgs.gov/amphibians/index.html
Eleutherodactylus
coqui
Common Name: coqui
Photo courtesy of: Hawaii
Department of Agriculture
Description
Eleutherodactylus
coqui is currently the most problematic amphibian in Hawaii. While
other species of Eleutherodactylus frogs, such as E. planirostris
and E. martinicensis, have been introduced to Hawaii, E.
coqui is the only frog that is considered a threat and a danger to
Hawaii’s ecosystem (Krause et al. 1999).
What
do they look like?
E. coqui is a small, non-descript, brownish-gray colored frog.
Females with a snout-vent length of 35-52 mm. are larger then males that
have a SNV of 30-38mm. (Townsend et al. 1984; Woolbright 1985). E.
coqui is most easily recognized by its piercing two-note call of
“co-qui” (Stewart and Rand, 1991) (Click
here to listen to the call of E. coqui)
Where
and when can they be found?
E. coqui is nocturnal and active throughout the year (Woolbright
1985; Stewart and Rand 1992). During the day E. coqui take refuge
in leaf debris and under bark, roots or rocks (Woolbright 1985; Stewart
and Rand 1991). Being close to the ground provides moisture, which is
essential for this terrestrial frog to avoid desiccation (Miyamoto 1982;
Stewart and Rand 1992; Woolbright 1996). The diurnal retreat of E.
coqui is considered a limiting resource, because no two frogs share
a retreat and both males and females actively defend their retreat (Stewart
and Pough 1983; Stewart and Rand 1991; Woolbright 1996). Woolbright (1991)
demonstrated that E. coqui populations increased after Hurricane
Hugo increased the available retreat sites. This may indicate that in
forests with high amounts of leaf litter on the forest floor, E. coqui
populations will be larger then in forests with relatively small amounts
of leaf litter.
At dusk, E. coqui moves from their diurnal retreat into the canopy
to begin calling (Townsend et al. 1984; Stewart and Rand 1991). They begin
their chorus at dusk, which continues, though progressively less intensively,
until dawn (Woolbright 1985). E. coqui are mainly sedentary and
can be found calling from the same place throughout the evening and even
from season to season (Woolbright 1985). Male E. coqui are extremely
territorial, and aggressively defend their territory by biting or wrestling
with their conspecifics (Townsend et al. 1984)
What
do they eat?
E. coqui is a sit and wait predator (Woolbright 1985) Males and
females, due to differences in body size feed on different sizes in prey
(Woolbright and Stewart 1987), which reduces intraspecific competition.
A single E. coqui on average consumes 3.3 prey per night, though
the number varies with the size of prey (Woolbright 1985).
Reproduction
and Development
E. coqui breeds continuously throughout the year (Townsend et
al. 1984; Stewart and Rand 1991; Townsend and Stewart 1994). Females exhibit
mate choice and initiate courtship, by making contact with a calling male
(Townsend and Stewart 1986).
They have internal fertilization (Townsend and Stewart 1986) and the eggs
are deposited terrestrially in or near the male’s territory (Townsend
1989; Townsend and Stewart 1994).
Clutch size varies with an average of 26 eggs per clutch and a female
can lay 4-5 clutches a year (Townsend et al. 1984; Townsend and Stewart
1994). The eggs are adhesive and so can be deposited on vertical surfaces
(Townsend et al. 1984). Males guard the nest against conspecific cannibalism,
and frequently only attends one clutch (Townsend et al. 1984; Stewart
and Rand 1991; Townsend and Stewart 1994).
The eggs undergo direct development (Townsend and Stewart 1985). Direct
development, is development that contains no free living or larval stage
and so 17-26 days when the eggs hatch, tiny froglets emerge (Townsend
et al. 1984; Townsend and Stewart 1985; Stewart and Rand 1991). (The male
continues to defend the nest site 1-6 days after the froglets hatch (Townsend
et al. 1984). Male parental care has been shown to greatly increase the
survivorship of the eggs and tadpoles (Townsend et al. 1984). Townsend
(1989) also showed that the survivorship of a clutch increased when eggs
were deposited closer to the area where the male calls.
Geographic
Range
There are representatives of Eleutherodactylus
species throughout Central and South America with some populations also
in the West Indies (Townsend and Stewart 1994). E. coqui is native
to Puerto Rico (Townsend et al. 1984).
On the islands of O'ahu, Maui, Kauai, and Hawaii, E coqui is
established in areas such as plant nurseries or in ornamental foliage
around hotels and in residential areas (Krause et al. 1999; USGS 2002).
Vectors
The earliest record of E. coqui’s presence
in Hawaii is from 1992, though the frog may have been there earlier (Krause
et al. 1999). It most likely arrived as a stowaway in potted plants being
delivered from the Caribbean to Hawaii (Krause et al. 1999). Transportation
of non-indigenous plants, most commonly palms, bamboo, and Dracaena, between
the Hawaiian Islands and to Hawaii from other locations continues to spread
E. coqui (Krause et al. 1999). E. coqui has also been
distributed throughout the islands, by well meaning individuals who collected
the frogs from plant nurseries and introduced them into local neighborhoods
(Krause et. al 1999).
Why is it successful
Natural populations of E. coqui can reach densities
of 20,570 adults per hectare (Stewart and Rand 1991). Krause et al. (1999)
observed that in Hawaii the E. coqui has been able to reach densities
ten times greater than has been found in its natural environment. This
may be due to a lack of predators, plentiful nesting and retreat sites,
and increased moisture, all of which are limiting factors in its native
environment.
E. coqui populations expand rapidly, increasing from just a few
individuals in an area, to hundreds within a couple years (Krause et al.
1999). The E. coqui’s direct development and ability to
breed continuously may contribute to its rapidly expanding populations
in Hawaii.
Impacts
While the only known populations of Eleutherodactylus
frogs are located in residential areas, hotel grounds or in plant nurseries,
if they were to spread to Hawaiian forests their impact on the native
community would be great (Kraus et al. 1999). Their presence could apply
predation pressure on the already stressed populations of native arthropods
leading to increased pressure on the native avifauna, which depends solely
on a diet of native insects (Krause et al. 1999). Also, because there
are no native predators of Eleutherodactylus in Hawaii, the frogs
would act as a nutrient sink, and would only succeed in increasing the
populations of introduced predators such as rats or mongoose (Krause et
al. 1999).
H umans have become tired of the novelty of having calling frogs in their
neighborhoods, especially now that the populations of E. coqui
have grown substantially. The once quiet peaceful nights on the Hawaiian
Islands have given way to the chorus of frogs. This may sound trivial,
but it could have a huge impact on Hawaii's economic structure, especially
because local residents have vowed to move if the frogs don't stop calling,
and visitors in hotels complain about the noise keeping them awake at
night (Kraus et al. 1999). Residents are encountering lower property values
and increased difficulty selling property that is infested with E.
coqui (USGS 2002).
Management
As with most environmental problems, education is the
most effective tool for the eradication of E. coqui. As people
become more aware of what to look for and why it is a problem, they can
more readily assist scientists in locating the frogs and exterminating
them. Hawaii is aware of their need for public education and has been
running radio and newspaper ads describing the E. coqui and what
individuals should do if they suspect to have one in the neighborhood.
Posters can be printed off the internet to educate and alert communities
to the presence of E. coqui. (Click
here to view an E. coqui Alert Poster)
In addition to general public education, on June 28, 2000 the Hawaiian
Department of Land and Natural Resources distributed a letter specifically
to the members of the horticulture industry alerting them to the presence
of E. coqui as a troublesome species and outlining the illegality
of distributing the species. (Johns 2000 cited by HEAR)
On September 27, 2001, the Hawaiian Board of Agriculture meeting declared
E. coqui a pest under section 150A-2, Hawaii Revised Statutes
(HRS) (HDOA 2001a).
Around the same time, the EPA approved use of a caffeine solution to be
sprayed on ornamental plants in residential areas, plant nurseries and
on hotel or resort property for the purpose of exterminating E. coqui
and E. planirostris. Caffeine has been noted to cause molecular
abnormalities in plants, and is usually a restricted chemical, but the
EPA gave permission for it to be used during a one year period, from 27
September 2001 to 27 September 2002, for the control of Eleutherodactylus
frogs. Prior to the use of caffeine, the only control methods were to
gather frogs by hand, or eliminate suitable habitat. (HDOA 2001b as cited
by HEAR) Directions for use of the caffeine spray include spraying at
night, and thoroughly covering all foliage. (Click
here for more information about the caffine solution used to conrol
E. coqui)
Other suggestions for control of E. coqui are to exclude them
by burying shade cloth below ground level, thus eliminating day time retreats,
remove the vegetation they prefer, such as Bromeliads, or collecting the
frogs by hand.
Links
For more information
on Eleutherodactylus coqui check out these websites:
Hawaiian
Department of Agriculture
This website provides general information on E. coqui, along
with information for what to do and whom to contact if E. coqui are
found near your residence in Hawaii.
http://www.hawaiiag.org/hdoa/coqui.htm
Institute
for Biological Invasions
Todd S. Campbell provides detailed information on E. coqui and
addresses its impacts on the Hawaiian Islands.
http://invasions.bio.utk.edu/invaders/coqui.html
Hawaiian
Ecosystems At Risk- Eleutherodactylus Frogs
This website provides general information about both E. coqui,
and E. planirostris. It also provides an extensive list of links
to obtain more information.
http://www.hear.org/frogs
Dendrobates
auratus
Common Name: Greeen and Black Dart-Poison Frog
Photo courtesy of:
Reptile and Arachnokulture
Photograph taken by Thomas Villegas
Description
The Dendrobatid
family owes its common name, dart-poison frog or poison arrow frog, to
the Indians of western-Columbia who used the skin secretions as a mild
toxin to poison weapons used for hunting animals (Maxon and Myers 1985).
What
do they look like?
Dendrobates auratus despite being a small frog, only about 30-39
mm. in length, is the largest member of its family (Savage 1968; Wells
1978). It is aposematically colored, with black blotches on a bright bluish-green,
jade or dark green background (Savage 1968). They have a 3-5 note call,
that has a high pitch (Savage 1968). The call can be described as insect-like,
somewhat musical and sounds like cheez-cheez-cheez (Savage 1968).
Where
and when can they be found?
D. auratus is a diurnal, terrestrial frog (Savage 1968; Graves
1999). While D. auratus is active throughout the whole day, they
are most active in the early morning and late evening (Graves 1999).
Males usually call from partially concealed locations such as bushes or
in holes near the base of a tree (Savage 1968; Wells 1978 ). Males defend
territories and are aggressive toward conspecific males (Wells 1978; Summers
1989; Pough and Taigen 1990).
What
do they eat?
D. auratus forages actively in leaf litter (Pough and Taigen
1990; Toft 1995). While D. auratus feeds on mites, beetles, spiders,
and lepidopteran larvae, it is primarily considered an ant specialist
(Toft 1981; Toft 1995). D. auratus will return to feeding sites,
where it has found plentiful prey previously (Pough and Taigen 1990).
Reproduction
and Development
Females initiate courtship, and then follow the male as he leads her through
the leaf litter engaging in courtship (Wells, 1978). Once an oviposition
site is chosen, the female lays a clutch of 5-13 eggs (Wells 1978). The
male provides partental care for the eggs and the tadpoles, which hatch
in 10-13 days (Wells 1978). Males will transfer moisture to the eggs,
which are laid in leaf litter, by sitting on a moist surface and then
sitting on the eggs, periodically shifting his position and moving the
eggs around (Wells 1978). When the eggs hatch, the male will carry the
tadpoles on his back to any available water source, including small pools
of water in bromeliads, shells of fallen fruit, dead leaves and holes
in trees (Savage 1968; Wells 1978). A male can care for more then one
clutch at a time (Wells 1978; Summers 1989; Pough and Taigen 1990). Females
can lay a clutch of eggs every 5-10 days, continuously throughout the
breeding season (Wells 1978).
Geographic
Range
Dendrobatid
frogs occur throughout Central America (Savage 1968). D. auratus
inhabits wet evergreen forests in Nicaragua, Costa Rica, and Panama (Savage,
1968). In their native range D. auratus only occur below 800
m. (Savage 1968).
In Hawaii, D. auratus are found primarily on O’ahu but
have also been found on the island of Hawaii (USGS 2002)
Vectors
The Territory of Hawaii, employed David T. Fullaway,
an entomologist, to bring in animals that would consume non-native insects
(McKeown 1996). In 1932, he successfully introduced 206 individuals of
D. auratus to O’ahu from an island off the coast of Panama,
for the purpose of mosquito control (McKeown 1996). Another source of
D. auratus in Hawaii is as escaped pets (USGS 2002).
Why
is it successful
D. auratus may be so successful in Hawaii, because
the habitat is very similar to that of its native habitat. Male parental
care and the ability for tadpoles to utilize a wide variety of water sources
also give this frog an advantage.
Impacts
The impacts of D. auratus on Hawaii are relatively
unknown. D. aurartus, like the other frogs mentioned in this
overview, eats insects and may decrease native populations of insects.
However the impact that D. auratus is probably much less severe
then the impacts of Eleutherodactylus coqui (Wright 2001).
Management
Rana catesbeiana
Common Name: Bullfrog
Photo
courtesy of: Cortland
Herpetology Connection
Description
Bullfrogs are North America’s largest,
most abundant, and perhaps best-known frog (Bury and Whelan 1985). They
are served in restaurants in a variety of styles, and are frequently used
for dissection in middle school science classes. On the darker side, Bullfrogs
have also been nominated as one of the 100 "World’s Worst"
invaders (ISSG 2002).
What
do they look like?
R. catesbeiana is a large frog with a varied appearance. Most
adults are between green and brown in color with mottled or striped patterns
on the legs (Bury and Whelan 1985). The ventral surface is lighter in
color and is often white mottled with gray (Bury and Whelan 1985). Males
can be distinguished from females by their brightly colored, usually yellow,
gular sac, which males display while defending their territory (Emlen
1968; Ryan 1980). The males are slightly smaller than the female. Males’
SVL measures 180 mm. while females are 200 mm. (Bury and Whelan 1985).
Both male and female R. catesbeiana vocalize, but the breeding
call of the male is the most apparent (Bury and Whelan 1985). The males
produce a deep call which has been described in various ways. Wiewandt
(1969) describes the call as “gronk,” or “bonk.”
(click here to
hear the call of a bullfrog)
Where
and when can they be found?
Bullfrogs are always found near water. They are mostly aquatic and require
a permanent water source (Bury and Whelan 1985). While R. catesbeiana
is frequently found in ponds, especially human altered ponds such as those
found in cattle fields, they can also been found along intermittent streams
(Moyle 1973; Bury and Whelan 1985). They prefer habitats with dense vegetation
and relatively calm water (Moyle 1973; Bury and Whelan 1985).
R. catesbeiana can survive in areas with a wide temperature range,
however they are very susceptible to the cold, and hence usually go into
hibernation before the first frost and do not emerge until late in the
spring (Bury and Whelan 1985). Bullfrogs are active both day and night
(Bury and Whelan 1985). R. catesbeiana may be limited to lower
elevations due to colder temperatures found at higher elevations, and
usually occur below 1,900 meters (Bury and Whelan 1985).
Bullfrogs are mainly solitary, but will form chorus groups (Bury and Whelan,
1985). In a pond there will be territorial males, non-territorial males,
and females (Emlen 1968; Bury and Whelan 1985) Males actively defend territories
with flexible boundaries that are very susceptible to change (Bury and
Whelan 1985). When confronted by a conspecific male a territorial male
will give aggressive vocalizations, wrestle with, shove or jump onto the
intruder (Wiewandt 1969; Emlen 1968; Bury and Whelan 1985).
What
do they eat?
R. catesbeiana is very opportunistic and is able to utilize a
wide variety of food sources. They are omnivorous and will eat anything
smaller than they are (Bury and Whelan 1985). Bullfrogs are sit and wait
predators (Bury and Whelan 1985). Their diet consists mainly of insects
and crustaceans, but has also been known to eat snakes, mice, birds, fish,
and turtles (Bury and Whelan 1985).
Reproduction
and Development
Size is the main determinant of sexual maturity (Bury and Whelan 1985).
Growth is faster in warmer climates and hence individuals reach sexual
maturity sooner (Bury and Whelan 1985). Males are reproductively active
all season, but females are only reproductively receptive for a short
period of time once in the breeding season (Emlen 1968). The mating system
has been described as a lek by numerous scientists (Emlen 1968; Ryan 1980;
Bury and Whelan 1985).
Females exhibit mate choice, and initiate courtship with a male (Emlen
1968; Ryan 1980). Eggs are deposited near or among vegetation on the water
surface (Ryan, 1980; Bury and Whelan 1985). A single female is capable
of depositing between 1,000 and 25,000 eggs at a time (Bury and Whelan
1980).
There is no known parental care for the eggs which hatch in 3-5 days (Bury
and Whelan 1985). The tadpoles are voracious omnivores that feed mainly
on aquatic plants, but may also feed on small vertebrates or scavenge
on dead fish (Bury and Whelan 1985). The tadpoles grow to become very
large before metamorphosis, reaching lengths of 152-178 mm. (Bury and
Whelan 1985). Metamorphosis takes two years to complete (Wright and Wright
1949 in Moyle 1973).
R. catesbeiana has been known to live 8-10 years in the wild
(Bury and Whelan, 1985).
Geographic
Range
R. catesbeiana is native to the eastern
United States and the Great Plains region, with some populations reaching
into Nova Scotia, Canada (Moyle
1973; Bury and Whelan 1985).
Introduced populations occur in California, British Columbia, Mexico,
the Caribbean Islands, Brazil, the far East, Europe, and Hawaii (Moyle
1973; Bury and Whelan 1985).
In Hawaii, R. catesbeiana occurs on the island of Hawaii, Maui,
O’ahu, Molikai, Lanai, and Kauai (USGS 2002).
Vectors
R. catesbeiana were first introduced to Hawaii in 1899
from a frog farm in California (Bryan 1931). The frogs were released and
quickly became established in the Hilo region of Hawaii (Bryan 1931).
Another account from 1902 tells of a shipment of R. catesbeiana sent to
Hawaii for the intention of release into local ponds and streams to help
control the Japanese beetle population (Bryan 1931). R. catesbeiana were
probably also the predominant frog used for restaurant preparations of
frog legs and other such frog dishes. Bullfrogs are currently being stocked
in Hawaii as a food source and as a control of pests (USGS 2002).
Why
is it successful
Bullfrogs have been very successful in establishing new
populations where they are introduced(Moyle 1973). This may be because
they have such flexible habitat and food requirements (Bury and Whelan
1985). They can utilize any water source, as long as it is within their
temperature range, and can even survive in wells found in the desert (Bury
and Whelan 1985). R. catesbeiana have also been found in underground
caves (Bury and Whelan 1985).
Impacts
In other areas where the bullfrog has been introduced,
the main impact it has on the native ecosystem is the elimination of the
native frog species (Hayes and Jennings 1986). For example, bullfrogs
were introduced to California in the early 1900’s and have since
led to the local extinction
of the red-legged frog, Rana aurora, and a reduction in the population
of the yellow-legged frog, R. boylii
in the San Joaquin Valley (Moyle 1973). Bullfrogs are now the most commonly
encountered frog in the Sierra Nevada foothills of California (Moyle 1973).
The impact that R. catesbeiana has on local amphibians is so
extensive, that it has led to the development of the bullfrog hypothesis,
which states that through competition and predation bullfrogs lead to
the decline of native frog populations (Hayes and Jennings 1986).
Management
Bufo
marinus
Common Name: Giant Toad
a.k.a. Giant Neotropical Toad
Photo courtesy
of: USGS,
Nonindigenous Aquatic Species
Photograph taken by Mike Pingleton
Description
What do they look like?
The world’s largest toad was a Bufo marinus, with a snout
vent length of 230 mm. (Zug and Zug 1979). While not all toads reach that
size, they are on average 100-150 mm. (Zug and Zug, 1979). B. marinus
is a heavy bodied toad that is almost as wide as it is long, and has been
described as "mobile cow patties" (Zug and Zug 1979). Both sexes
are covered with warts, though the male has a horny spine on each wart
(Zug and Zug 1979). Females are dusky brown with mottled blotches of tones
of beige, with a beige mid-dorsal stripe, while males are cinnamon in
color also with a mottled pattern (Zug and Zug 1979).
Where
and when can they be found?
B. marinus are most commonly found near human dwellings, and
in human altered habitats such as open grass fields (Zug and Zug, 1979).
They are rare in forested habitats, and swamps, rivers and rainforests
may act as dispersal barriers (Zug et al. 1975; Zug and Zug 1979). B.
marinus is a lowland toad, and occurs below elevations of 1000 m.
(Zug and Zug 1979).
B. marinus is a nocturnal toad that is primarily active right
after dusk, and ceases activity by dawn (Zug and Zug 1979). During the
day they take shelter in burrows on the forest floor (Zug and Zug 1979)
B. marinus is not territorial and can be found feeding in groups
at a single food source (Zug and Zug 1979). They have a large home range
of 160 m. or more, and can be found feeding anywhere throughout that range
(Zug and Zug 1979). Despite the large home range of the toad, it stays
in relatively the same location for one evening but will feed in different
locations throughout the course of the season, such that the toad may
not be found feeding in the same spot from one evening to the next (Zug
and Zug 1979).
In their native environment, densities range from 50-140 toads per hectare,
though in one evening only 31-50% of that population will be active (Zug
and Zug 1979).
What
do they eat?
B. marinus is an opportunist and a generalist; they can and do
eat anything they can catch, including nonfood items such as glass (Zug
and Zug 1979). For the most part, they feed on ants and mites, but also
eat other terrestrial arthropods, earthworms, and even other small vertebrates
(Zug and Zug 1979). In urban settings, which they are extremely well adapted
to, B. marinus has been observed eating dog and cat food that
has been left outside (Zug and Zug 1979). Another way they have adapted
to urban settings is by feeding on the insects that gather around artificial
light sources (Zug and Zug 1979).
In one evening, a single B. marinus may eat over 50 prey, with
an average of 43.2 prey eaten per evening (Zug and Zug 1979). However,
B. marinus does not feed every evening, rather they gorge themselves
in one evening and then are inactive for 2-4 days (Zug and Zug 1979).
Reproduction
and Development
Size is the main determinant of sexual maturity in B. marinus
(Zug and Zug 1979). Most individuals reach sexual maturity within a year
(Zug and Zug 1979). In a given population there are twice as many females
as there are males.
The breeding season begins in April and usually ends in May, but can be
extended into June and July if conditions are right (Zug and Zug 1979).
Females deposit between 8,000- 35,000 eggs, once or twice a year, in streams
or ponds (Zug et al. 1975; Zug and Zug 1979). These eggs hatch within
36 hours, but may be delayed to 4 days (Zug and Zug 1979). The tadpoles
metamorphose within one to two months depending on the environmental conditions
(Zug and Zug 1979). The tadpoles are toxic to predators. The adults also
produce a toxic skin secretion that is toxic to numerous vertebrates (Covancevich
and Archer 1975).
Geographic Range
Natural populations of B. marinus occur
from southern Texan, through Mexico, and into central Brazil (Zug and
Zug 1979).
Introduced populations occur in southern Florida, Louisiana, the Carribean,
Hawaii and in Queensland and New South Wales in Australia (Covancevich
and Archer 1975).
In Hawaii, populations of B. marinus occur on Maui, O’ahu,
Molikai, Lanai, Kauai, and the island of Hawaii (USGS 2002)
Vectors
B. marinus has been purposefully introduced to
many islands in the South Pacific for the purpose of controlling insects
that damage the sugar cane crop grown in those regions (Zug et al. 1975).
B. marinus was brought to Hawaii for just that purpose, in addition
to the hopes that its presence would control populations of rats and snails
(Covancevich and Archer 1975). In 1932, Dr. Cyril E. Pemberton introduced
148 toads to the sugar cane and taro fields in O’ahu, Hawaii (Zug
et al. 1975; McKeown 1996)
Other populations of B. marinus have become established outside
of their home range, when released from laboratories that were using the
toads in pregnancy tests (Zug et al. 1975).
Why
is it successful
· B. marinus has a broad
diet and is capable of utilizing the most abundant food resource in any
environment (Zug and Zug 1979).
· B. marinus has
an incredible reproductive capacity: producing 8,000 - 35,000 eggs twice
a year (Zug et al. 1975). They are able to use a wide range of water resources
for reproduction, including brackish water (Covancevich and Archer 1975).
An example of their success is that two years after Pemberton released
his 148 toads, he collected 103, 517 juveniles from the same area for
further distribution throughout the island (Zug et al. 1975). They are
currently the most common small vertebrate found in Eastern Queensland
(Covancevich and Archer 1975).
· In Hawaii, B. marinus
grows very rapidly and reaches sexual maturity in half the amount of time
as in their native environment (Zug et al. 1975).
· B. marinus is
very well adapted to human altered habitats (Zug and Zug 1979) and as
human habitat expands the populations of B. marinus will continue
to increase.
· Toxic skin secretions,
release B. marinus from predation pressure (Covancevich and Archer
1975).
Impacts
In Queensland, B. marinus has the potential to
cause the extinction of terrestrial predators that are not adapted to
handle its toxicity (Covancevich and Archer 1975). Animals that have died
due to the ingestion of B. marinus include the Austrailian native
cat, Dasyurus geoffroii, numerous snake species, crows, kookaburras,
and the Tasmanian devil (Covancevich and Archer 1975).
In Hawaii, the exact impact of B. marinus is unknown. Animals
in Hawaii that would be poisoned due to the ingestion of B. marinus
are all introduced species as well, which would not cause further detriment
to the ecosystem. The largest impact that B. marinus might have
on Hawaii’s ecosystem is a decline in insect populations due to
its high rates of consumption. Another area of concern is that B.
marinus may also consume small vertebrates and may eat endemic vertebrates.
However, because of B. marinus’ habitat near human dwellings,
they are only likely to encounter introduced insects and other introduced
vertebrates. An indirect impact that B. marinus may have on the
Hawaiian ecosystem is that mongoose and rats can prey on B. marinus,
creating another food source for these invasive predators (Zug et al.
1975). More research needs to be done in order to determine the exact
impacts that B. marinus may have on Hawaii’s ecosystem.
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Designed by: Stephanie Danyi
Contact: danyist@earlham.edu
last revised: 9 December 2002