Senior
Seminar 2002
Introduced Species in Hawaii
Brown Tree Snake Soft Shell Turtles Mourning Gecko House Gecko
Introduction
Hawaii is home to 28 species
of reptiles, of which only six are indigenous (a sea snake and five species
of sea turtles). Of the introduced reptiles, man is thought to be the
primary cause of their establishment on Hawaii (McKeown 1996). Man began
this long process of introduction with the arrival of Polynesian settlers
in the fifth century and it continues even now through the illegal pet
trade. Additionally remains of geckos and skinks have been found in prehistoric
avifaunal deposits on Oahu (Kirch 1982), however further genetic studies
need to be conducted in order to determined their relatedness to current
species of geckos in Hawaii. It is also very possible that some individuals
rafted to the Hawaiian islands independent of man, but it is hard to say
to what extent this is possible. The most likely candidates for this type
of travel are geckos and skinks, due to their ability to withstand adverse
conditions and the durability of their eggs (McKeown 1996) .
Understanding
the effects of these various reptiles is quite complicated. There are
a variety of reasons why these reptiles were brought to the Hawaiian Islands
and there are many different ways that these species could and do affect
the endemic species. Reptiles have been brought to Hawaii for food in
the case of the soft shell turtles, for aesthetic value in the case of
the Jackson's chameleon and various geckos, and simply by accident. The
effect that these various species can have are very different. Some species,
such as Iguana iguana and the soft shell turtles, Trionyx
sinensis and Trionyx steindachneri, have yet to be proven
invasive or harmful to native populations. In the case of the orange spotted
day gecko, Phelsuma guimbeaui guimbeaui, a federally listed endangered
species (CITES Appendix II) the introduction to Hawaii could prove beneficial
to the survival of this species. On the other hand this species feeds
on nectar and in many cases has specialized on invasive plants in Hawaii
and therefore could be aiding in the spread of non-native flora. Lastly,
a reptile like the Brown Tree Snake, Boiga irregularis, if fully
established on an island like Hawaii could destroy the flora and fauna
in a matter of years. In summary we must not take for granted these introduction
and be constantly making an effort to better understand the full effect
of these species on Hawaii and in other threatened environments of the
world.
Boiga irregularis
Common Name (Brown Tree Snake)
Picture courtesy of: http://www.geocities.com/zelresearch/BrownTreeSnakeLinks.html
Description
The Brown Tree
Snake, Boiga irregularis, was first described by Merrem in 1802.
This is a fairly large snake reaching a maximum length of about 2 meters,
but most commonly found around 1.4 meters in length. The Brown Tree Snake
is a rear fanged venomous snake but is not thought to be dangerous to
humans in terms of toxicity (Cogger 1992). These snakes reach maturity
at 3 years of age and are oviparous (Fritts 1988). Snakes lay approximately
4-12 eggs per clutch and are thought to reproduce 1-2 times per year,
though they may reproduce more often if the environmental conditions are
suitable, due to their ability to store sperm for several years (Mckeown
1996). Eggs are laid in caves, hollow trees or other areas that provide
protection from desiccation and over heating. Once eggs are laid they
are abandoned by the mother, so it is important that they are laid in
optimal areas (Fritts 1988).
This
is a nocturnal species and therefore does the majority of its feeding
at night using its incredible sense of smell and night vision.The
Brown Tree Snake has been observed foraging on birds, eggs, small mammals
and lizards (Macedon 1996). This species is highly adaptive in its feeding
habits and can be thought of as a generalist that has the ability to switch
its diet according to the avalibity of prey (Rodda and Fritts 1992). Unlike
most snakes this snake has a strike distance of half of its body length
and is very aggressive (McKeown 1996).
The
Brown Tree Snake can be found in a variety of habitats, including rainforests,
mangroves, dry forests and paperback swamps (Cogger 1992). These snakes
can also be found grassy fields, livestock areas, gardens, garages, and
roofs of houses. Due the wide variety in their diet and their nocturnal
behavior, Brown Tree Snakes are often found in living quarters in search
of food and shelter from sunlight and heat (McKeown 1996).
Geographic
Range
The Brown Tree Snake,
Bioga irregularis, is native to New Guinea, Indonesia and the Solomon
Islands. A related form also exist in northern and eastern Australia (Cogger
1992). The invasive nature of this snake has increased its range to include
Guam (McKeown 1996). Additionally sightings have occurred on Cocos Island,
Corpus Chirsti (Texas), Diego Island, Kwajalein, Pohnpei, Saipan, Rota,
Tinian, Wake Island, Mainland Japan, Taiwan, Darwin (Australia), Spain,
Okinawa, and Oahu (Hawaii) (USGS 2002), where the extent of establishment
is not yet known.
Vectors
All reports of Brown Tree
Snake sightings in Hawaii have reported snakes on airplane runways. Planes
coming in from Guam and landing at the Honolulu Airport, Barbers Point
Naval Air Station, and Hickam Air Force Base can account for all the sightings
in Hawaii. Additionally, it is thought that the cause of all dispersal
events to the areas listed above can be linked to snakes leaving Guam
on various planes . All of these introductions have occurred within the
past three decades with the exception of Wake Island that occurred in
1949 (USGS 2002).
The cause of the appearance of the Brown Tree Snake on Guam can be traced
back to US military forces flying from either New Guinea or Manus Island
(off the coast of New Guinea) to Guam, during or shortly after WWII (McKeown
1996).
Why
is it successful
The success of the Brown
Tree Snake comes from its generalistic ability. Rodda and Fritts (1992)
have shown that this species is able to feed on almost anything that it
encounters due to its size and aggressive yet secretive behavior. This
generality allows for the rapid growth in population size of this species.
Additionally, since this species has such a wide range in habitat use
(McKeown 1996) it
is able to occupy and populate almost any area. Therefore
even though Hawaii has a wide range of habitats the Brown Tree Snake is
not obligated to one of them as are many of the native species. The population
size on Guam was estimated as 12,000 snakes per square mile in 1988 (Fritts
1988), and is most likely much larger at this time. Introductions are
going to keep occurring with a host population of this size and since
females of this species can store sperm for years mating does not necessarily
have to occur after the snake is introduced. This means that populations
can begin the thrive rapidly and without a large number of introduced
individuals.
Impacts
The impact that this species
can have on the endemic species in any area is so vast that it is almost
incomprehensible. On Guam alone this snake has managed to extirpate the
majority of avifauna, which has meant extinction in many cases. Savage
(1987) demonstrated that this snake has cause the extirpation of 11 out
of the 12 native birds, three native skinks and two geckos. Though it
is thought that this snake is not directly harmful to humans, it has caused
repeated power outages and extensive electrical damage in Guam, due to
its arboreal nature. Snakes climbing on electrical lines have become a
huge economic burden for all residence of Guam (Fritts 1988).
If this snake is not intercepted and denied
habituation it can literally destroy an ecosystem. This snake comes from
a area of the world that has very low resources so it can survive on very
little and reproduce year round if conditions are favorable. Hawaii has
no natural predators for this snake so if it is allowed establishment
it will destroy what is left of Hawaii's endemic flora and fauna.
Management
An effective control mechanism
has not yet been established though many groups are beginning to take
action in response to this supertramp species. In 1995, the US Department
of Agriculture listed this species as "among the top three species
requiring control and eradication." The Global Invasive Species Program
has also begun to gather information from leading officials in Guam, Australia,
Hawaii, and mainland USA in hopes of constructing a plan of action (Atkinson
and Atkinson 2000). For the time being the most effective and important
thing to be done is careful inspection and precaution to all in coming
aircrafts, especially those from Guam. This must be a joint effort from
all citizens as well as officials. Everyone must do there part in order
to take control of this situation (Fritts 1988).
The Brown Tree Snake Control Group, established
by Paul Breese in 1990, is dedicated to evaluating control mechanisms
and making the legislation aware of the options at hand. Members of this
group make frequent trips to invaded sites and talk to officials to learn
what the most effective actions to be taken are. From the work of this
group dogs have been implemented as search devices which has proven to
be very beneficial in the prevention of the spread of the Brown Tree Snake
(McKeown 1996).
If you see a Brown Tree Snake or unsure of the species, report the sighting to U.S. Fish and Wildlife Service, Fish and Wildlife Enhancement, P.O. Box 50167, Honolulu, Hawaii, 96850, U.S.A. (Fritts 1988).
For additional information on control methods go to the summary project at Columbia University.
Trionyx
sinensis
Chinese Softshell Turtle
Trionyx steindachneri
Wattleneck Softshell Turtle
Pictures courtesy of:http://www.e2121.com/food_db/viewherb.php3?viewid=336
Description
T. sinensis
: The Chinese Soft Shell Turtle was first described by Weigmann
in 1834. The carapace, head, and limbs of this species are grayish green
in adults and patterned in juveniles, whereas the plastron is white in
adults and has black blotches in juveniles. This
is the closest living relative to the Wattle Neck Soft Shell Turtle and
can be differentiated by the presence of a dark line radiating from the
eye and the absence of a dark bordered pale strip passing through the
eye. Characteristic of soft shell turtles males
are smaller than females and have longer thicker tails. Though little
is known about the natural behavior of this species, individual in captivity
hibernate in the mud at the bottom of ponds from October to April or May,
however it is thought that in Hawaii hibernation is not necessary. Sexual
maturity occurs between 4 - 6 years depending on the region of origin.
Breeding usually occur in March or April and nesting follows in May (approximately
60 day incubation). Mean clutch size is approximately 75 eggs. One turtle
raised in captivity lived for 24 years, however longevity in natural populations
is unknown. This species feeds primarily on fish, crustaceans, mollusks,
insects, and seeds of marsh plants. Little is known of natural predators
though it is thought that there are predators of both juveniles and eggs.
Additionally adults have been observed eating their young. The Chinese
soft shell turtle is most commonly found in rivers, lakes, ponds, drainage
canals, creeks, and marshlands (Ernst et al. 1994).
T. steindachneri : The Wattle Neck Soft Shell Turtle was first described by Siebenrock in 1906. Color variation in this species ranges from brown to olive in adults and orangish brown with black a pattern in juveniles (carapace). As the name suggests the most distinctive characteristic of this species is the presence of a large clump of course tubercles at the base of the neck, known as wattles. Basking behavior is not well studied though it seems that these turtles are active year round. Nesting occurs in June and hatching follows in late August and September. Clutch size varies from 3 - 28 eggs. This species is primarily carnivorous, specific dietary needs are only known to individuals in captivity. Little else is known about this species and extensive studies need to be conducted in order to better understand the natural history of the Wattle Neck soft shell turtle (Ernst et al. 1994).
Geographic
Range
T. sinensis
: The original range of this species covers
central and southern China, Vietnam, Hainan, and Taiwan. Additionally,
this species has been introduced to Bonin Islands, Timor, Japan, and Kauai
and Oahu and the Hawaiian Islands (Ernst et al. 1994).
(Note:
this is thought to be the rarer of the two introduced species of soft
shells in Hawaii)
T. steindachneri : The original distribution of this species in China ranges from Guangdong, Guangxi, and Hainan Island southward into Vietnam. Additionally this species has been introduced on Mauritius and Kauai and Oahu of the Hawaiian Islands (Ernst et al. 1994).
Vectors
Soft shell turtle meat is
a delicacy in both China and Japan. It is thought that early immigrants
from China brought both live Wattle Neck and Chinese soft shell turtles
for food into the Hawaiian Islands prior to WWII, and as far back as 1850.
Chinese immigrants to Hawaii brought numerous exotic species with them,
mostly arriving in Honolulu
(McKeown and Webb 1982).
Why
is it successful
Both
species of soft shell turtles, T.
sinensis and
T. steindachneri, once imported were sold to individuals for
pets and well as to restaurants for food. Large rearing ponds were also
created so that these turtles could be farmed and sold. The feral populations
of soft shell turtles are most likely the result of both accidental and
intended release (McKeown and Webb 1982). As stated earlier little is
known about the natural history of these species so it is hard to say
what has made these turtles persist even after introductions have ceased.
Being purely speculative I would say that these turtles have lasted through
out the decades since introduction due to the fact that hibernation is
unnecessary in Hawaii, possibly allowing for year round reproduction.
Clutch size is also fairly large so the chances of genetic survival is
high. Additionally both species seem to have a wide range of habitats
in which they are found, giving them great versatility in habitat and
increased chances of survival.
Impacts
To date the softshell turtles
introduced to Hawaii have not been found as a threat to the endemic species.
This is not to say however that through increased research this species
will not be found to be invasive. It is important to remember that many
species of the world as well as relationships between these species is
yet to be discovered. We must not take the lack of evidence thus far to
suggest that these soft shell turtles have no harmful effect but take
this information to mean that there is much more research that needs to
be done on these turtles and on their introductions.
Management
Soft shell turtles have
not posed a threat thus far so there is little being done to eradicate
those populations that have been introduced in various parts of the world.
Preventing the spread of exotic species is however something that is always
being addressed, therefore inspection, search and regulations on the pet
trade have been used to stop the spread of all exotics, including soft
shell turtles. In many cases this is done to prevent over harvesting and
extirpation of natural populations.
Lepidodactylus
lugubris
Mourning Gecko
Picture Courtesy of:http://www.ribbitphotography.com/lizards/geckos.html
Description
The Mourning gecko
is a small gecko,reaching a total length of 105 mm. Males are smaller
than females in this species (male = 35-39 mm SVL; female = 31-45 mm SVL).
Coloration ranges from light pinkish gray to light brown dorsally with
a creamy white venter. This species is primarily nocturnal and forages
on insects attracted to lights. The
Mourning gecko is parthenogenic and a communal nester. Eggs are
laid in pairs on the ground under the bark of mangroves, bananas, palms,
and pines. Eggs are commonly laid under tree bark or similar areas (Savage
2002). Sexual maturity is reached with 8 - 10 months and the average life
span is approximately 5 years. These geckos are most commonly found on
trees, hiding under bark and on the sides of peoples houses (McKeown 1996).
Additionally Mourning geckos prefer the more mesic, cooler, windward sides
of islands, and can be found in high density inside the forests of larger
islands (Case et al. 1994).
Geographic
Range
The original distribution
of this species encompasses Sri Lanka, northern India, Burma, Southeast
Asia, Malaysia, and northern Australia. Introduced populations can be
found in New Zealand, the Galapagos Islands, Costa Rica, central Panama,
Columbia, Ecuador, Nicaragua, and Florida (Savage 2002). Additionally,
the Mourning gecko can be found on nearly all of the Hawaiian Islands
including: Niihau, Kauai, Oahu, Lanai, Molokai, Maui, Kahoolawe, and Hawaii
(McKeown 1996).
Vectors
The Mourning gecko most
likely arrived in Hawaii as a stow away aboard an early Polynesian canoe
or have rafted across the sea from nearby islands on storm debris (eggs
have been shown to be salt water tolerant) (McKeown 1996).
Why
is it successful
The fact that this species
arrived to the Hawaiian islands very early on may be one of the reasons
why this species has been so successful. When early Polynesian settlers
arrived on Hawaii there was very little, if any, reptilian life there
and so what ever arrived with them or shortly after had an opportunity
to establish with out the risk of high predation. Additionally this species
is parthenogenic so very few individuals could start a thriving population
since there is no mating requirement.
Impacts
The
impacts of this species have not been well documented and it seems that
to date that the Mourning gecko does not pose a threat to any native populations.
It should be considered however, that this species is capable of having
an impact on insect populations, since insects are their primary form
of nutrience. This could be both negative and positive. Negative effects
would occur if this species over forages on native insect populations
and positive effects may occur if this gecko specializes on an introduced
insect. Since this species has not to date devasted any known native populations
it is most likely that this gecko may have beneficial effects if any.
Management
Current management for this
species is to protect it from the common house gecko that was more recently
introduced to Hawaii and has had a hughe negative effect on the Mourning
gecko (see profile of House gecko for more details). Of course prevention
of additional Mourning Geckos making there way into Hawaii is always monitored
through inspection and pet trade regulation.
Hemidactylus
frenatus
House Gecko
Picture courtesy of: http://www.wildherps.com/species/H.frenatus.html
Description
H. frenatus
was first described by Schlegal in 1836. Males of this species are larger
than females (males = 52-65 mm SVL; females = 50-60 mm SVL). Coloration
varies from grayish brown, pink, red, to dark brown dorsally and yellow
ventrally. This gecko is most active during the night, but can be found
in the day. Eggs are most commonly laid in pairs and females often breed
communally. Deposition of eggs usually occur in crevices or under ground
debris. Incubation time depends on temperature but ranges from 45-90 days.
The primary diet of this gecko are insects which are most commonly gathered
around lights during the night. The House gecko is found in a variety
of coastal areas, most commonly associated with human habitations (Savage
2002).
Geographic
Range
The original distribution
of this species encompasses India, Sri Lanka, Burma to southern China,
the Malay Peninsula, and the Philippines. Human
caused introductions have extended the distribution of the House gecko
to New Guinea, the Solomon Islands, northern Australia, Samoa, Guam, islands
adjacent to southern China and in the Indian Ocean, Madagascar, East Africa,
the Cape of Good Hope, St. Helena, the Mediterranean region, Mexico, Central
America, Panama, Texas, Florida, and the Hawaiian Islands (Savage 2002).
The
first House gecko identified in Hawaii was on Oahu in 1951 by Paul Breese.
By 1962
the House gecko was established in Honolulu, and by 1967 the it was reported
as replacing other edificarian geckos in Oahu, and Maui (McCoy and Busack
1970). The House gecko is now well established on all the major islands
of Hawaii, as well as the smaller islands of Lanai and Kahoolawe (McKeown
1996), and has been documented as outcompeteing other geckos on many of
these islands (Case et al. 1994).
Vectors
It is thought that the House
gecko arrived as stowaways among equipment and materials brought from
Pacific Islands to Hawaii shortly after WWII (1940's) (McKeown 1996).
Why
is it successful
The clumped resources that
humans provide, in the form of light aggregated insect populations, is
according to Petren and Case (1996) largely responsible for the success
of the House gecko. These clumped resources allow for increased exploitative
interspecific competition, and the decrease in both insect and Mourning
gecko populations. The larger size of the House gecko, in comparison to
the Mourning gecko, also allows for increased competitive success. The
dominance achieved by the House gecko at feeding sites allows for not
only increases food but also increased predator detection abilities, both
of which increase the House gecko populations and the decrease the Mourning
gecko populations (Case et al. 1994). Predation of juvenile Mourning geckos
by adult House gecko also gives the House gecko an advantage over the
Mourning gecko and allows to its increased success (McKeown 1996).
Impacts
Petren
and Case (1996) demonstrated that the House gecko has a much more devasting
effect on insect populations than does the Mourning gecko. From
this we can hypothesize that this newly introduced House gecko may have
a much greater and possibly detrimental effect on native insect populations,
as well as on the formerly introduced, more passive Mourning gecko.
Management
No
special procedures have been developed to eradicate this population of
House geckos, most likely because this is a new population that has just
recently been discovered for its competitive and exploitative effects.
Extensive research is being conducted by Petren, Case and most recently
Decker, Powell and Bauer, on this species and other geckos so that they
can be monitored and the better effects known. As always this species
is controlled through inspection and pet trade regulations, as is all
incoming cargo to the Hawaiian Islands.
Bibliography
Cogger. 1992. Reptiles and Amphibians of Australia. Cornell University Press. Ithica, New York
Case, T. J., D. T. Bolger, and K. Petren. 1994. Invasions and competitive displacement among house geckos in the tropical Pacific. Ecology 75:464-477.
Decker, H., R. Powell, and A. Bauer. Gecko populations on Coconut Island, Hawaii. Manuscript.
Ernst, Lovich and Barbour. 1994. Turtlrs of the United States and Canada. Smtihsonian Institutional Press. Washington and London.
Fritts, T. H. 1988. The Brown Tree Snake: A harmful pest species. U.S. Department of the Interior Fish and Wildlife Service. U.S. Government Printing Office.
Lovich, J.E. 1989. Another Exotic turtle record for Hawaii. Elepaio 49:86
McCoy, C. J. and S. D. Busack. 1970. The lizard Hemidactylus frenatus and Liolopisma metallica on the island of Hawaii. Herpetologica 26:303.
McKeown. 1996. Reptiles and Amphibians in the Hawaiian Islands. Diamond Head Publishing, Inc. Los Osos, California.
McKeown, S. and R. G. Webb. 1982. Softshell turtlesin Hawaii. Journal of Herpetology 16:107-111.
Petren, K and T. J. Case. 1996. An experimental demonstration of exploitation competition in an ongoing invasion. Ecology. 77:118-132.
Petren, K., D. T. Bolger, and T. J. Case. 1993. Mechanisms in the competitive success of an invading sexual gecko over an asexual native. Science 259:354-357.
Rodda, G. H. and T. H. Fritts. 1992. The impact of the Colubris snake Boiga irregularis on Guam’s lizards. Journal of Herpetology 26:166-174.
Savage. 2002. The amphibians and reptiles of Costa Rica. The University of Chicago Press. Chicago and London.
Savidge, J. A. 1987. Extinction of an island avifauna by an introduced snake. Ecology:68:660-668.
USGS. 2002. Brown
Tree Snake colonization and dispersal events on Guam. URL www.mesc.usgs.gov/resources/education/bts/invasion/colonize.asp
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