Senior
Seminar 2002
Introduced Species in Hawaii
Miconia calvescens Myrica faya Psidium cattleianum
Photo courtesy of Scott and Susan Whigham
Introduction
There are more invasive
plants in Hawaii than any other group besides arthropods. Over the
last 200 years, over 4,500 species have been introduced into Hawaii. Smith
(1985) categorizes 2%, or 86 species, as being problem species. Most
alien plants are introduced through human movement or botanical gardens.
Thus, humans are responsible for the severe invasive plant problem.
Since the problem is so severe, a full understanding of plant ecology
and methods of invasion is needed to combat it.
Some of the characteristics of plants that lead to their success as invasives include dispersal of seeds by animals, large seed banks, and high competitive ability (Cronk and Fuller 2001; Pattison et al. 1998). Most invasives produce lots of seeds that are usually dispersed by birds or other animals. This allows for dispersion both close to and far from the original site of infestation. Also, seeds from invasive plants appear to remain dormant for longer periods of time and develop quickly when they do sprout. Thus, an area could still be susceptible to invasion long after the parent plants are removed. Invasive plants show high competitive ability and are able to colonize areas sooner than native species. Reproducing through vegetative means allows some plants to colonize areas even if con specifics aren't present.
These plant invasives can have severe effects upon the natural ecosystems. Fragmentation of native ecosystems, due to human activities, has enabled invasives to reach areas that might not have been accessible before. Invasives usually act by displacing native individuals, using up nutrients through higher efficiency, and/or act as vectors or hosts of diseases and pests (Smith 1985). These mechanisms can lead to formations of monotypic stands, changed fire regimes, altered soil-water regimes, altered nutrient cycling, extinction of other species, and/or creation of mutually beneficial interactions between plant and animal invasives (Smith 1985; Cronk and Fuller 2001). Invasives can form monotypic stands that decrease the diversity in an area. For example, Psidium cattleianum can form dense stands of trees that shade out and kill other species that were living in the area (Smith 1985). Invasive plants can also alter fire regimes. Evidence suggests that fire was rare in the Hawaiian islands before the arrival of humans. However, this is now changing with the activities of humans and the introduction of invasive grasses that provide fuel for the fires. These grasses are also the first to recolonize an area after a fire and keep other native species out and thus the fire cycle is perpetuated (Cronk and Fuller 2001). Certain invasive plants can alter the soil chemistry of areas. For example, Myrica faya is a nitrogen-fixing plant and levels of nitrogen increase where this invasive is present. This increase in nitrogen can allow other invasives, that wouldn't ordinarily have enough nutrients, to invade an area. Also, some invasive plants can either take water away from other plants or they can allow water to runoff and cause erosion (Smith 1985). Finally, invasive plants are often spread by feral pigs or invasive birds, which eat the fruit. This increases the number of plants in other areas and increases the food supply for invasive pigs and birds which will lead to an increase in their populations. Thus, invasive animals and plants can perpetuate each other's existence.
There are three basic things that need to be done to stop the introduction or spread of invasive species. First, further introductions should be prevented. Second, disturbances of already introduced species need to be stopped. Finally, native plants need to be allowed to re-establish themselves (Smith 1985).
The first step, preventing further introductions, should be implemented through regulations and education. People need to be educated about the impact of introduced species and what plants pose problems. In the case of certain harmful plants, an outright ban may be beneficial. Also, inspection and quarantine of imports can decrease the number of invasives and potential pests they may carry.
One of the largest problems with post-introduction management is finding cost effective solutions. Many measures could be taken, like ripping out all members of the invasive species in an area. However, not all solutions can be implemented due to cost constraints. The first question is to decide where control measures should be concentrated. Not all infested areas can be targeted because many will be in inaccessible or hard-to-locate regions. In the past, control efforts have been directed at regions with severe infestations that are easily accessible. This is because volunteers often provide the labor and want to see the most obvious results. Recently it has been shown that small founding populations should be targeted to stop the overall spread of the invasive (Goodland et al. 1998). These scattered populations, though not a problem on their own, can help to spread the invasive to areas that are more vulnerable. Another method is to clear designated areas of the invasive. For example, in Hawaii Volcanoes National Park a Special Ecological Area (SEA) has been created (Goodland et al. 1998). The main benefits of SEAs include, an area that is easier to maintain, control and that still contains intact, native species, an ability to fence out feral animals, and an area that can be expanded as invasives are brought under control. Clearing large, heavily invaded areas should not be a top priority though it is often cheaper to do. This will not stop the spread of the invasive, and may not even eliminate it in the area, due to large seed banks. This method should only be undertaken if the native populations become unlikely to re-establish due to loss in their seed bank numbers.
Once an area has been targeted for removal the next question is what individuals should be removed. It is very expensive to remove all plants, so certain individuals that would have the greatest effect should be targeted. It is obviously necessary to concentrate removal efforts on sexually reproducing individuals to stop the spread of the invasive. In the case of dioecious species only the females need to be targeted (Goodland et al. 1998). All individuals may be targeted if the invasive spreads through vegetative means. However, there are drawbacks to removing lots of individuals from an area. This can open up habitat that other invasives can colonize because they invade more quickly than native species. This just changes the problem from one invasive species to another. A slow removal of individuals can improve re-establishment of natives species, but this is not very economical.
Natives need to be helped to re-establish self-sustaining populations. As already mentioned above, the removal of invasives can lead to establishment of other invasives rather than natives. It is clear that restoration of native ecosystems is not as simple as just removing the invasive individuals. A full understanding of the invasive, natives, and ecosystems is necessary in order to develop re-establishment strategies.
Because of human activity, it is extremely unlikely that the introduction of alien species will be stopped. However, with the right strategies the impact, spread and numbers of new introductions could be brought under control. Below are descriptions of some of the more problematic invasive plants in Hawaii.
Miconia
calvescens
Common Name: velvet tree, green cancer
Photos courtesy of The Nature Conservancy
Description
Miconia calvescens
(Melastomataceae) is a shade-tolerant, small tree that can grow 4-15 m
tall. It has large (80 cm in diameter), tri-veined leaves that are
dark green above and purple-blue underneath. It becomes sexually
mature after 4-5 years and can self-pollinate. The flowers of M.
calvescens are white to pink in color, abundant and very short-lived
(12-24 hours after opening) (Wise and Lyons 1998). Its dark purple
fruits are about one-half inch in diameter and contain many (140-230)
seeds per fruit (Binggeli 1998). In its native range, M. calvescens
has at least three reproductive peaks per year and an older individual
can produce over 5 million seeds a year.
Geographic
Range
M. calvescens
is a native of Tropical America where it extends from southern Mexico
to northern Argentina and Chile (Medeiros et al. 1997). It
grows well in elevations of 300-1830 m. It has a wide climatic tolerance,
but prefers areas that have distinct seasonality. M. calvescens
favors wet habitats with at least 1,800 mm annual rainfall (Baruch et
al. 2000). Though it usually invades disturbed areas, occasionally
it invades disturbed areas. M. calvescens is considered
an invasive in Tahiti, Hawaii, Moorea and may now be invading Australia
(Cronk and Fuller 2001; Csurhes 1997). In Hawaii, it is found on
four islands (Hawaii, Maui, Oahu, and Kauai).
Vectors
The velvet tree was introduced
to Hawaii as an ornamental in 1961 (Medeiros et al. 1997). In
Hawaii, M. calvescens is dispersed by invasive birds including
the Japanese white-eye, red-billed leiothrix, and the common mynah.
Why
is it successful
M.
calvescens produces copious numbers of seeds that can be disperse
to near and far areas by introduced birds (Medeiros et al. 1997).
Once it reaches an area, M. calvescens grows very rapidly
and reaches sexual maturity early. It forms dense, monotypic stands
that shade out any plants growing beneath its canopy. M. calvescens'
numerous seeds form large seed banks and can lie dormant for many years
until increased light levels stimulate germination. This can reverse
the gains from manual removal, because once older trees are removed, increased
light levels will stimulate the growth of new individuals. Other
reasons for its success include its shade tolerance, continuous seed production
and ability to regenerate quickly after disturbance.
Impacts
The greatest concern about
M. calvescens' presence in Hawaii is that it could take over
the islands like it has in Tahiti. Currently, M. calvescens
covers 70% of Tahiti's forests and is a threat to over a quarter of its
endemic flora and fauna (Association for Biodiversity Information 2001).
The dense canopies that M. calvescens forms can suppress
the growth of any plant living in the understory. This leads to
a reduction in biodiversity in infested areas and if not stopped, could
lead to the extinction of many endemic plants (Meyer and Florence 1996).
Not only are other plants negatively effected, but dense stands
of M. calvescens can lead to a loss of habitat for other animals.
Some research suggests that the shallow root system of M. calvescens
doesn't hold the soil together and may cause erosion in areas it inhabits.
This soil can then enter the ocean and damage coral (Association
for Biodiversity Information 2001). If left unmanaged, M. calvescens
has the possibility of dramatically changing the landscape of Hawaii much
like it has in Tahiti.
Management
Officials and organizations
appear to understand the possible impact of M. calvescens and
are mobilizing to deal with it before Hawaii becomes another Tahiti. More
time and money has been spent of stopping M. calvescens then
on any other invasive plant in Hawaii. Public awareness programs
have been started and a hotIine was created to take calls concerning M.
calvescens sightings. In 1996, "Operation Miconia"
was launched. This program combines federal, state, businesses,
and nonprofit organizations to combat the spread of the velvet tree (Tavares
1997). A three person team has been created, who's sole function
is to stop the spread of M. calvescens. Today, chemical
controls and manual removal are the only measures being taken against
M. calvescens. However, like many other invasives, biocontrol
seems to be the only long term option. Hopefully the controls currently
in place can keep M. calvescens in check until a feasible biocontrol
is found. Currently, research is being performed on the possibility
of using Colletotrichum gloeosporoides f. sp. miconiae, a
fungal pathogen, as a biocontrol, though more research needs
to be completed (Medeiros et al. 1997).
Myrica
faya
Common Name: firetree
Photo courtesy of Plant Conservation Alliance
Description
Myrica faya
(Myricaceae) is an evergreen shrub or tree that ranges in height from
4-16 m. It has oblanceolate, alternate leaves that are shiny, dark
green and range from 4-12 cm in length. Though M. faya
is wind pollinated in its native range, in Hawaii, it is also pollinated
by introduced honeybees (Vitousek and Walker 1989). There is some
evidence that the leaves of M. faya may have allelopathic activities
(Smith 1985). This is a dioecious species that may have some flowers
with both male and female flowers. Its fruit are small, edible drupes
that are red to purple and form dense clusters. Its seeds are primarily
dispersed by invasive birds and feral pigs.
Geographic
Range
The firetree is
a native of the Azores, Madeira, and the Canary Islands and is considered
an invasive in Hawaii where it has invaded all the major islands. It
grows well in warm temperatures and can grow in both dry and moist environments.
In its native range, M. faya occurs from sea level to 610
m in elevation and is often found in pastures (Cuddihy and Stone 1990).
Within Hawaii, M. faya grows between 300-1210 m and usually
in areas with annual rainfall above 875 cm (Cronk and Fuller 2001).
Vectors
Myrica faya was
introduced in Hawaii in the late nineteenth century by Portuguese immigrants
as an ornamental, source of fruit for wine-making, medicinal plant and/or
for firewood. Later, the Hawaiian Sugar Planters Association obtained
seeds from a Portuguese farmer and began to plant M. faya for
reforestation purposes. In the 1920s and 1930s the Territorial
Department of Forestry began to plant M. faya as a way to reclaim
watersheds (Vitousek and Walker 1989). These plantings continued
until 1937 when the invasibility of M. faya was finally recognized.
By 1980, over 34,000 acres were infested with firetree. It
has spread to many habitats and though it invades mostly disturbed areas,
it can also invade undisturbed habitats.
Why
is it successful
Myrica faya is
primarily a wind pollinated plant and is thus able to reach distant areas.
Since it can often self-pollinate, a single individual is all that
is needed to start a new colony. Also, M. faya produces
copious amounts of fruit. On average a male tree can produce more
than 40,000 fruits per year, while females can produce more than 400,000
(Vitousek and Walker 1989). Though it grows better in habitats with
high light levels, M. faya is able to invade almost any area.
It can grow very rapidly in open-canopied regions and is able to
shade out native plants. One of the main reasons for the success
of M. faya as an invasive is that it has a symbiotic relationship
with nitrogen-fixing bacteria. This allows it to invade early successional
areas and to grow in regions where there would normally not be enough
nitrogen to support abundant plant life (Cronk and Fuller 2001).
Impacts
Myrica faya is
considered an invasive plant only in the Hawaiian archipelagos, elsewhere
native predators are able to keep it under control. Within
Hawaiian habitats, M. faya is able to form monotypic stands which
prevent the recruitment of and shade out natives. This leads to
huge decreases in diversity in invade areas and if not dealt with, could
lead to extinction of native flora. M. faya also changes
the nutrient levels of the regions it invades. It has a symbiotic
relationship with nitrogen-fixing bacteria and much of this nitrogen remains
in the soil. This allows M. faya and other plants (especially
invasive grasses) to grow in areas they wouldn't normally be able to inhabit.
This increase in grass density can lead to more intense and frequent
outbreaks of fire to which natives are ill adapted (Lenz and Taylor 2001).
There is also recent evidence that suggests that M. faya
may increase the abundance of the invasive insect Sophonia rufofascia
(Homoptera: Cicadellidae). Lenz and Taylor (2001) found that there
were huge increases in the abundance of S. rufofascia in areas
where M. faya was present, as compared to areas without the tree.
Damage was not just concentrated on M. faya foliage, but also
on native species found in the same area. However, the extent of
damage on M. faya was not sufficient to consider the use of S.
rufofascia as a biocontrol. In fact, more effort should be
concentrated on removing M. faya as its presence enables S.
rufofascia to attack vulnerable native species (Lenz and Taylor 2001).
Management
At the moment, M. faya
is mostly controlled by removing individuals by hand. Chemical controls
have been investigated, but these have the potential for killing native
plants as well and cannot deal with the problem in the long term (Markin
2001). For long term management, biocontrol seems to be the best
answer. This would allow for management of the species even in hard
to locate areas. However, the introduced biocontrol must only target
the invasive plant and not other species. Knowledge of which organisms
will do this takes time and research. Preliminary research showed
that the lepidopteran, Caloptilia schinella, a native predator
of M. faya in its native habitats, showed potential as a biocontrol
(Markin 2001). In 1991, a permit was issued for the release of these
moths into infested areas. Later research showed that the C.
schinella had established populations in infested areas. However,
funding was soon withdrawn from the project and no new individuals were
released. Evidence of herbivory due to C. schinella is
still apparent on M. faya leaves, but further introductions of
the moth are needed if its population is to reach a level that could control
M. faya.
Psidium
cattleianum
Common Name: strawberry guava
Photo courtesy of University of Hawaii Botany Department
Description
Psidium cattleianum
(Myrtaceae) is a large shrub or small tree that can grow to 6 meters in
height (Cronk and Fuller 2001). It has shiny, obovate leaves 4-8
cm in length with many small, white flowers. Its fruit is reddish-purple,
contains numerous seeds and is produced throughout the year. It
can also regenerate and spread through clonal suckers. Smith (1985)
characterized P. cattleianum as "the worst pest in Hawaii's
rain forests."
Geographic
Range
Psidium cattleianum
is a native of South American and Brazil in particular. Not much
is known about its ecology in its natural range (Cronk and Fuller 2001).
It has been introduced and is invasive in Hawaii, Tropical Polynesia,
Norfolk and Maurilius. It is found on all major islands of Hawaii,
from sea level to 1300 meters in elevation. It thrives well in wet
habitats with at least 1,250 mm of annual rainfall and does especially
well in rain forests (Smith 1985).
Vectors
Strawberry guava was introduced
to the Hawaiian islands by European settlers in 1825. It was originally
intended to be cultivated as an edible fruit. However, it soon escaped
cultivation and began to spread to surrounding habitats (Cronk and Fuller
2001). Today it is often spread to new areas in the feces of invasive
feral pigs and birds like the myna and Japanese white eye. It was
calculated that in densely populated areas, individual feral pigs were
responsible for dispersing about 8,000,000 seeds per month during the
height of the fruiting season (June to October) (Tunison 1991).
Why
is it successful
P. cattleianum
forms monotypic stands, in disturbed or undisturbed habitats, that push
other native species out and shade out the understory plants. There
is also some evidence that strawberry guava is allelopathic and the toxins
from its leaves prevent other species from growing. The production
of numerous seeds allows P. cattleianum to successfully take
over an area and increases its likely hood of being spread to new ones
(Tunison 1991). The rapid growth of P. cattleianum allows
it to quickly overgrow and shade out other species in an infested area.
Another characteristic that leads to the success of P. cattleianum
is its ability to withstand many environmental conditions (Cronk and Fuller
2001).
Impacts
P. cattleianum
is invasive in Hawaii, Tropical Polynesia, Norfolk Island, and Mauritius.
As mentioned above, one the the most severe effects of P. cattleianum
is that it forms monotypic stands that shade out other native species
(Tunison 1991). Feral
pigs, which like to eat the fruit of P. cattleianum, may disrupt
the substrate of an area by digging up the ground searching for fruit
(Cuddihy and Stone 1990). These
disturbances can prevent other plants from growing in the area.
Management
Currently,
P. cattleianum is being controlled through manual removal and
chemical controls (Cuddihy and Stone 1990). Many chemical combinations
have been shown to be effective. However, this is very labor intensive
and cannot be applied to densely infested areas. Thus, biocontrol
seems like the best answer to controlling P. cattleianum. Presently
research is being done on possible biocontrols, but nothing has been proven
to only target strawberry guava. This task is very difficult because
a close relative of strawberry guava, common guava, is a cash crop in
Hawaii and may also be effected by biocontrol (Cronk and Fuller 2001).
Further research needs to be done considering the extent of the
problem.
Links
Department
of Land and Natural Resources and Department of Forestry and Wildlife.
Good overview of invasive plants in Hawaii and more detailed information
about what makes invasives so successful. Also provides a link to
a complete list of invasives plants in Hawaii.
http://www.state.hi.us/dlnr/dofaw/hortweeds/
Operation
Miconia Hawaii
This is a site devoted to stopping the spread and impact of the invasive
Miconia calvescens.
http:www.hear.org/operationmiconia/index.html
Special
Ecological Areas
Brief overview of SEAs, what they are and what they are meant to do.
http://www.nps.gov/redw/sea.htm
Literature
Cited
Association for Biodiversity
Information. 2001. The Dirty Dozen: America's Least Wanted.
http://www.natureserve.org/publications/leastwanted/miconia.html
(19 September 2002).
Baruch, Z., R. R. Pattison, and G. Goldstein. 2000. Responses to light and water availability of four invasive Melastomataceae in the Hawaiian Islands. International Journal of Plant Sciences 161: 107-118.
Binggeli, P. 1998. Miconia calvescens DC. (Melastomataceae). http://members.lycos.co.uk/WoodyPlantEcology/docs/web-sp10.htm (19 September 2002).
Cuddihy, L. W. and C. P. Stone. 1990. Alteration of native hawaiian vegetation: effects of humans, their activities and introductions. Honolulu, HI. University of Hawaii Press.
Cronk, Q. C. B. and J. L.Fuller. 2001. Plant invaders: the threat to natural ecosystems. Sterline, VA: Earthscan.
Csurhes, S. M. 1997. Miconia calvescens, a potentionally invasive plant in Australia's tropical and sub-tropical rainforests. Proceedings of the First Regional Conference on Miconia control: 72-77.
Goodland, T.C.R., J. R. Healey, and P. Binggeli. September 1998. Control and management of invasive alien woody plants in the tropics. School of Agricultural and Forest Sciences number 14. University of Wales, Bangor.
Lenz, L. and J. A. Taylor. 2001. The influence of an invasive tree species (Myrica faya) on the abundance of an alien insect (Sophonia rufofascia) in Hawai'i Volcanoes National Park. Biological Conservation 102: 301-307.
Markin, G. P. 2001. Notes on the biology and release of Caloptilia sp. nr. schinella (Walsingham) (Lepidoptera: Gracilariidae), a biological control moth for the control of the weed firetree (Myrica faya Aiton) in Hawaii. Proceedings of the Hawaiian Entomological Society 35: 67-76.
Medeiros, A. C., L. L. Loope, P. Conant and S. McElvaney. 1997. Status, ecology, and management of the invasive plant, Miconia calvescens DC (Melastomataceae) in the Hawaiian Islands. Bishop Museum of Occasional Papers 48: 23-36.
Meyer. J. Y., and J. Florence. 1996. Tahiti's native flora endangered by the invasion of Miconia calvescens DC. (Melastomataceae). Journal of Biogeography 23: 775-781.
Pattison, R. R., G. Golstein, and A. Ares. 1998. Growth, biomass allocation and photosynthesis of invasive and native Hawaiian rainforest species. Oecologia 117: 449-459.
Smith, C. W. 1985. Impact of alien plants on Hawaii's native biota. Pages 180-250 in C. P. Stone and J. M. Scott, editors. Hawaii's terrestrial ecosystems: preservation and management. University of Hawaii Press for Cooperative National Park Resources Study Unit, Honolulu, Hawaii, USA.
Tavares, K. 1997. Big Island Melastome Action Committee: Miconia calvescens control and program overview. Proceedings of the First Regional Conference on Miconia Control: 52-64.
Tunison, T. 1991. The Nature Conservancy Element Stewardship Abstract for Psidium cattleianum. http://tncweeds.ucdavis.edu/esadocs/documnts/psidcat.html (10 October 2002).
Vitousek, P. M. and L. R. Walker. 1989. Biological invasion by Myrica faya in Hawai'i: Plant demography, nitrogen fixation, and ecosystem effects. Ecological Monographs 59: 247-265.
Wise, A. and R. E. Lyons. November 1998. Velvet Tree: Miconia calvescens. http://www.nps.gov/plants/alien/fact/mica1.htm (11 October 2002).
Designed by: Heather
Meckel Contact:
meckehe@earlham.edu
Last revision date: 9 December 2002