Ryan Seibold
Myrica
faya, or firetree, is an invasive exotic plant
species in the Hawaiian Islands. Firetree is native to the Azores, Madiera, and
the Canary Islands off the northwest coast of Africa in the Atlantic Ocean.
Firetree alters ecosystem processes in ways that allow it to out compete the
native vegetation of affected ecosystems in Hawaii. By primarily changing the
nutrient balance of affected ecosystems, invasion of firetree leads to the
alteration of vegetation succession. Ultimately, firetree increases the
invasibility and disrupts native community dynamics of Hawaiian natural areas,
and as a result, jeopardizes the survival of Hawaii’s native plant communities.
The system most affected by firetree is the seasonal sub-montane rainforest
which is an early successional and open-canopied ecosystem; however, firetree
will colonize a wide range of habitat.
Vitousek and Walker (1989) state that colonists from Europe and Asia, beginning in the 18th century, introduced many plant species into Hawaii for agricultural, medicinal, and ornamental purposes. Firetree was among these many introductions and was probably used for ornamental or medicinal purposes. In the 1920s and 1930s, the Territorial Department of Forestry planted firetree for watershed reclamation until its invasive qualities were recognized; unfortunately, firetree had already colonized five of the six main islands of Hawaii (Vitousek and Walker 1989). The invasion of firetreee into Hawaii Volcanoes National Park (HAVO) began in 1961 and became extensive enough for intensive control to be initiated. By 1977, firetree infestation had spread across 12,200 hectares in HAVO and 34,365 hectares in the Hawaiian Islands (Vitousek 1990).
In 1955, the search for biological control agents began with the entomologist, F.A. Bianchi, under sponsorship from the Hawaii Territorial Board of Agriculture, who traveled to firetree’s native habitat. However, research was abandoned because the most promising agent was found to attack valued trees in Hawaii, such as mango and avocado (Gardner 1992). Later, research involved herbicidal programs, however, these programs involved too much labor and had other inherent disadvantages. Biological control is a current focus of research. This paper will discuss the biology that allows Myrica faya to be a successful invader and the resulting ecological impacts in the invaded Hawaiian ecosystems. The approaches to control this invasive in Hawaiian natural areas, primarily, the effectiveness and feasibility of chemical and biological methods will be characterized.
The Hawaiian archipelago is a small chain of
volcanic formed islands located in the Pacific Ocean. Hawaii consists of 6 main
islands. The climate of Hawaii is highly influenced by its proximity to the
ocean (about 95 percent of Hawaii is within 20 miles of the coast) and its
mountainous terrain (Blumenstock and Price 1994). Marine affects (mainly
precipitation) are lessened on the leeward side of mountains, resulting in
drier ecosystems. Fifty percent of Hawaii‘s elevation reaches 600 meters and 10
percent reaches 2100 meters, therefore, the landscape is characterized as
mountainous. Hawaii is within the tropical zone, however, its climate occupies
a wide range of world climatic zones. Lastly rainfall and temperature
characterize the most variable climatic features (Blumenstock and Price 1994).
Hawaii’s geographical pattern of climates
inherently affects the diversity and composition of its ecosystems. These
ecosystems can be ordered into the following vegetation zones: strand, coastal,
dryland forest and shrub, mixed mesic forest, rain forest, bogs, subalpine
woodland, shrubland, and desert, and cliffs. The result is a unique assemblage
of vegetation on the Hawaiian landscape. According to the Hawaii Tropical
Forest Recovery Task Force (1994) rainforest ecosystems dominated the main
islands of Hawaii in pre-settlement time, approximately 1500 years ago.
However, by the late 1700s, most of these forests had become severely altered
or stressed through nearly 1000 years of agricultural disturbance and species
introductions by Polynesians. These disturbances were then followed by European
settlement, which brought degradation to an explosive scale through further
introductions and more direct exploitation of the land.
In Hawaii, the problem of exotic invasive plants
and animals is highly detrimental to native populations. Approximately 86
exotic species pose a considerable threat to native biota (Vitousek and Walker
1989). The ecosystem of most concern to firetree invasion is primarily the dry
seasonal sub-montane rainforest, although, a wide range of ecosystems and
production landscapes are also negatively affected by firetree’s invasion
(Vitousek and Walker 1989). The rainforests that are most invaded are early
successional sites, disturbed primarily by volcanic lava flows and cinder ash
deposits (Vitousek and Walker 1989). These sites often have an open-canopied or
interspersed pattern due to the uneven occurrence of disturbance. According to
Mueller-Dombois (1981) evergreen species (Metrosideros
polymorpha), along with the legume tree (Acacia koa) are the dominant vegetation types in the rainforests;
also present are the native tree ferns (Cibotium
spp.). Metrosideros’ distribution
pattern has a wide habitat range which includes the warmer tropical-dry
climate, where it is the dominant native species (Mueller-Dombois 1981). Metrosideros is also a primary successional
pioneer species and would likely colonize volcanic cinderfalls and lava flows
first, which, like most early primary successional sites, are nitrogen-limited
(Vitousek 1989). However, firetree is also a primary successional pioneer and
would also colonize these disturbed sites.
The effect of Metrosideros’
early establishment has been found to create a safe habitat for firetree.
Firetree is then able to occupy these open-canopied sites of Metrosideros.
The successful invasion of firetree is primarily
due to its biology and Hawaii’s high invasibility. Vitousek and Walker (1989)
state that every successful invader interacts with its site, often exploiting
or altering its environment for the benefit of its survival. This affected site
may be conducive to invasion through its available open niches or safe places
for which the invader can easily colonize. According to Loope (1992), the
Hawaiian Islands provide a powerful example of islands’ high invasibility, with
the idea (held common among natural scientists, including Darwin) that island
biota are lacking in adaptive capacity. Loope (1992) outlines these following
four theoretical reasons as to why islands are highly invasible: (1)
archipelagos are isolated from powerful selective forces of continents; (2) the
Hawaiian Islands have been highly altered by humans; (3) Hawaii has low species numbers relative to continental
systems, and also, important taxonomic [or functional groups] are entirely
absent, such as large mammals, ants [and nitrogen-fixing plants]; and (4)
species in Hawaii have low aggressiveness and are highly vulnerable to
extinction. The early successional sites invaded by firetree are lacking the
nitrogen-fixing functional group (Vitousek and Walker 1989). Nitrogen is the
limiting resource in these ecosystems, and therefore, any species that has the
ability to fix nitrogen could potentially out compete other pioneer species.
Firetree’s capacity to fix nitrogen is aided by
its actinorrhizal symbiosis with Frankia,
and provides the biological advantage that allows it to out compete and
dominate other early successional native species. Nitrogen fixation is the conversion of di-nitrogen gas to
ammonium, which is the inorganic form of nitrogen. Ammonium is the most usable
form by plants and micro-organisms. Once ammonium is released into the soil it
undergoes many transformations through processes such as nitrification and
denitrification. Most importantly, nitrogen fixation transforms unavailable
nitrogen into a form that can be exploited for energy, which eventually will be
released into the soil environment to become available among many organisms as
an energy source.
Vitousek (1990) has suggested the importance of
individual species as controlling factors on whole ecosystems. He relates
firetree’s ability to alter the sites it invades through the alteration of the
nutrient balance by increasing nitrogen inputs 4-fold. This added nitrogen to
the system increases firetree’s rate of growth and the total pool size of available
nitrogen in the site, which ultimately allows further invasion by other
opportunistic and invasive species. Increased rates of nitrogen mineralization
and nitrification indicate that the total pool of nitrogen available in the
soil increases under firetree (Matson 1990). Matson (1990) found that
colonization of other invasives was not correlated with firetree’s invasion.
Apparently, the dense canopy cover of firetree shades out the entire
understory. It seems likely, however, that with any amount of firetree dieback,
other invasive species would rapidly exploit the increased nitrogen and
available light energy.
In addition to firetree’s ability to fix
nitrogen, the several other characteristics make firetree a successful invader.
Wind-pollination allows firetree’s independence from pollen-vectors; production
of male and female flowers on one plant allows one species to found a whole
community; and, prolific seed production (ranging from 40,000 to 400,00
fruits/year) and rapid growth allows the rapid spread of firetree.
According to Vitousek and Walker (1989), several
factors make the open-canopied sites invasible to firetree. Metrosideros sp. are good perch trees
for birds, which are firetree’s primary seed dispersers. The primary dispersing
agent of firetree fruit is the exotic Japanese white-eye (Zosterops japonica). Seed drop also leads to population growth by
nucleation. Further, there is partial shade at ground level allowing for
firetree’s successful germination. At the same time, there is enough light at
ground level for firetree to grow fast and fix nitrogen (Vitousek and Walker
1989). The population growth of firetree has been evaluated and it is estimated
that in 1 year >150 mature firetree would establish from seeds of 21 adults
(Vitousek and Walker 1989). This estimation expresses how rapidly firetree
spreads. Ultimately, firetree creates dense monospecific stands and sparse
understory; this homogenization of the landscape is a significant alteration in
ecosystem dynamics (Whiteaker and Gardner 1992).
Currently there are no proven long-term control
methods for firetree in Hawaiian natural areas (Whiteaker and Gardner 1992).
The need for a long-term control method assumes a very high cost when considering
any method that is labor-intensive.
Physical and chemical control methods have been successful in certain
situations, however, because these methods are too labor-intensive they are not
currently considered for control in the natural areas most affected by firetree
(the sub-montane rainforests). Firetree’s most successful invasions occur in
upper-elevation rainforests that are highly inaccessible in terms of
transporting equipment and machinery. Physical removal is most common in
agricultural landscapes where machinery can access firetree.
Biological control offers certain potential
advantages over herbicidal and mechanical methods including longevity and
self-dispersal of the agent into the environment. Biological control, although
initially research intensive, could potentially offer long-term control which
over time, would diminish in the amount of overall management and labor input
(Gardner 1998). There are disadvantages of biological control however,
mortality is often slower and less perceptible, and there is less complete
control of the agent (Gardner 1998). The latter concern is the cause for so
much required initial research and laboratory testing.
Research by Donald E. Gardner from Hawaii
Volcanoes National Park in the early 1980s focused on finding the least
labor-intensive method of chemical treatment on firetree. Herbicidal control
agents that have been tested include: 1) 4% Kuron in diesel oil; 2) Tordon 22K;
and 3) Roundup (glyphosate). Chemical
control of firetree is often not feasible because the sites most highly invaded
are within Hawaiian Volcanoes National Park where herbicide is avoided at all
costs. However, Gardner and Kageler (1982) investigated the efficiency and
environmental soundness of herbicidal treatments in the early 1980s,
acknowledging that the intensity of firetree invasion allows permission of
chemicals as appropriate control agents. This research found that injection of
undiluted Roundup was a more successful treatment to previous herbicidal
control programs. The previous control program composed a treatment of 4% Kuron
in diesel oil, which involved spraying the solution on the lower tree stems so
that a 0.5 meter wide area around the stem would be treated. Although this
program was effective, Gardner (1982) acknowledged that transporting the
required large amount of solution became difficult and costly, the use of oil
as opposed to a water-based solution was an inconvenience, and further,
treatment was limited to windless and dry days to avoid exposure to non-target,
native species. However, the practice of spraying inevitably exposes native
plants, because firetree often grows right next to or is interspersed with
native vegetation.
Roundup was found by Gardner (1982) to be the
most efficient herbicidal treatment because of its effectiveness in undiluted
form and through its rapid absorption rate (30-40 minutes). In its undiluted
form, Roundup can be used in small quantities (5-10 ml per tree). Tordon 22K
was also effective in small quantities of undiluted form, however, absorption
rate was intermediate (24-48 hours). Kuron absorption rate was slow (more than
1 week). Treatment of undiluted Roundup or Tordon 22K allowed for the reduction
in treatment quantity. The smaller quantities of treatments necessary due to
the elimination of a solution reduced the amount of total treatment needed out
in the field, therefore reducing labor and transportation costs. The absorption
rate of Roundup allowed for the rapid reuse of tube sections, which affected
the amount of equipment needed in the field. Also, the absorption rate (30-40
minutes) allowed the field workers to leave the site shortly after application
allowing for quicker site-to-site application (Gardner 1982).
Gardner’s research results concluded that injection
of undiluted Roundup provided the least exposure to nearby non-target species.
Environmental soundness is related to the chemical’s rapid inactivation in the
soil by micro-organisms, which is well proven in the appropriate soil
conditions of the target ecosystem (Gardner 1982). The appropriate soil
conditions for rapid inactivation often are is the result of temperature and
pool size of nutrients already present, thus providing an inviting and optimum
habitat for micro-organisms. It seems an appropriate assumption that firetree
the soil these conditions. Therefore, trace amounts of chemical treatment
locked within dead plant material will most likely be degraded by soil fauna.
It is highly improbable then, that Roundup will enter the soil environment
(Gardner 1982).
The goal of biological control is often not to
eliminate the invasive, but to lessen its impact, by suppressing its
aggressiveness and allowing the native flora to compete. Invasive species
control in Hawaii Volcanoes National Park has focused on “Special Ecological
Areas,” or areas that are ecologically important and/or intact, in a report by
Tunison and Stone (1992). Firetree was included as being one of three most
costly target species in this paper. The prioritizing of sites into “Special
Ecological Areas” seems to be a feasible goal towards firetree control because
this process acknowledges the limitations of current control of firetree. In
addition, these priority sites may provide the suitable scale for field experiments
and research.
Classical biological control involves importing
the target-species’ native enemy (disease or insect) into the nonnative target
ecosystem. This method involves intensive research on the foreign control agent
in quarantine and has proven to be very expensive and time-consuming. Insects
as control agents are a common practice, although the use of plant pathogens is
relatively new in the field. Diseases or insects that already exist in Hawaii
and that cause target species’ decline are manipulated to intensify their
effect. As a result, the insect or pathogen does not have to be imported or
quarantined (Gardner 1998). This “alternative” approach to control holds most
promise in future control research (Duffy and Gardner 1994).
Botrytis
cinerea is the first locally established pathogen that
has been found to diminish the survival of firetree communities. Duffy and
Gardner (1994) report B. cinerea as
one of the more promising control agents because it poses little or no threat
to native species in firetree’s range. This fungus causes fruit rot, which
ultimately affects the reproductive capacity and spread of firetree
populations. Infection of B. cinerea
was found by Duffy and Gardner (1994) to significantly reduce firetree seed
viability from 66 to 16.8% in 1992 in a site in Hiawaii Volcanoes National
Park. The infected fruit were also found to be less attractive to birds,
therefore lessening the spread of firetree. Interestingly, firetree’s spread
would be much more widespread without B.
cinerea having a control factor on it already; apparently, the effect of
this fungus, until now, was not easily perceptible among firetree populations.
Firetree is infected by Botrytis cinerea through fruit-feeding adult insects and larvae,
which, as vectors, carry the infection from plant to plant. Two compatible
vectors, Amorbia emigratella and Cryptoblabes gnidiella. These vectors
can be artificially-reared through the inoculation of Botrytis in the laboratory (Duffy and Gardner 1994). Duffy and
Gardner (1994) have suggested that the level of control on firetree could be
enhanced through the introduction of a high number of Botrytis infected vectors into stands of firetree during the early
fruiting season. In fact, they urge that locally established agents be
considered before foreign insects or pathogens because these agents have
already established themselves in a niche, which expresses their compatibility
to the community and environmental dynamics of the target system. Furthermore,
introduced foreign agents could weaken the effects of locally established
agents on the target species, therefore diminishing the naturally occurring
control of firetree (Duffy and Gardner 1994).
The invasion of exotic species in Hawaiian
natural areas is a topic of great concern among preservationists and conservation
scientists. The invasion of the introduced exotic species, Myrica faya, or firetree, typifies the right situation for which
research and methodology can be tested because its cause-effect relationships
are so well understood; furthermore, firetree exemplifies a powerful force of
change on ecosystem and community dynamics. Through its ability to fix
nitrogen, firetree alters the natural nitrogen dynamics of the early
successional submontane rainforest ecosystem in Hawaii, characterized by
frequent volcanic disturbances. These sites are naturally nitrogen-limited
because they lack a native nitrogen-fixer, and therefore provide an open niche
for which firetree can invade. Through increasing nitrogen input 4-fold,
firetree increases the total available nitrogen pool, which ultimately will
allow other invasive species to exploit the affected ecosystems and out compete
the native species on these sites. Funding for the control of firetree and
other invasives is necessary if the future of Hawaii’s unique assemblage of
ecosystems is to be preserved. Funding should encourage research that aims
towards long-term control and preservation of ecologically important systems in
Hawaii. This analysis of control on Myrica faya expresses promise that
biological control will suppress firetree’s continued and pervasive spread upon
the sub-montane rainforest landscape.
Through understanding the controlling factors of individual species on whole
ecosystems, the importance and devastation of invasive species will become more
apparent, although currently, their impacts in the future are largely
unpredictable.
Blumenstock, D.I. and Price S. “Climates of the states: Hawaii.” in Kay, E.A. editor. A natural history of the Hawaiian Islands. University of Hawai’i Press; 1994. p 94-114.
Duffy, B.K. and Gardner, D.E. 1994. Locally
established Botrytis fruit rot of Myrica faya, a noxious weed in
Hawaii. Plant Disease 78:919-923.
Gardner, D.E. 1998. <http://www.botany.hawaii.edu/faculty/gardner/biocontrol/myrica%20faya/ myrica.htm> Accessed 26 November 2000.
Gardner, D.E. and Kageler, V.A.D. 1982.
Herbicidal control of firetree in Hawaii Volcanoes National Park: a new
approach. Ecological Services Bulletin 7.
Available from: U.S. Department of the Interior; National Park Service;
I29.3/3:7
Hawaii Tropical Forestry Recovery Task
Force. Hawaii Tropical Forestry
Recovery Action Plan; 1994.
Loope, L.L. 1992. “An overview of problems with introduced plant species in
national parks and biosphere reserves of the United States.” in Stone, C.P., Smith, C.W., and Tunison,
J.T. editors. Alien plant invasions in
native ecosystems of Hawaii: management and research. University of Hawaii,
Cooperative National Park Resources Studies Unit, 1992.
Matson, P. (1990). Plant-soil interactions in
primary succession at Hawaii Volcanoes National Park. Oecologia 85:241-246.
Mueller-Dombois, D. “Understanding Hawaiian forest ecosystems: the key to
biological conservation.” in Mueller-Dombois, D., Bridges, K.W. and
Carson, H.L. editors. Island ecosystems:
biological organization in selected Hawaiian communities. The Institute of Ecology; 1981. p 502-520.
Tunison, J.T. and Stone, C.P. (1992). “Special
ecological areas: an approach to alien plant control in Hawaii Volcanoes
National Park.” in Stone, C.P., Smith, C.W., and Tunison, J.T. editors. Alien plant invasions in native ecosystems
of Hawaii: management and research. University of Hawaii, Cooperative National
Park Resources Studies Unit, 1992.
Vitousek, P.M. 1990. Biological invasions and
ecosystem processes; towards an integration of population biology and ecosystem
studies. Oikos 57:7-13.
Vitousek, P.M. and Walker, L.R. 1989. Biological
invasion by Myrica faya in Hawaiíi: plant demography, nitrogen fixation,
ecosystem effects. Ecological Monographs
59:p 247-265.
Whiteaker, L.D. and Gardner, D.E. “Firetree (Myrica faya) distribution
in Hawaiíi.” in Stone, C.P., Smith,
C.W., and Tunison, J.T. editors. Alien
plant invasions in native ecosystems of Hawaii: management and research. University of Hawaii, Cooperative National
Park Resources Studies Unit, 1992.
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