Non-native Plants and Animals in the Hawaiian Islands

Carolyn Reilly 22 May 2001 Department of Environmental and Plant Biology Pbio 405, Island Biology Ohio University

Invasive plants and animals have been a problem in Hawaiian ecosystems since the discovery of the islands by the Polynesians 1500 years ago.  Huge increases in the numbers of non-native species, many of which are invasives, have occurred with the arrival of Europeans in the Hawaiian islands about 200 years ago to take part in the sandalwood trade (Vitousek et al. 1987).  As human mobilization around the globe continues to increase, many non-native plants and animals will continue to be transported to the islands, some of which will become invasives and continue to threaten natural ecosystems unless something is done to prevent spread of non-natives to the islands.

There are several terms used to define plants and animals that are not native to a specific geographic area.  Before discussing such organisms, it is useful to define the various terms and to use them consistently.  In the definitions, and throughout, I will use the term "non-native" to indicate species that are currently found in but are not native to the Hawaiian Islands.  According to the Hawaii Department of Land and Natural Resources (2001), an alien species is a species that has been transported to and established in a place outside its native habitat due to the activities of humans.  This definition is used by the department because dispersal by humans is much faster than other methods of dispersal, which are usually considered to be more natural.  They go on to define invasive species as those alien species that cause significant disruption to an ecosystem or community.  Disruption is not defined.  Smith (1998) defines aliens and several other terms, but does not use the term "invasive."  He discusses exotic as a literally correct definition of a non-native species, but one that connotes something as exciting as it is strange.  "Introduced" implies a specific action of deliberately placing a non-native species in an area and has no negative or positive connotations.  "Adventive" is a botanical term used to define a non-native species that has been introduced and dies out after persisting for only a short while in the new environment.  The term "alien," which Smith favors, indicates a non-native species and has a negative connotation, as in something strange to an area that is also not wanted in that area (as opposed to "exotic").  A "naturalized" species is a non-native species that has become established and self-sustaining in a new environment.  This is different from "invasive" in that it does not have any known detrimental effects upon native ecosystems.  The last definition Smith (1998) discusses is that of "weed," which refers to the functional aspect of a non-native plant and includes such things as those plants that compete with native Hawaiian plants by physical displacement, acting as hosts to pests and disease, and to some extent by allelopathy.  Cronk and Fuller (1995) add to this list an excellent definition of invasive plants:

"An alien plant spreading naturally (without the direct assistance of people) in natural or semi natural           habitats, to produce a significant change in terms of composition, structure or ecosystem processes" (p. 1).

In the context of this paper, the same definition can be applied to non-native animals in the Hawaiian Islands.  Throughout, I will use the terms non-native (as mentioned above) and Cronk and Fuller's (1995) definition of invasive to refer to any such plants and animals.

The ecosystems of the Hawaiian Islands are very vulnerable to invasion by non-native species.  Invasion by just one non-native species has the potential to change the islands' characters forever (Hawaiian ecosystems 2001).  This is the case because of various characteristics of the Hawaiian Islands, such as relative species poverty, loss of competitive ability (i.e. loss of thorns on plants, flightlessness in birds and insects, etc.), evolution in isolation (i.e., in the absence of many herbivores or other predators, fires, etc.), and small geographic area.  All of these are likely to contribute to the invasibility of Hawaiian ecosystems (Cronk and Fuller 1995).  In addition, the islands were colonized relatively early.  Polynesians arrived 1500 years ago, bringing with them dogs, pigs, rats, chickens, and about thirty plant species (Vitousek et al. 1987), and this was followed by European settlers arriving from 1815 onward with the onset of the sandalwood trade.  This early colonization allowed for early arrival of many non-native animal and plant species.  The latter have then had time to progress quite far through the stages of invasion outlined by Cronk and Fuller (1995).  These include introduction of the non-native species by humans; naturalization of the species; and facilitation of the species via pollination and dispersal.  This is aided by certain characteristics of the non-native species, such as seed dispersal by birds (a common disperser of non-native seed in the Hawaiian islands), the ability to reproduce by self-pollination as well as cross-pollination, seed longevity, and copious seed production.  Following facilitation leading to spread of the non-native plant, species interactions with other plants and animals are important.  The final stage in non-native species invasion is stabilization, the establishment of single-species stands that take the place of a formerly species-rich habitat, as is seen in the photo in Figure One (Cronk and Fuller 1995).  As non-native species progress through these steps, which they are apt to do fairly quickly in such invasible ecosystems as are found on the Hawaiian Islands, they become an increasing problem for the natural ecosystems.

Figure 1.  Stand of Andropogon virginicus after it has invaded  seasonal montane forest of Hawaii Volcanoes National Park (Vitousek et al. 1987).

There are several reasons that non-native species can become invasives.  These depend upon the characteristics of that non-native plant and how those fit into the surrounding new environment.  Not all non-native species become invasives.  In the Hawaiian Islands, 2,600 non-native plants have been introduced over the past two hundred years, but only 86 (2%) of these have become invasive (Smith 1998).  Non-native species, and potential invasives, arrive and become established at a rate of about 20-30 species per year, whereas prior to human colonization a new species arrived and became established at a rate of about one species every 35,000 years.  It is projected that in the near future there will be approximately 200-300 invasives within the Hawaiian Islands. A non-native species is considered an invasive if it is showing dominance in any one area or ecosystem.  If the species is causing changes in water, nutrient, or disturbance cycles it is also classified as an invasive (Hawaii department 2001).  This is the case with Myrica faya, discussed in depth below, an invasive evergreen tree that changes levels of available soil nitrogen by fixing atmospheric nitrogen, a trait that most native Hawaiian plants do not possess (Vitousek and Walker 1989).  Invasive plants also have several characteristics that contribute to their status as invasives.  They tend to have easy long-distance fruit or seed dispersal, usually by animals or wind.  They usually produce abundant seeds and have a fast rate of growth (Hawaii department 2001).   Vitousek et al. (1987) state that invasive organisms in general are the result of ecological success in competition, grazing, predation, parasitism, disease, and facilitation.  For example, feral goats graze and browse, leading to increased soil erosion and decreased native species populations.  Feral sheep eliminated reproduction of Sophora chrysophylla, a native legume, and lowered the tree line on Mauna Kea.  Feral pigs alter the soil nutrient availability, Myrica faya alters the soil nutrient cycling, and Andropogon and Pennisetum, grass species, alter the fire ecology of the islands.  Invasives are also damaging because they decrease the diversity of the area they invade (Cronk and Fuller 1995).  This leads to the current situation, in which many endemic and native Hawaiian species are endangered.

Currently in the Hawaiian Islands, 40% of all of Hawaii's birds are extinct, including 20 species of flightless birds (Anon 1993) and 10% of Hawaii's native flora is extinct (Vitousek et al. 1987).  In addition, 297 organisms are federally listed as endangered species (Stone 1999).  Vitousek et al. (1987) add that 40-50% of all native flora is endangered or threatened, and this accounts for 1/3 of all endangered species in the United States despite the fact that the Hawaiian Islands consist of only 2% of the land area of the United States.  These figures are greatly attributed to introduction of new species of plants, animals, and other organisms, in addition to the obvious factors of habitat loss due to human activities such as urbanization and land clearance for agriculture.  Non-native species, including potential invasives, are brought to the Hawaiian Islands in a variety of ways.  Approximately 70% of all non-native plants are imported as horticultural plants, while others arrive for use as crop plants, livestock forage, forestry plants, and by accidental introduction, sometimes by seeds being mixed in with other seeds being imported (Hawaii department 2001).  Non-native animals are also introduced accidentally, some of which have been discovered and removed before becoming established, such as the Harlequin stinkbug, cougars, alligators, piranhas, and scorpions (Anon 1993).  The brown tree snake, a known invasive venomous snake in Guam that has caused many problems for humans as well as for ecosystems and now populates that island at a concentration of 30,000 snakes per square kilometer, has been brought to Hawaii accidentally several times.  It is not yet established in the Hawaiian Islands, but is easily transported in ship and airplane holds.  Another example of a potentially devastating organism that has been brought to Hawaii is the mosquito.  One hundred and four mosquito species have been collected from the outer surfaces of aircraft landing in Hawaii, and any of these could have been carrying malaria, a disease that has thus far been avoided in the Hawaiian Islands.  In addition to those organisms that have arrived but are not established, there are several species that have become established and are causing problems.  These include the Formosan ground termite, which causes $150 million in damage and treatment per year.  Of the 2,000 known termite species in the world, only four of them are currently established in the Hawaiian Islands and many more of them could thrive in the Hawaiian climate (Hawaiian ecosystems 2001).  In addition, twenty new arthropod species are introduced every year (Anon 1993, Hawaiian ecosystems 2001, Vitousek et al. 1987).  Half of these are known invasive species in other parts of the world (Hawaiian ecosystems 2001).  This is an introduction rate of greater than one million times the background rate of invertebrate introduction (Vitousek et al. 1987).Rabbits, feral pigs, Argentine ants, yellow-jacket wasps, fruit flies, the banana poka vine, the rosey cannibal snail, and miconia plants are only a few examples of invasive species in the Hawaiian Islands (Anon 1993).  Invasive species are clearly a problem for the Hawaiian Islands, resulting in the islands having the highest numbers of species on the federal list of endangered species.     The listing in Table One gives further numbers for introduced organisms compared to native and endemic species within the Hawaiian Islands.

Table 1.  Numbers of endemics, non-endemic natives, and established non-native organisms in the Hawaiian Islands over the past two hundred years (from Vitousek et al. 1987).

Number of   non-endemic native species

Number of   endemic species

Number of   established non-native species

Groups

Flowering plants

~1100

87

800

Ferns and allies

105

38

21

Resident birds

44

12

38

Land mammals

0

1

18

Reptiles

0

0

13

Amphibians

0

0

4

Freshwater fish

6

0

19

Arthropods

~8000

~160

~2000

Because invasive species are so successful, Pattison et al. (1998) did a study to examine characteristics of Hawaiian invasives that lead to their ecological success.  Native Hawaiian forest plant species of Pipturus albidus, Hedyotis terminalis, Psychotria mariniana, and Bidens sandwicensis, were compared to the following invasive plant species:  Schinus terebinthifolius, Citharexylum caudatum, Cestrum nocturnum, Psidium cattleianum, and Bidens pilosa.  All species were grown under light conditions to simulate forest edge and center as well as under complete shade.  Relative growth rates (RGR), leaf area ratios (LAR), and carbon dioxide assimilation for both sets of plants under all growth conditions were examined.   It was found that invasive species had overall higher RGRs in all light conditions, a higher LAR in total shade, and more efficient CO2 assimilation (i.e., at a lower respiratory cost) than did any native species.  These factors indicate that the invasive species utilize light, which tends to be the limiting factor in Hawaiian rainforests, better than native species, leading to higher growth rates. A higher growth rate is an important characteristic of invasive plants, as mentioned above, because they then tend to physically replace native species.  Invasives appear to be especially effective in disturbed areas with high light levels, as are commonly the effect of invasive animal species such as feral pigs.  If feral pigs and other invasive animals were removed, this would decrease ecosystem disturbance and so decrease success of invasive plant species (Pattison et al. 1998).

Pattison et al.'s conclusions on the ability of invasives to better utilize light are further demonstrated in a study of tree ferns by Durand and Goldstein (2001).  Three native species of tree ferns that grow in wet forests of the Hawaiian Islands, Cibotium chamissoi, Cibotium menziesii, and Cibotium glaucum, were used in the study.  They were compared with the invasive tree fern species Sphaeropteris cooperi.  This invasive, originating in Australia and imported as a horticultural plant, tends to displace the native tree ferns.  The tree ferns are a very important member of Hawaiian ecosystems because they support native epiphytic growth and sequester nitrogen and phosphorus in their leaves, thereby influencing nutrient cycling.  In the study, possible reasons for S. cooperi success as an invasive are examined.  Again, effects of light utilization were examined as was photoinhibition under various light levels for all species studied.  They found that light is indeed a limiting factor in Hawaiian rainforests.  Cibotium species spend most of their life cycle as sporophytes in the understory and subcanopy before reaching heights of 3-7m at which point they are able to reach more light.  S. cooperi, on the other hand, grows up to 8m in the subcanopy, gaps, or forest edges, and so is able to reach more light than Cibotium.  S. cooperi  has a higher leaf surface area per plant than Cibotium, which can at least partly account for higher growth rate. Photosynthetic rates of S. cooperi were nearly double that of the native tree ferns.  S. cooperi was also less subject to photoinhibition, which together with other stresses (water shortage, etc.) can lead to damage.  Short leaf spans are associated with a high photosynthetic rate and high leaf nitrogen content?as is the case with the S. cooperi (50% shorter leaf life span than Cibotium).  In accordance with the findings of Pattison et al. (1998), high disturbance levels again contribute to the success of the invasive:  "The invasive tree fern appears to be more suited than the native Cibotium species for capturing and utilizing light resources, particularly in environments with changing light levels, such as those characterized by relatively high levels of disturbance" (Durand and Goldstein 2001, p.353).

Another aspect of damage to Hawaiian ecosystems is that of fire.  Although the islands were formed by volcanic activity, the wet forests form a natural vegetative fire break and protect ecosystems from fire damage from further volcanic eruptions.  However, invasive grasses are now instigating a change in the fire ecology of the islands.  Land clearing in what Hughes et al. (1991) have termed the seasonal submontane climatic area, tends to lead to initial grass colonization.  Prior to land clearing, the area studied consisted of open woodland with shrubs.  If disturbances occurred, native shrub species would recolonize the area.  The area now contains invasive grasses that prevent native shrub colonization, Melinis minutiflora and Shizachyrium condensatum.  These grasses are known to be excellent fuel for fires.  Before fires, cleared land is now invaded by Shizachyrium condensatum.  After a fire, Melinis becomes dominant, increasing in numbers while Schizachyrium numbers remain constant.  After a second fire on the same site, Melinis continues to increase in numbers while Schizachyrium numbers decrease.  This increase in spread of Melinis can be devastating, because as a fuel it causes fires to become more intense and to spread more rapidly.  In addition, this species spreads by seeds and by rhizomes, which can overgrow and smother other species.  Together, these two grass species incur greater frequency of fire on the Hawaiian islands where there was an almost non-existent fire cycle before (Hughes et al. 1991).

Native land snails are also affected by introduction of non-native species.  As discussed earlier in the seminar, land snails are an important and interesting part of Hawaii's ecology.  They are now disappearing due to predation by carnivorous snails, habitat destruction and modification, introduction of 81 non-native snails and establishment of 33 of those (12 freshwater, 21 terrestrial).  One particularly devastating introduction was that of Euglandina rosea, the rosey cannibal snail.  This is a carnivorous snail species that was introduced to the Hawaiian islands as biological control for non-native Achatina fulica, the giant African snail.  Unfortunately, Euglandina preferred eating the native land snails instead of the African snail, detrimentally impacting the native land snails while having no effect on the African snail, which was merely a garden pest (Anon 1993, Cowie 1998, Ezzell 1992).

Another problem plant in the Hawaiian islands is Myrica faya (Figure 2).  This small evergreen tree, native to the

Figure 2.  Myrica faya (Cronk and Fuller 1995).

Canary Islands, Azores, and Madeira in the north Atlantic, was brought to the Hawaiian islands by the Portuguese in the late 1800s as an ornamental and medicinal. It was planted for watershed reclamation in the 1920s and 30s  until it was realized that the plant aggressively took over areas in which it was used.  By that time, it was established on five of the six large islands.  Control measures were implemented in 1973, but abandoned by 1977 because of its continued rapid spread.  It is a problem not only because of its ability to rapidly and aggressively colonize open-canopied forests, but also because it is an actinorhizal nitrogen fixer in its native range.  It is able to form a symbiotic relationship with the actinomycete Frankia, which is also a non-native, but it is unknown how it reached the islands.  No nitrogen fixers occur elsewhere in Hawaiian primary succession, and Myrica tends to invade young volcanic sites, in which primary succession would occur, and which are low in nitrogen.  By fixing nitrogen, Myrica greatly alters the nutrient availability in the soils of these primary succession sites, increasing opportunities for further invasion by non-native plants (Vitousek and Walker 1989).  In addition, there is evidence for Myrica allelopathy, a factor that affects plants growing within close vicinity of Myrica.  The plant's dispersal agent, the bird Zosterops japonica (Japanese white-eye), is also present in Hawaii and greatly contributes to its spread throughout the islands (Beard 1990).

Native bird populations are also affected by non-native species.  In a study by Mountainspring and Scott (1985), interspecific competition among birds for food was examined.  They observed birds in pairs of natives, non-natives, and one native paired with one non-native.  The species chosen include the Hawaiian thrush, elepaio, common amakihi, Anianiau, Kauai creeper, Maui creeper, Akepa, apapane, crested honeycreeper, and iiwi.  Non-native species used were the melodious laughing-thrush, red-billed leiothrix, Japanese white-eye, and northern cardinal.  Using the pairing approach to examine competitive effects within a geographic area, they statistically removed any effect of habitat gradient and evaluated associations between the paired species as negative or positive.  They found that in native/native pairs and in non-native/non-native pairs there were virtually no negative associations.  This is due to the fact that the native species have evolved together on the Hawaiian Islands.  Non-native pairs are relatively few and are established in few geographically overlapping areas, so negative associations have not yet developed.  When native and non-native pairs were evaluated, however, it was found that "nearly half of the species-pairs of native/exotic [non-native] primary potential competitors in any given forest modify their distribution across the landscape in apparent response to interspecific competition" (p. 237).  There were clearly significant negative correlations due to direct and diffuse competition that led to modification of geographical distribution of bird species (Mountainspring and Scott 1985).  Without the presence of non-native species, this would not occur.  It is due to this factor of re-distribution (in combination with habitat loss, etc) that most likely leads to the threatening and endangerment of so many native Hawaiian birds.

The above case studies are only a few examples of invasive plants and animals in the Hawaiian Islands.  There are many more examples that could be listed here, all of which contribute to the status of so many of the Islands' native organisms as endangered.

As non-native organisms continue to decimate Hawaiian Island ecosystems, there is work being done to reduce their impact.  Ecologists and environmentalists from universities, botanical gardens, and other locations are working together in attempts to eradicate non-natives, while at the same time legislation is being introduced to prevent their spread (Ezzell 1992).  Environmental groups such as the Sierra club coordinate expeditions in which members remove invasives within state parks (Hattam 1999).  Smith (1998) suggests that via education and inspection of incoming travelers and mail deliveries to the Hawaiian Islands the influx of non-native species can be slowed, and at the same time ecosystem disturbance should be minimized.  The Hawaiian Ecosystems  At Risk Project (HEAR, 2001) has come up with a ten-point plan to reduce introduction of non-native species.  This plan includes items such as aircraft inspection, increasing public awareness, and ensuring federal support for the plight of the Hawaiian ecosystems.  In the meantime, they give some suggestions as to what people living in Hawaii and people traveling to Hawaii can do.  For Hawaiian residents, they suggest learning to identify non-native plants and animals and reporting sightings of them to the state department of agriculture, keeping pets at home, mail-ordering wisely, landscaping with native plants, and not sharing diseased plants.  For those traveling to Hawaii, they ask that we not bring plants, fruits, vegetables, or illegal animals into the state, in addition to keeping our hiking gear clean both before entering the state and between islands.  In addition, everyone should help by spreading the word about the effect non-native species, especially invasives, are having on the fragile ecosystems of the Hawaiian Islands.

It is clear from various case studies that invasive plants and animals are very damaging to Hawaiian ecosystems.  They compete with native and endemic species for resources and tend to crowd out native species physically in addition to otherwise altering the environment in which native species live by changing the fire and nutrient cycles.  Non-native animals encourage invasion by non-native plants by creating disturbances in forests in which non-native plants, often good utilizers of light, can grow quickly and overshadow native species.  Non-native species are clearly a main cause for the endangered status of many of the Hawaiian Islands' native and endemic species.  There is some work being done to combat the spread of non-native species and this should be continued in the political and educational arenas.

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