Threats to the Parks: Hawaii Volcanoes National Park Struggles with Invasive Plants

Though a handsome ornamental, fountain grass is one of the exotics being combated at Hawaii Volcanoes National Park. Photo by Gerald D. Carr, University of Hawaii.

Many nonnative plant and animal species that have invaded our national parks pose a danger to the native species. It’s a system wide problem, and it’s getting worse.

At Everglades National Park, biologists worry about the potentially devastating impact on native fauna accompanying the unchecked spread of the Asian swamp eel, the Burmese python, and other exotic critters. On the other side of the continent, volunteers have already spent over 50,000 hours removing nonnative plants at Point Reyes National Seashore. At Great Smoky Mountains National Park, Congaree National Park, and even in the National Capital Parks, the story is pretty much the same.

The worst of the invaders threaten ecological havoc, and to fight back effectively park managers must be vigilant, resourceful, and persistent. This article takes a look at some invasive plants in one of the most vulnerable of our national parks, Hawaii Volcanoes National Park, and examines measures used to deal with them.

Nearly 200 species of plants that grow in Hawaii have been classified as invasive and noxious weeds. For a comprehensive listing, see this site.

Many plant species introduced to the Big Island through human activities, especially ranching and the cultivation of ornamentals and edible fruits, have made their way into Hawaii Volcanoes National Park. There they are causing grievous harm to the native species.

Invading plants have long been a serious problem at Hawaii Volcanoes. Among the main culprits are African and South American grasses that were introduced to Hawaii to improve grazing for cattle and sheep. Grazing is an important industry on the Big Island, so it is not surprising that ranching practices have impacted the park’s native vegetation. For a list of Hawaii’s native plant genera and their respective Hawaiian names, see this site.

The seeds of introduced grasses, such as fountain grass (Pennisetum setaceum) and molasses grass (Melinis minutiflora), are mostly spread by the wind, so it is virtually impossible to keep them out of the park. The big problem is, these invaders grow more vigorously and are more fire tolerant than most native grasses.

Molasses grass, in particular, produces a thick, fire resistant mat that spreads rapidly and smothers the native grasses in its path. While these characteristics might be desirable for cattle grazing, they are not good for the native vegetation of Hawaii. In addition to smothering native vegetation, the lush growth of the invaders increases fuel loads (causing fires to burn more frequently and much hotter) and makes it very difficult for fire intolerant species to thrive.

The increase in the frequency, intensity, and distribution of wildfires has proven harmful to the native vegetation in some areas of the park. Few problems exist in the eastern coastal lowlands, where more frequent fire may actually stimulate native species regeneration.

The native grass tanglehead (Heteropogon contortus) in particular responds well to frequent fire and thrives in its aftermath. It is in the seasonal submontane zone that the greatest negative impact occurs. Nearly all the native species there are fire intolerant, including the dominant native shrub, pukiawe (Styphelia tameiameia). By quickly reestablishing after fire, the fire tolerant invaders replace the natives. For more information about the fire-related spread of noxious plants in the park, see this site.

Grasses are not the only invasive plants plaguing Hawaii Volcanoes, nor even the worst. Among the many other unwelcome exotics that have made their way into the park are various shrubs, trees, and vines, including the banana poka, the firetree, the prickly Florida blackberry, the strawberry guava, the miconia, and two glory bush variants, the cane tibouchina, and the princess flower.

The prickly Florida blackberry or ohelo 'ele'ele (Rubus argutus) is a shrub that was introduced to the Big Island from the eastern United States around 1900. This plant, which has long, prickly stems, aggressively invades disturbed areas and can grow very thickly. Cattle grazing facilitated its spread, and by the 1930s it was labeled a serious pest. When it invaded Hawaii Volcanoes, spread there by frugivorous birds, it took root in disturbed areas of moist or wet forests and created impenetrable thickets in some places

The banana poka (Passiflora tarminina), is a high climbing vine with showy pink flowers. It grows best in full sun, but can tolerate some shade. Cultivated elsewhere for its edible fruit, this plant is primarily used as an ornamental in Hawaii. Its seeds are easily spread in the droppings of frugivorous animals (especially feral hogs), and it has become well-established in both open and partially closed canopy forests in Hawaii Volcanoes. When trees die or fall in the forest, this invader can quickly move into the newly sun-drenched space of the canopy gaps and grow vigorously there. Its potentially heavy impact on the native forest vegetation makes the banana poka very unwelcome, indeed.

Imported from Brazil for its edible fruit and as an ornamental, the strawberry guava or waiwi `ula-`ula (Psidium cattleianum) sports aromatic, leathery leaves and fragrant white blossoms. It grows in wet forests, where it is easily spread by fruit loving birds and pigs. This is a very serious invader. Though sometimes found growing individually, the strawberry guava more frequently creates dense thickets that suppress native species so thoroughly that almost nothing else can grow.

The cane tibouchina or glory bush (Tibouchina herbacea) is a semiwoody upright shrub that can grow as tall as nine feet. It is used for ornamental purposes because it is a handsome plant with fuzzy leaves and showy pink flowers. Spread by birds, this very prolific plant is now growing wild in various moist or wet forests on the Big Island, including some parts of Hawaii Volcanoes. It is a serious pest because it outcompetes the native vegetation and forms dense monotypic stands. A related invasive species, Tibouchina urvilleana, is a purple-flowered plant that is also called the glory bush (or princess flower). Like the cane tibouchina, this close cousin is an ornamental escapee that infests some wet lowland parts of Hawaii Volcanoes.

The firetree (Myrica faya aiton), was introduced to the Big Island in the late 1800s, apparently as an ornamental, and was used for reforestation in the 1920s. This was a big mistake, since the firetree is an aggressive and very noxious plant that can grow tall (up to 50 feet) and form a dense canopy and thick understory that is nearly devoid of other plant life. An eradication campaign initiated in the 1940s was unsuccessful, and there was no stopping the plant’s spread into vulnerable areas such as Hawaii Volcanoes National Park. The Park Service has been trying to control this invader for several decades.

Vilified as the “purple plague” or “green cancer,” miconia (Miconia calvescens) is even worse than the firetree and may very well be the single worst of all the invasive plant species. The miconia, a tree native to tropical America, was introduced to Hawaii in 1959 as an ornamental. However, it spread very rapidly to wet forests where it is most emphatically unwanted. Unfortunately, the miconia is not only very aggressive, but also very ecologically harmful. Its seeds lie dormant in the soil throughout the forest. Any gap in the rainforest canopy caused by dead or fallen trees creates a patch of sunlight that this plant can swiftly occupy and dominate. The miconia’s 40-inch long leaves shade out other plants, effectively obliterating the native flora. If the spread of this fearsome invader isn’t contained, it will eventually turn huge swaths of richly diversified native rainforest into monotypic stands of miconia.

Controlling the invaders is a complicated and expensive business. Achieving permanent eradication is an unrealistic goal for most invasive species, so the Park Service and its partner federal, state, and international agencies mainly hope to keep the invaders from spreading to new unspoiled areas and getting completely out of hand. Control measures include physical removal, herbicide spraying, and using biological controls.

Manual or mechanical removal methods, such as hand-pulling plants and removing roots and stumps, is often too costly or impractical because of the large scale of the infestations, the need for conscientious followup work, and the difficulty of working in remote areas. Removal may work for minor infestations in their early stages, but for the most part the invaders simply cannot be uprooted and destroyed fast enough and thoroughly enough to halt their spread.

Park managers are always reluctant to use chemical solutions for their problems, since this approach is environmentally risky as well as expensive. However, it was long ago deemed appropriate to use herbicides on a carefully controlled basis to curb the spread of invading grasses, shrubs, and trees at Hawaii Volcanoes. Herbicide spraying has been effective in many applications, but is not viewed as a sole strategy or a long term solution.

Biological agents -- insects, mites, and pathogens that feed on or infect noxious weeds – are playing an increasingly important role in controlling invasive plants. The biological control of weeds has been practiced in Hawaii with some degree of success for over a century, and very good results have been obtained with the use of biocontrols on infestations of prickly pear cactus, lantana, Hamakua pamakani, emex, St. John’s wort, banana poka, and ivy gourd.

Much work remains to be done, however. Combating the advance of the dreaded miconia offers a good example. Herbicide applications are of limited value for miconia control, and long term control can only be assured by using biocontrols. The first step was to release a fungal agent. Now, aided by collaborators in Brazil and Costa Rica, scientists in Hawaii have been working to develop a suite of other biocontrol agents that attack miconia plants. The ultimate goal is to reduce seed production, slow growth, and increase miconia mortality. For an excellent report on the biological control of invasive plants in Hawaii, see this site.

The Institute of Institute of Pacific Islands Forestry and its quarantine facility play key roles in the development of effective biocontrols. The ISPIF reports that:

Successful biological control of weeds depends upon a network of international cooperators, sustained funding of projects for up to 10 years or more, and the commitment of trained personnel and specialized quarantine facilities. The Institute of Pacific Islands Forestry operates the Hawai`i Volcanoes National Park Quarantine Facility, one of only two USDA-APHIS-certified insect containment laboratories for biocontrol in the state. Our ability to develop safe, effective agents is strengthened by partnerships with the National Park Service, US Geological Survey—Biological Resources Division, University of Hawai`i, Hawai`I Department of Agriculture, Hawai`i Division of Forestry and Wildlife, the US Fish and Wildlife Service, and the Hawai`i Invasive Species Council. We also collaborate with researchers outside Hawai`i, including scientists working where our target weeds are native.

It remains to be seen whether invasive plants species will ever be fully brought under control at Hawaii Volcanoes. Each scientific discovery or managerial innovation that sparks new hope for success seems to be offset by reports of yet another noxious invader. In this situation, as in so many others, it may be necessary to run faster and faster just to stay in place.