Introduced Plants in the Hawaiian Islands

Many thanks to Charles “Chuck” Chimera for allowing us to reprint the following article, originally published in the January/February 2017 issue of Hawaii Landscape. Chimera is a Weed Risk Assessment Specialist at University of Hawai‘i Manoa, Pacific Cooperative Studies Unit and Botany.

Introduced Plants in the Hawaiian Islands – Some Pros and Cons of Dynamic Diversity

by Charles Chimera

How many plant species are in the Hawaiian Islands? That’s a question I’ve been regularly asked during my tenure as a weed risk assessment specialist for the state. I spend a lot of time reading about and evaluating non-native plants for their potential to become invasive in Hawaii, and am often buried in books, scientific journal articles, databases, and websites to find the best and most current information about plants. Despite all that time surrounded by reference materials, however, I still don’t have a precise number. My answer is usually “That’s a good question.” (Then I say, “What’s that?” and as the person looks to where I’m pointing, I run off). Seriously though, if it’s a question about how many plants are present in the islands, without regard to whether they are native, non-native, naturalized, invasive, or just cultivated, my somewhat offhand answer is usually that there are more than 10,000. This number is roughly based on the Bishop Museum’s Annotated Checklist of the Hawaiian Islands, which lists the 10,226 plants (a total compiled between 1984 and 1999) deliberately grown by home gardeners for ornamental and landscape purpose. This total does not include weeds, forestry tree plantings, pasture and forage grasses, or native and naturalized species that exist outside of cultivation. For that number, I use the “Hawaiian Native and Naturalized Vascular Plants Checklist,” another Bishop Museum resource edited by botanist Clyde Imada in 2012. At that time, the total number of native and naturalized non-native plants was reported to be 2,983. Another good reference for a list of native and naturalized plant species is the Smithsonian Institution’s online Flora of the Hawaiian Islands. Possibly excluded from these references are the 1,026 taxa of plants that have been planted in forest reserves or on certain offshore islands, as reported by Roger Skolmen in the 1980 publication “Plantings on the Forest Reserves of Hawaii: 1910–1960.” For those keeping score, and without accounting for some overlap between lists, there are approximately 14,000 plant species in the Hawaiian Islands, or about 3.5% of the Earth’s estimated total. With between 1,355 and 1,386 native plant species (depending on sources), this means that recorded plant diversity of the Hawaiian Islands has increased by over 900% since humans first set foot on the islands. (These totals are also an underestimate, and somewhat outdated, as new plants have continued to be introduced every year since publication of the original lists.)

The term biodiversity is almost always associated positively with nature, and with a healthy, thriving environment, most people familiar with the natural history of the Hawaiian Islands know that many of our native plants have become quite rare or endangered, and several have gone extinct, due to loss of habitat, damage from invasive pests and pathogens, and competition with invasive plants, among other factors. So even though the Hawaiian Islands are more diverse in terms of total numbers of plant species than they have ever been, native biodiversity has declined as overall species diversity has increased.

Of course, this does not mean that all non-native plants are bad, or are equally responsible for the rarity and extinction crisis of Hawaii’s native flora. Of the roughly 12,000 to 13,000 non-native plants present, only about 12% have become naturalized, or have formed self-sustaining populations in the wild, and fewer still have become invasive weeds of agriculture or the natural environment. In other words, the overwhelming majority of non-native plants introduced to the islands are generally benign, and in many cases, have provided us with valuable food, medicine, building resources, ornamental beauty, and other uses that enrich our lives and landscapes.

blackburn

Endangered Blackburn’s Sphinx Moth caterpillar (Manduca blackburni) getting its nicotine fix on invasive tree tobacco (Nicotiana glauca). Photo: Forest & Kim Starr

Despite the negative impacts they have on the environment, many of Hawaii’s invasive plants were also originally introduced with good intentions, and may still provide certain benefits. Some may even be providing valuable habitat or food resources for native Hawaiian species. In one such example, the Blackburn’s Sphinx Moth (Manduca blackburni), Hawaii’s largest native insect, was the first in the state to be listed as an endangered species. This was due to many factors, including the rarity of ‘aiea, its native host tree (species in the genus Nothrocestrum). Fortunately for the moth (if not for the habitat invaded by the weed), its caterpillars can survive by feeding on the leaves of the invasive tree tobacco (Nicotiana glauca). So even though this plant can become quite invasive in dry and disturbed habitats, it may be helping to prevent the extinction of the Blackburn’s Sphinx Moth while ‘aiea and its habitat are restored. There are other examples of native species utilizing introduced plants, including Hawaiian honeycreepers sipping nectar from the flowers or eating the fruit of non-native trees, or our state bird, the Nene, feeding on invasive grasses and herbs in Haleakala crater.

For the most part, however, it would be preferable to preserve and protect Hawaii’s remaining native habitats for the species that need them to survive, rather than relying on introductions of non-native, and potentially invasive, species to hopefully save the day. When possible, use appropriate native plants, or non-invasive introduced species, in the landscape, and avoid importing or planting highly invasive weeds, especially those targeted for eradication or control by conservation land managers, the agricultural industry, and invasive species committees. By making responsible and informed planting choices, we can both preserve our native plant diversity, and continue to enhance our introduced biodiversity and enjoy all the associated benefits it brings. To learn more about both invasive and non-invasive plants in the Hawaiian Islands, or to have your plants screened for risk of invasiveness, please visit the Plant Pono website.

References

Flora of the Hawaiian Islands. Smithsonian Institution, Washington, D.C. http://botany.si.edu/pacificislandbiodiversity/hawaiianflora/

Imada, C.T., Staples, G.W. & Herbst, D.R. 2005. Annotated Checklist of Cultivated Plants of Hawai‘i. http://www2.bishopmuseum.org/HBS/botany/cultivatedplants/

Imada, C. 2012. Hawaiian Native and Naturalized Vascular Plants Checklist (December 2012 update). Bishop Museum Technical Report 60. Bishop Museum, Honolulu

Skolmen, R.G. 1980. Plantings on the forest reserves of Hawaii: 1910–1960. Institute of Pacific Islands Forestry, Pacific Southwest Forest & Range Experiment Station, US Forest Service, Honolulu.

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