The Hawaiian Islands are home to an array of native plant species that has attracted the attention of botanists, naturalists, horticulturists and world travelers ever since Europeans first visited the islands near the end of the 18th century. Not only is the native flora–combining endemic and indigenous species–unique in its richness, its study has been the source of highly significant insights into the very essence of isolated island floras including such evolutionarily important questions as (very) long distance dispersal, the rate of evolution of an island's biota, and the nature of adaptive radiation. Of intense interest is the ongoing study of the origin of the individual elements of the flora. Situated in the midst of the vast Pacific Ocean, over 4,000 km distant from the nearest continent, the question of origins has been a major driving force in addressing the broader issues. It is easy to see why the Hawaiian Islands have been described as a ‘natural laboratory’ of unsurpassed significance.
But all is not well in the Rainbow State, at least when considering its natural biological treasures. Agriculture, both contemporary and historical; increasing population pressure; nearly unchecked commercial development; the importation of alien species for horticultural and/or agricultural purposes; and the inadvertent arrival of alien species as hitchhikers, have all had, and in most instances continue to have, serious impact on the natural laboratory. Many programs have been undertaken to return critical areas to some semblance of their original constitution including extensive fencing to keep ungulates and pigs from sensitive areas, out-planting seedlings of rare and endangered species, efforts to restore the integrity of streams, eradication of weedy species through physical removal, application of herbicides, and use of species-specific insect herbivores. There has been progress with each of these general approaches, but there is much to be done in order to protect the unique flora of the islands.
It is my purpose in this series of articles to share with the reader a sample of the flowering plants of the Hawaiian Islands, both native and introduced, through short descriptions of each of the families represented on the islands accompanied by images of the plants. Most of the plants were photographed in the field, but a few of the rare and/or endangered ones were seen only in cultivation. From time to time images from other sources will be used.
We begin with an overall picture of the flora. The Manual of the Flowering Plants of Hawai`i (Wagner et al., 1999), hereafter referred to as the Manual, reported that the native flora consists of 87 families (none endemic), 216 genera, and 956 species, of which 850, or nearly 89% are endemic. Of the 216 genera recognized in the flora 32, or nearly 15%, are endemic. The number of naturalized species–alien species that reproduce on their own–was given as 861, which gives a total of 1,817 species on the islands.
Plant families represented on the Hawaiian Islands will be discussed in alphabetical order as they appear in the Manual (1999). Entries are grouped in sets starting with the dicotyledonous (Dicot) families in Parts 1-50 and the monocotyledonous (Monocot) families in Parts 51-65. No relationships are implied for members of a set. Family discussions vary from brief, for those families for which I have no images or for which little information is available, to quite long, as in the case of the Asteraceae and Campanulaceae, both of which are well represented on the islands and about which much is known.
These numbers reflect the situation at the time of publication of the Manual and should only be taken as an indication of the overall situation. Relevant changes, at least in the status and/or number of native genera, will be noted below. The status of several traditionally recognized families with island representatives has also changed owing to recent DNA-based research. These will be commented on under the traditional family name as used in the Manual. Despite these changes, the critical message is the fact that the native flora amounts to slightly less than 50% of the total number of flowering plants on the islands. In other words, half of the islands' plants are alien!
One of the interesting facets of the native Hawaiian flora is that the majority of endemic species–although many are tropical and exotic in appearance–belong to families that would easily be recognized by visitors from temperate North America. The five families that make the greatest contribution in species to the flora are, in descending order: Asteraceae, the sunflower family, with at least 92 native species; Poaceae, the grasses, with 47; Campanulaceae, home to the campanulas and lobelias, with 110; Fabaceae, the legumes, with 20; and Lamiaceae (Labiatae), the mints, with 54 native species. An additional feature is the proportion of endemic species within the total representatives from these families. The 92 endemic species within Asteraceae, for example, represent slightly over half of the total number of species of the family present. (This is a conservative figure insofar as newly described species have been added since publication of the Manual, from which source these numbers have been taken.) In the Campanulaceae, 110 out of 114, or 96.5%, species are endemic; in the Lamiaceae, 54 of 70, or 77.1% occur nowhere else. Other major contributing families include the Rubiaceae (coffee family) where 54 of 66 species, or 81.8%, are endemic; Rutaceae (citrus family) where 55 of 56, or 98.2%, are endemic; and Gesneriaceae, the home of gloxinia and African violet, which is represented on the islands by 55 species of Cyrtandra, all of which are unique to the islands.
The number of endemic genera contributed by common families is indicative of the level of differentiation that has occurred throughout the comparatively short evolutionary history of the Hawaiian Islands. Six genera within Asteraceae are unique to the islands: Argyroxiphium, Dubautia, Hesperomannia, Lipochaeta, Remya, and Wilkesia. Campanulaceae contribute six: Brighamia, Clermontia, Cyanea, Delissea, Rollandia, and Trematolobelia. Traditionally the Araliaceae was thought to contribute three: Munroidendron, Reynoldsia, and Tetraplasandra. Recent research (in discussion of Araliaceae) suggests that they, and several other Indo-Pacific genera, more correctly belong in the genus Polyscias. The result is a loss of three genera (two of them endemic), and the gain of the new one (Polyscias). This change will be discussed below. Lamiaceae contribute two endemic genera (Haplostachys, Stenogyne ) and a near-endemic (Phyllostegia) which has one species in Tahiti. Malvaceae (cotton family) and Urticaceae (the nettles) each contribute two, Hibiscadelphus and Kokia in the case of the former, Neraudia and Touchardia in the latter. The following families also contain endemic genera (noted in parentheses): Amaranthaceae (Nototrichium); Apocynaceae (Pteralyxia); Begoniaceae (Hillebrandia); Caryophyllaceae (Alsinodendron = Schiedea); Hydrangeaceae (Broussaisia); Loganiaceae (Labordia); Rubiaceae (Bobea); Rutaceae (Platydesma); and Violaceae (Isodendrion).
The increased scrutiny of plants by contemporary botanists using sophisticated macromolecular technology coupled with statistical analyses, has resulted in the reassessment of relationships within and among members of many groups of plants including several in the Hawaiian flora. Already mentioned above is the change in status of Munroidendron, Reynoldsia, and Tetraplasandra. Another example from within the Hawaiian flora that resulted in the loss of an endemic genus involved the genus Pelea–named after the Hawaiian Goddess of Fire. The genus was defined in the 19th century by Harvard University botanist Asa Gray to accommodate a group of Hawaiian endemic species within Rutaceae. A reappraisal of the island group (in discussion of Rutaceae) revealed that the Hawaiian species of Pelea fit more properly within the widespread (Madagascar to New Zealand and Australia) genus Melicope. The unique status of the Hawaiian species is recognized, however, by placing them together in Melicope section Pelea. We await Pele's response to this obvious slight; she is well known for her bursts of temper. All members of the widespread genus Chamaesyce are now considered to be species of Euphorbia (spurge family).
Many island endemic species are known that belong to families that would be familiar to visitors from temperate regions of the world. For example, Solanaceae, the potato family, is represented by the rare and beautiful Solanum nelsonii, largely restricted to a small area on Moloka`i. There are no endemic roses per se on the islands but the family does have endemic species of Fragaria (strawberry) and Rubus (blackberry). The islands have their own unique cotton, Gossypium tomentosum, and sandalwoods (Santalum, Santalaceae) can still be found despite serious over-harvesting of the popular wood in the early years of the 19th century. Three genera of orchids, each with its own unique species, occur on the islands; the genera are known elsewhere, however. Three endemic species of Vaccinium (blueberries) occur on the islands, one of which belongs to the group of pioneer plants that are first to colonize new lava.
Before starting our tour of the islands' plants, it is useful to take a short geographical tour to remind readers where our subjects live, and to provide a time line along which colonists arrived on the islands and the time during which the flora evolved, for much has happened in the realm of plant evolution during the five or so million years during which the current high islands have existed. The Hawaiian Archipelago is the product of a “hot spot” on the floor of the Pacific Ocean at approximately 19° N, 155°5 W (Clague and Dalrymple, 1987) that has been active for approximately 80 million years. The earliest product of the process has been identified as the Meiji Seamount, which lies in the distant northwestern Pacific at 53°12'N, 164°30'E. The main set of islands–sometimes referred to as the 'tourist' group–extends some 350 miles (565 Km) from the Big Island at its southeastern end to Kaua`i and Ni`ihau at the northwestern end. Our tour starts with the Island of Hawai`i, known familiarly as the Big Island. [I use this name to avoid confusion with Hawai`i, the state.]
The Big Island has a larger surface area than all of the other islands combined. Its two main landmarks are Mauna Kea (13,796', 4,205m) and Mauna Loa (13,679', 4,169m). The former is rated as dormant–it's likely to have at least one more episode of eruptive activity before it is considered extinct–while the latter is very much active having produced significant lava flows in recent years. The major active volcano on the island, however, is Kilauea which has been highly active during recorded history as well as in native Hawaiian memory, and is a major tourist attraction. There are two other volcanoes on the island, the northernmost, and oldest, Mt. Kohala, and Hualalei which rests on the northwestern flank of Mauna Loa. Rock from Mt. Kohala has been K-Ar dated at 0.43 million years. [K-Ar refers to potassium-argon dating .] Using the same method, Mauna Kea has been dated at 0.38 million years, while ages of both Mauna Loa and Kilauea are given as 0-0.4 million years (Carson and Clague, 1995). Before leaving the Big Island it is necessary to point out that the next island in line is alive and well. Lo`ihi lies to the southeast of the Big Island but is not expected to break the surface of the ocean for several thousand years. There is some thought that it may coalesce with the Big Island.
Next in the chain is the island of Maui, which is really two volcanic islands joined by a low isthmus. East Maui consists of the single, massive volcanic mountain that we know as Haleakalä. Haleakalä translates as House of the Sun (hale is house), although I have seen it written Heleakalä, which means Path of the Sun (hele is path). At one time in its youth, sometime between three quarters of a million and a million years ago, Haleakalä reached perhaps 16,000' (ca. 4,850m) elevation; today it reaches 10,023' (3,055 m) towering above everything else on Maui, East and West included. The West Maui mountains, whose tallest peak is Pu`u Kukui (5,788', 1,764m), have been K-Ar dated at 1.32 million years. Arising at about the same time, 1.28 million years ago, was Läna`i. Earlier still, at about 1.03 million years ago, one could have witnessed the birth of Kaho`olawe, which is more closely related to East Maui than it is to West Maui. Kaho`olawe and Läna`i lie in the rain shadows of the much larger mountains to the east and are thus much drier than the windward sides of all the other islands. This is reflected in their sparse vegetation as noted in numerous places in the text.
Two eruptions yielded the island of Moloka`i: East Moloka`i has been K-Ar dated at 1.76 million years, West Moloka`i at 1.9 million years. A third small eruption later in the island's history produced the Kalaupapa Peninsula whose fame–perhaps infamy is the better term–came about owing to the establishment of the colony for people suffering from leprosy (Hansen's disease) and the ministrations of the Belgian priest Father Damien.
During the period of Pleistocene glaciation, when sea levels were significantly lower than they are now, Maui, Moloka`i, Läna`i, and Kaho`olawe were joined as a single land mass. This mega-island, larger at its maximum emergence than the Big Island is today, is referred to as Maui Nui, literally, big Maui. The juxtaposition of these separate islands into one massive tract explains why the flora and fauna of the four are more similar to each other than they are to the other islands in the chain. The Maui Nui Botanical Garden, located in Kahului, Maui, maintains a collection of plants that reflect the evolutionary history of Maui Nui.
The next island on our tour is also the result of two eruptions. The eastern mountain range of O`ahu, the Ko`olau range, has been dated at 2.6 million years; the western range, the Wai`anae mountains, at 3.7 million. The geography is complicated somewhat by the existence of the southwestern rift zone on Moloka`i, which continues some distance forming a structure known as Penguin Bank, which Carson and Clague (1995) list in their tabulation of island volcanoes. They suggest that the high point in this area could have reached an elevation of 1,000m. Penguin Bank lies well below present sea level, but in the past may have been in physical contact with O`ahu joining the two islands.
Kaua`i, the larger of the pair of northwesterly islands, is also the oldest with an estimated K-Ar age of about five million years (values range from 4.7-5.1). Lying just to the southwest of Kaua`i is little Ni`ihau, sometimes referred to as the Forbidden Island (it is off-limits to non-Hawaiians). Rocks from Ni`ihau have been dated at 4.89 million years making it slightly younger than its larger cousin. There is evidence that Ni`ihau and Kaua`i were conjoined during Pleistocene glaciation.
Travelling northwest from Kaua`i for 130 miles (209 km) (but only with special permission), a traveler would encounter Nihoa, whose age has been determined to be approximately 7.3 million years. This remnant of a once larger island is estimated to have attained a height of 1,300 m (Carson and Clague, 1995) at its maximum. The island is home to seabirds and supports only a very limited plant community, although one of its species is the Nihoa endemic palm Pritchardia remota. The next jump on the archipelago would take our visitor about the same distance again to tiny Necker Island with an estimated age of about 11 million years. This remnant is all that is left of a volcano that at the high point in its life as reached an estimated 1,100m in height. And so it continues along the string of degraded volcanic remnants–many little more than sand islands now–passing by French Frigate Shoals, whose La Pérouse Rock has been dated at 12.8 million; Gardner Pinnacles (15.8 million); Maro Reef, awash at high tide (no age data); Laysan Island (20.7 million), whose large volcanic base–120 square miles–suggests the one-time existence of a very large volcano; Lisianski Island (23.4 million); Pearl and Hermes Reef, whose large base also suggests the existence of a large volcano; Midway Atoll, important as a stepping stone in early trans-Pacific air travel to Japan; and Kure Atoll, at 28°35'N, 178°10'W. Midway has been dated at about 28.7 million years, Kure Atoll at 29.8 million years. Although Midway and Kure are the last islands, or island remains, that still exist, the chain continues well beyond them in the form of submarine mountains (guyots) that extends far into the northwestern Pacific.
October 24, 2011: update March 24, 2013; March 21, 2016
Carson, H. and D. Clague. 1995. Geology and biogeography of the Hawaiian Islands. In W. L. Wagner and V. A. Funk (eds.) Hawaiian biogeography: Evolution on a hot spot archipelago. pp. 14-29. Smithsonian Institution Press, Washington, D.C.
Clague, D. A. and G. B. Dalryole. 1987. The Hawaiian-Emperor volcanic chain. In R. W. Decker, T.L. Writght anld P. H. Stauffer (eds.) Volcanism in Hawaii, 1-54. U. S. Geological Survey Professional Paper 1350. U. S. Government Printing Office, Washington, DC. 2 5, 13, 14.
Wagner, W. L., D. R. Herbst, and S. H. Sohmer. 1999. Manual of the Flowering Plants of Hawai`i. 2nd Ed. Vols. I and II. University of Hawai`i Press and Bishop Museum Press, Honolulu, HI.