Argyroxiphium sandwicense ssp. macrocephalum
|May 15, 1992
|Perennial with a rosette of daggerlike leaves and a slender stalk bearing pinkish flowers.
|Barren alpine scrub on volcanic slopes.
|Restricted range, predation by Argentine ants and yellow jackets.
The Haleakala silversword (Argyroxiphium sandwicense ssp. macrocephalum ) is a single-stemmed shrub of the aster family. Its rosette of narrow, sword-shaped leaves has a dense mat of silky, silvery hairs. The narrow, branched, flowering stalk is elliptic to lanceolate in outline, 9-31 in (22.9-78.7 cm) wide and 2.3-4.9 ft (0.7-1.5 m) long. This species can be branched or unbranched. The flowering heads are about 1 in (2.5 cm) in diameter and have 11-42 pink petal-like ray florets. Central disk florets, pink to wine-red at the tip and yellowish at the base, number 120-600 per head.
The monocarpic (a plant that flowers only once, at the end of its lifetime) Haleakala silversword matures from seed to its final stage in approximately 15-50 years. The plant remains a compact rosette until it sends up an erect central-flowering stalk, sets seed, and dies.
The silversword flowers from June to September, with annual numbers of flowering plants varying dramatically from year to year; 217 flowering plants were reliably recorded in 1935 and 815 were counted in 1941. Numbers recorded in recent years have ranged from zero in 1970 to 6,632 in 1991. The environmental stimulus for synchronous silversword flowering is as yet unknown. An apparent relationship of the 1991 mass flowering event to stratospheric alteration by the eruption of Mount Pinatubo in the Philippines is intriguing. Investigations are under way by R. Pharis of the University of Calgary and L. L. Loope to explore a mechanism for enhanced silversword flowering related to increased UV-B radiation due to temporary reduction of stratospheric ozone.
Flying insects—especially native bees, moths, flies, bugs, and wasps (many of which are pollinators)—are attracted in large numbers to the giant, aromatic inflorescences. It has been demonstrated that the Haleakala silversword is self-incompatible and must have insect pollinators for reproduction.
Hybrids between the Haleakala silversword and Dubautia menziesii have been observed only rarely. Primarily found within Haleakala Crater, especially on Puu o Pele and Puu o Maui cinder cones, these hybrid individuals flower for several years before dying.
This species typically grows on barren cinder cones and young 'a'a lava flows in dry alpine areas at an elevation of 7,200-9,800 ft (2,195-2,987 m). The precipitation in these areas is only 30-50 in (76.2-127 cm) annually. This plant community is subjected to frequent frosts and arid extremes that limit grasses, mosses, and alpine-adapted shrubs to bare gravel, debris, and cinders near the lower boundary of the regime. The substrate has almost no soil development and is subject to frequent formation of ice at night and extreme heating during cloudless days.
Associated native species include Agrostis sandwicensis, Deschampsta nubigena, Dubautia menziesii, Silene struthioloides, Styphelia tameiameiae, Tetramolopium humile, and Trisetum glomeratum. Alien species occupy little territory in Haleakala silversword habitat, but those that do include Hypochoeris radicata, Heterotheca grandifiora, and Rumex acetosella.
The Haleakala silversword, a classic example of a species considered rare because of its highly restricted habitat, is endemic to a 2,500-acre (6,175 hectares) area at 6,900-9,800 ft (2,103-2,987 m) elevations in the crater and outer slopes of Haleakala volcano. This very small portion of Haleakala National Park on Maui appears to be most of its historic range.
Near extinction in the 1920s due to human vandalism and browsing by goats and cattle, the Haleakala silversword has increased greatly under protection and deserves attention as one of the most dramatic conservation success stories of the Hawaiian Islands.
The first reliable information on Haleakala silversword numbers is from the summer of 1935. In that year, Ranger S. H. Lamb tallied 1,470 plants, 88 of which were flowering, on the Ka Moa o Pele cinder cone within Haleakala Crater. Since about 217 plants were flowering within the crater, a reasonable estimate of the total population at that time was approximately 4,000 individuals. Lamb talked with numerous knowledgeable individuals about the status of the plant and concluded that it was as prevalent in 1935 as it had been 30 years before. Fairly accurate silversword counts are possible because the plants occur on barren cinder; information gathered since Lamb's time illustrates the silversword trend over about 60 years of protection.
Plants have been counted by successive investigators on Ka Moa o Pele, where the largest number of plants survived in 1935. By 1979 the population on this volcanic cone had increased by a factor of about 4.4, from 1,470 to 6,528 individuals. The silversword has also increased in numbers and extent elsewhere in Haleakala Crater, with large local populations in areas where few plants survived in 1935. Counts of the entire Haleakala silversword population in 1971, 1979-1980, 1982, and 1991 revealed respective populations of 43,262, 35,000, 47,640, and 64,800.
The current population of silversword is approximately 16 times larger than the estimated population in 1935. Annual trends in 11 fixed 16.4 by 65.6-foot plots (5 by 20-meter plots) from 1982-89 suggest that substantial annual fluctuations occur in the recruitment and survival of seedlings. The other subspecies of A. sandwicense —ssp. sandwicense, endemic to Mauna Kea on the island of Hawaii—is federally listed as endangered, with only several hundred surviving individuals.
The Haleakala silversword receives more attention from visitors to Haleakala National Park than any other plant species because of its striking appearance and its limited distribution. There is ample evidence that it attracted attention from indigenous Hawaiians. Prior to 1960 (when the park was established), plants were often removed by visitors to Haleakala volcano to prove that the party had reached the summit, a practice that eventually had a serious impact on the silversword population. At one time the silverswords on Haleakala were uprooted and rolled down cinder cones for sport. Browsing by goats and cattle was also undoubtedly a factor in its decline, especially at the margins of its range. Silversword numbers were so depleted by the 1920s that the Maui Chamber of Commerce sent a petition to Washington, D. C., requesting that a serious effort be made to save the species.
The main current threats to A. sandwicense ssp. macrocephalum include loss of pollinators, native seed-eating and herbivorous insects, a limited natural range, and various human physical impacts.
The silversword has a combination of life-history traits and physical attributes that make it quite vulnerable. It is a slow-growing plant that flowers only once and dies yet is self-incompatible. As a result, it is dependent upon the availability of pollinating insects, primarily localized endemic species, for seed set.
The greatest threat to the pollinators of the silversword appears to be the Argentine ant (Iridornyrrnex hurnilis ). This introduced species occupies two disjunct areas at elevations of 6,792-9,350 ft (2,070-2,850 m), totaling about 400 acres in Haleakala National Park. Because the ant queens can't fly, the spread of the species is relatively slow. This predaceous ant harms the endemic arthropod fauna, including pollinators, that evolved in the absence of ant predation. A marked expansion in the ant's range was noted in 1993, especially at the higher elevation areas. Unless this ant species is controlled, it appears capable of spreading widely, with potentially catastrophic effects on the silversword and other endemic biota. Experimental control efforts are under way using a hydromethylnon/protein bait and techniques developed for Argentine ant control in agricultural sites in California.
Alien yellow jackets (Vespula pennsylvanica ) pose a lesser but significant threat toward elimination of silversword pollinators.
The silversword is dependent upon continuing seed production for its survival. The developing seeds are fed upon by the tephritid fly Trupanea cratericola. On average, 60% of the seeds produced by the silversword are destroyed by the small, white, grublike larvae of this fly. Developing seeds are also fed upon, sometimes extensively, by the larvae of Rhynchephestia rhabdotis, a native phycitid moth. In addition, there is an endemic cerambycid beetle, Plagithmysus terryi, which bores in roots and stems, sometimes causing silversword plants to fall over. The impacts of these insects have probably been overestimated historically. In earlier times, these insects were perceived as posing a serious threat to the Haleakala silversword, and perusal of park files reveals that application of DDT to protect the plants was contemplated by park managers as recently as the 1960s. The locally endemic insects that evolved with the silversword are currently regarded by Haleakala National Park managers as an essential part of the silversword ecosystem.
The limited natural range of this plant makes it vulnerable to extinction due to a single catastrophic event such as a natural disaster or alien plant or animal introduction.
Possible future threats include competition from the alien plants fountain grass and mullein (Verbascum thapsus ), along with the human impacts of possible overcollection and consequent site degradation. The human threats are currently controlled within the park but may become more serious as the number of visitors increases.
Conservation and Recovery
The Haleakala silversword represents one of the most dramatic conservation success stories of the Hawaiian Islands. As a result of management within Haleakala National Park, human vandalism and feral ungulate browsing—formerly the most serious threats to the Haleakala silversword—have been virtually eliminated. Almost all subpopulations of this species are within Haleakala National Park, a successful protector of the plant since the 1930s, and only a few individuals survive just outside the boundaries of the park. This species has been successfully propagated at the National Tropical Botanical Garden (NTBG).
The Haleakala silversword is a highly appealing, attention-seizing species for both the casual park visitor and the evolutionary biologist. Continued protection from hoofed mammals and human vandalism is essential, while potential threats from the Argentine ant and alien plants must be addressed. Even given its limited range and precarious life cycle, the long-term prognosis for survival of this species now appears remarkably favorable.
Without intervention, it is likely that the Argentine ant will continue to spread slowly and may eventually come to occupy much of the range of the silversword. Such an infestation is likely to deplete pollinator populations on which the silversword is highly dependent. The result of such a reduction in pollinators is a reduced silversword reproductive capacity and lessened chances for long-term survival. Chemical control using a bait-toxicant appears to be the best chance to restrict or eliminate high-elevation populations of the Argentine ant on Haleakala. Research is ongoing in cooperation with scientists of the Clorox technical center.
Another important consideration in control of the Argentine ant is the prevention of further spread. Queens of the Argentine ant often forage with workers and are quick to establish small satellite nests. Such behavior facilitates potential transfer of queens in trash, roadfill, potted plants, and firewood.
One of the chief impacts of the long-term degradation of high-elevation habitat of silversword on Haleakala volcano is the elimination of silversword populations in areas on the periphery of Haleakala Crater. As a result of fencing the boundary of Haleakala National Park in the mid-1980s, these areas are now protected from feral goats, which had extirpated the silversword from certain peripheral areas. Now that the habitat of these sites is protected, they are prime candidates for reintroduction. The best documented examples of appropriate sites are upper-central Kaupo Gap, Kalapawili grasslands, and Puu Nianiau. Other areas such as the outer leeward slopes and southwest rift of Haleakala should be considered if protection from feral goats can be achieved there. Extreme caution should be taken not to introduce the Argentine ant with planted materials, as the species frequently nests in potted plants grown in the headquarters area of Haleakala National Park
U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
Eastside Federal Complex
911 N.E. 11th Ave.
Portland, Oregon 97232-4181
Pacific Remote Islands Ecological Services Field Office
300 Ala Moana Blvd., Room 3-122
P. O. Box 50088
Honolulu, Hawaii 96850
Telephone: (808) 541-1201
Fax: (808) 541-1216
Carr, G. D. 1982. "Status Report on Argyroxiphium sandwicense var. sandwicense. " Research Corporation of the University of Hawaii for U.S. Fish and Wildlife Service, Honolulu.
Kobayashi, H. K. 1974. "Preliminary Investigations on Insects Affecting the Reproductive State of the Silversword (Argyroxiphium sandwicense D.C.) Compositae, Haleakala Crater, Maui, Hawaii." Proceedings of the Hawaiian Entomological Society 21: 397-402.
Meyrat, A., G. D. Carr, and C. W. Smith. 1983. "A Morphometric Analysis and Taxonomic Appraisal of the Hawaiian Silversword Argyroxiphium sandwicense D. C. (Asteraceae)" Pacific Science 37 (3): 211-225.
Siegel, S. M., P. Carroll, C. Cron, and T. Speitel. 1970."Experimental Studies on the Hawaiian Silverswords (Argyroxiphium spp.): Some Preliminary Notes on Germination." Botanical Gazette 131 (4): 277-280.