The Hawaiian Island Environment

Item Type text; Article

Authors Shlisky, Ayn

Citation Shlisky, A. (2000). The Hawaiian Island environment. Rangelands, 22(5), 17-20.

DOI 10.2458/azu_rangelands_v22i5_shlisky

Publisher Society for Range Management

Journal Rangelands

Rights Copyright © Society for Range Management.

Download date 30/09/2021 01:33:31

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Link to Item http://hdl.handle.net/10150/639245 October 2000 17

The Hawaiian Island Environment

Ayn Shlisky

aradise: the universal vision we Each island is the result of accumula- most of their moisture. The driest areas have of Hawai‘i. Hawai‘i’s habi- tions of successive volcanic eruptions at are the upper slopes of high mountains, Ptats are diverse, unique, and love- the Hawaiian hot spot. The older volca- where a trade wind inversion tends to ly—a land of flowing red-hot lava, and noes have been transported from the suppress vertical lifting of air, or in lee- at the same time, delicate pastel orchids. Hawaiian hot spot to the northwest by ward positions at the coast or inland. Yet the Hawai‘i of today is much plate movement. Through time, they Winter cold fronts moving in from the changed from that discovered by the erode and subside to become a mere northwest may infrequently travel far Polynesians, or more than 1,000 years pinnacle of rock, then an atoll of accu- enough south to drop snow on the upper later, by Captain Cook. Over time, mulated coral, and finally a submerged slopes of Haleakala (Maui), Mauna Loa Hawai‘i has been discovered and re-dis- guyot (flat, reef-capped volcano) and Mauna Kea (Hawai‘i). covered by different groups of people of (Normark et al. 1982). Only volcanoes Geographic isolation is another impor- widely varied goals and aspirations. of Hawai‘i and Loihi are still active. tant factor influencing Hawaiian plant Partly because of this, the introduction Nowhere else in the United States are and ecosystem diversity. The nearest of exotic species, an important concern rainfall gradients so steep. Annual rain- continent, North America, is 2,300 miles to range managers worldwide, is one fall averages 70 inches, but ranges from away, while Japan, Australia and the major challenge to rangeland and 5 to 470 (Blumenstock and Price 1972, Philippines are 2,500, 4,000 and 5,300 ecosystem management and conserva- McNab and Avers 1994). The highest miles away, respectively. Never linked tion in Hawai‘i. annual rainfall is on the eastern, wind- geographically with continental land ward sides or crests of mountains, where (Carlquist 1982), the isolation of The Hawaiian Ecosystem almost year-round trade winds shed Hawai‘i dictates that, before human ar- Geologic evolution, tropical climate, and island isolation underlie the diversi- ty and distribution of island vegetation (Hubbell 1968, Sohmer and Gustafson 1987). The eight islands considered within the State of Hawai‘i are those be- tween 19° N to 22° N latitude, although the entire Hawaiian Island chain follows a 1,600 mile arc from 9° N to 28° N lati- tude. Some islands rise to more than 32,000 ft, but the oldest island of the chain is low-lying Kure Atoll to the northwest, and the youngest, Loihi, is Fig. 1. Climatic vegetation still below sea level, to the southeast. zones of the eight main Island ages range from over 75 mil- Hawaiian Islands (from lion years to less than 500,000 years Mueller Dombois and (Clague and Dalrymple 1989, Walker Fosbert 1998 with permis - 1990). Erosion increases from east to sion from Springer-Verlag). west with island age, as evidenced by the contrast between deeply incised Waimea Canyon on Kaua‘i, and the slightly eroded lava beds of Mauna Loa and Mauna Kea on Hawai‘i. This pro- gressive aging has been largely attrib- uted to the hypothesis, first proposed by Wilson (1963), that the Hawaiian Islands are formed by the movement of the sea floor (Pacific plate) over sources of lava called "the Hawaiian hot spot". RANGELANDS 22(5) 18 rival, only species capable of long-dis- tance dispersal, and pre-adapted to tropi- cal climates, could become established. Major agents of dispersal for Hawaiian plant species are migratory birds (Carlquist 1982), led to the islands by chance or storm movements. Air and water currents also disperse plants, but are less important due to distance and the amount of non-coastal habitat in Hawai‘i. The largest proportion of na- tive plants are related to plants in tropi- cal Asia and Indonesia, and more than 20 percent to plants of North and South America (Carlquist 1982). Once established, isolation and high habitat diversity favors speciation, as evidenced by the high degree of en- demism in the Hawaiian Islands. The Islands contain 960 flowering plants and 168 ferns and fern allies (Wagner et al. 1990). Endemism within the Islands is estimated to be between 86 and 96 per- cent (Mueller-Dombois 1975, Mueller- Dombois and Fosberg 1998). Conversely, the small size of island land masses supports small populations, in- creasing extinction rates (MacArthur and Wilson 1967), and reducing re- silience to disturbance (Loope and Mueller-Dombois 1989).

Plant Communities Humans have altered the Hawaiian landscape. Nonetheless, island habitats remain highly diverse, and so are the plant communities that occupy them (Figure 1). The Hawai‘i Natural Heritage Program recognizes 150 distinct natural co m m u nity types, including aquatic and subterranean types like caves. Most communities can be classified into nine broad types: tropical coastal vegetation, Fig. 2. Waipo Valley, Hawai’i, on the northeastern coast. Courtesy of Paul F. Starrs. lowland grasslands and savanna, mon- tane moist forests, lowland rain forest, lehua) or Diospyros sa n d w i c e n s i s Native lowland rain forests occupy montane wet forests and bogs, subalpine (Figure 1, zones 1 and 2). Hawai‘i’s windward low elevations, and are domi- vegetation, alpine vegetation, and mon- northwestern side at Pu’u Wa’a Wa’a nated by open forests of lehua (Figure 1, tane dry forests (McNab and Avers contains a notable remnant of this type zone 4). Much has also been converted 1994). (Mueller-Dombois and Fosberg 1998). to other uses (e.g., development, agri- Coasts are rimmed by coral sand Between the dry leeward and wet wind- culture), and like other vegetation types beaches and associated strand vegetation ward climates, montane moist forests in Hawai‘i, has been invaded by intro- (Figure 2). Leeward slopes, mostly once dominated, but today most of this duced species, including Casuarina eq - below 1500 m, are dry, and were for- type has been converted to other uses u i s e t i f o l i a and P a r a s e r i a n t h e s merly covered by dry grassland, or open (Figure 1, zone 3). This type is either falcataria. These lowland rainforests in- forest/woodland of predominantly broad dominated by l e h u a, the Polynesian-in- clude the conspicuous stoloniferous fern schlerophyll trees, such as Metrosideros troduced kukui nut tree (Aleurites moluc - mats of the native false staghorn fern p o l y m o r p h a (locally known as ‘ohi’a ca n a ), or a mix of le h u a and the non-na- (Dicranopteris linearis), which usually tive Myrica faya, or Acacia koa (k o a ) . indicate either advanced primary succes- October 2000 19 sion, or a breakdown stage of lehua for- duction of large grazing and browsing other non-native species (Vitousek est (Mueller-Dombois and Fosberg mammals (some of which have become 1986, Vitousek and Walker 1989). 1998). Above these lowland rainforests destructive feral populations), the ex- In dry and mesic vegetation, alien lie tropical montane cloud forests, which ploitation of lowland forests for sandal- grass establishment has increased fire as their name suggests, are enveloped wood (Santalum spp.), land clearing for frequency to a degree deleterious to na- with clouds at least part of the day. agriculture (especially sugarcane) and tive plants (Cuddihy and Stone 1994). Similar to lowland rainforests, they are development, koa logging, and the con- The eastern North American grass dominated by l e h u a in addition to version of upland forests into cattle Andropogon virginicus has spread Cheirodendron trigynum, although they ranches (Cuddihy and Stone 1994, across denuded lowland rain forest habi- differ in that they receive a large amount Mueller-Dombois and Fosberg 1998) tats on windward Oahu, now forming of additional precipitation as fog drip, have left few island ecosystems in their the dominant herbaceous cover. In addi- contributing to their mossy character. native state. The introduction of non-na- tion to displacement of native species, Montane bogs mostly occur as rain for- tive plant species and the extinction of this fire-adapted bunchgrass accumu- est openings, and are dominated by a many native and endemic species have lates dead standing foliage, within a few mixture of sedges and grasses. Above been two of the most significant results years forming a straw-like mulch that 5,900 ft in the high elevations of of these activities. prevents evapotranspiration from the Hawai‘i and Maui, where conditions are Of course a detailed account of all of soil surface during winter months when relatively dry, and snow sometimes falls the ecological issues facing Hawai‘i excess water is a problem (Mueller- (Figure 1, zones 6 and 7), are found sub- today cannot be covered here, but intro- Dombois 1975). Consequently, this alpine and alpine vegetation; either duced plant species and feral pigs are habitat now shows more erosion and mountain parkland, schlerophyll scrub, two of particular concern to range man- runoff than forested sites where evapo- C h e n o p o d i u m scrub, or tussock grass- agers and scientists. transpiration is considerably greater. In land. Despite elevation and ruggedness, dry forest on the leeward slopes of historic overgrazing by feral and domes- Maui, African kikuyu grass (Pennisetum tic sheep and goats has contributed to . . .o v e rgrazing by larg e clandestinum) was introduced by ranch- exotic plant invasions and drastic com- ers for its forage value. The thick mat- position changes (Mueller-Dombois and feral populations of grazing ani- t h forming habit of this grass inhibits na- Fosberg 1998). mals in the late 18 c e n t u r y tive tree regeneration. On Oahu, over- Island age, because it is linked to soil caused invasion by non-native grazing by large feral populations of properties and topographical relief, in shrub species... grazing animals in the late 18t h c e n t u r y part determines the suite of species es- caused invasion by non-native shrub tablished on any particular island. There species, such as Prosopsis pallida, is a correlation between substrate age Acacia farnesiana, Lantana camara, and soil moisture regime in mid-eleva- Exotic Species Many ecological relationships on is- and Opuntia ficus-indica, which today tion rain forests, where soil water make up their own community types regimes tend to change from xeric to hy- lands involve the interaction between native and exotic species. Greater than within the dry lowlands (Mueller- dromorphic through time (Mueller- Dombois and Fosberg 1998). Dombois 1975). Tied to generally in- 8,000 plant species have been intro- duced (Yee and Gagne 1992), mostly During the 1960’s through 1980s, the creasing soil moisture regimes from the montane moist forest once dominated by younger island of Hawai‘i toward the within the last 200 years (Mueller- Dombois 1975). Of these, about 11 per- open l e h u a was invaded by Myrica faya older island of Oahu is a decrease in from the Macronesian Islands. M y r i c a range of the common forest tree l e h u a. cent now have reproducing populations (Wagner et al. 1990), and at least 86 of fa y a is a nitrogen-fixing tree, and its inva- Dieback of lehua on wet sites on the is- sion not only increases forest density, but lands of Maui and Hawai‘i have been at- these species pose threats to native ecosystems (Smith 1985). Smith (1985) also causes nitrogen accumulation. The tributed in part to this species’ inability nitrogen is used by the Myrica trees them- to adapt to aging soils (Mueller- lists 13 species, five of which are grass- es, as some of the worst weeds, includ- selves, allowing them to displace at least Dombois and Fosberg 1998). Island age partially, l e h u a (Walker and Vitousek also determines its relative degree of ing Myrica faya, Psidium cattleianum, Andropogon virginicus, S c h i z a c h y r i u m 1991). Effects of exotic species, in addi- erosion and its unique topography. For tion to community type and land use example, species such as C r y p t o c a r y a c o n d e n s a t u m, Melinis minutiflora, Pennisetum clandestinum, and P. se - changes, contribute to ongoing species m a n n i i and Elaeocarpus bifidus o c c u r extinctions, with nearly a quarter of native only in the mixed mesophytic forests on t a c e u m. Some are so competitive that they can convert native forest to single- plants proposed or listed as threatened or Kaua‘i and Oahu (Sohmer and endangered (McNab and Avers 1994). Gustafson 1987). species stands, particularly after distur- The past 200 years have witnessed bances such as a hurricane or ‘ohi’a drastic changes to native Hawaiian l e h u a dieback. Others can alter natural Feral Pigs ecosystems. On the heels of European processes such as nutrient cycling, European feral pigs have had substan- contact in the late 18th century, the intro- sometimes facilitating the invasion of tial effects on Hawaiian rain forests and RANGELANDS 22(5) 20 other ecosystems. In addition to eating Clague, D. A., and Dalrymple, G. B. 1989. Vitousek, P. M., and Walker, L. R. 1989. common foods such as tree-fern trunks, Techtonics, geochronology, and origin of Biological invasion by Myrica faya in strawberry guava fruits (Psidium cat - the Hawaiian-Emporer Volcanic chain. In: Hawai‘i: plant demography, nitrogen fixa- E. L. Winterer, D. M. Hussong and R. W. t l e i a n u m), and earthworms, pigs seek tion, ecoosystem effects. Ecol. Mono. Decker (eds.) The Eastern Pacific Ocean 59:247–265. out certain now rare plant species for and Hawai‘i., The Geological Society of Wagner, W. L., Herbst, D. R., and food (Loope 1998). Plants with fragile America, Boulder, Colo., pp. 188–217. Sohmer, S. H. 1990. Manual of the flow- stems and leaves have drastically de- Cuddihy, L. W., and Stone, C. P. 1994. ering plants of Hawai‘i. Bishop Museum clined. Pigs also are dispersal agents for Summary of vegetation aleration in the and University of Hawai‘i, Honolulu. non-native plant species. They carry Hawaiian Islands. I n : E. A. Kay (ed.) A Walker, G. P. L. 1990. Geology and vol- seed on their coats or in their digestive Natural History of the Hawaiian Islands: canology of the Hawaiian Islands. Pacific Selected Readings II. University of Science 44:315–347. tracts, depositing it on the exposed min- Hawai‘i, Honolulu, pp. 467–472. Walker, L. R., and Vitousek, P. M. 1991. eral soil of the forest floor where germi- Hubbell, T. H. 1968. The biology of islands. Interactions of an alien and a native tree nation is favored, often resulting in un- Proceedings of the National Academy of during primary succession in H a w a i ‘ i derstory thickets of strawberry guava Sciences 60:22–32. Volcanoes National Park. Ecology (Loope 1998, Mueller-Dombois and Loope, L. L. 1998. Hawai‘i and the Pacific 72:1449–1455. Fosberg 1998). Islands Status and Trends of the Nation's Wilson, J. T. 1963. A possible origin of the Biological Resources, United States The isolation, tropical location, and Hawaiian Islands. Can. J. Physics Geological Survey, Washington, D. C. 41:863–870. beauty of the Hawaiian Islands have re- Loope, L. L., and Mueller-Dombois, D. Yee, R. S. N., and Gagne, W. C. 1992. sulted in their uniqueness, but also con- 1989. Characteristics of invaded islands, Activities and needs of the horticulture in- tributed to their degradation. Few rem- with special reference to Hawai‘i. I n : J . dustry in relation to alien plant problems nants of natural vegetation are left in the Drake, F. di Castri, R. Groves, F. Kruger, in Hawai‘i. I n : C. P. Stone, C. W. Smith coastal and lowland areas, where resorts H. A. Mooney, R. Rejmanek and M. and J. T. Tunison (eds.) 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