Comments:
photo
The Vascular Plant Ecology
and Biological Observations of
Mohotani, Marquesas Islands
(0—520 m elev.)
K. R. Wood
P. O. Box 745, ‘Ele‘ele, Kaua‘i, HI 96705, kenwood@pelea.org
[region visited by the author 20—21 Jul 2004]
Mohotani. The Marquesan island of Mohotani is situated approximately 15 km to the south of Hiva Oa, 23 km to the west of Tahuata, and 48 km north-northwest of Fatu Hiva, all of which are included in the Marquesan southern group of islands. It is 12.2 km2 with a maximum length of approximately 8 km, width of 2 km, and elevation of 520 m (9°59.S/138°49.W). It is volcanic in origin and estimated to be an average age of approximately 2.15 million years old (Craig et al. 2001). Southeast of Mohotani and separated across a 200 m wide strait is Terihi, a small 150 m tall islet with steep to vertical slopes that may be the remains of what was the central vent of Mohotani’s original crater. Adamson (1936) quotes Lebronnec’s suggestion that Mohotani is a fragment of a volcano which had its center to the south and Terihi, which is composed of pitch-black rock quite different from the rocks of Mohotani, may be an infilling of the volcanic conduit.
Mohotani’s eastern side has steep precipitous slopes and cliffs culminating in elevation on the southeastern side at 520 m. These windswept eastern slopes are severely eroded and barren looking with exposed red, rocky soil and talus substrate. Only a few scattered groupings of Pisonia and Pandanus still remain as the result of the over-grazing by sheep. Mohotani’s northern end is also highly eroded and barren with few remaining shrubs, herbs and grasses. The southwestern region is forested with gentle slopes and sections of plateau leaving what appears to be around 30% of the island forested. Two springs are reported by Lebronnec, one on the western shore of Anauia and the other above that area at approximately 200 m elev. (Adamson 1936).
Formerly inhabited by a single tribe, Mohotani was evidently abandoned during the late prehistoric or early historic period (Linton 1925) and is now only visited on occasion by fisherman and hunters, in addition to wood gathers. While the author was on island, fishermen were diving with spear-guns along the northwestern coast and filled three large coolers with coastal species of fish. Hunters will come to shoot feral sheep which were introduced during the second half of the 19th century (Sachet et al. 1975), and wood carvers come to gather the prized wood of Thespesia populnea (mi`o) and Cordia subcordata (tou), both of which have grown to enormous size on Mohotani (i.e., up to 1 m in diameter). Evidence of chain saw logging along the southwestern coast was common.
Plant Communities. During the period of July 20—21, 2004 the author explored Mohotani with field associate S. Perlman. We gained access with the assistance from the Service du Developpement Rural (SDR) who dropped us off and picked us up the following day with their 8 m long twin-engine aluminum boat. The author explored the southern half of the island during those two days and only viewed the northern end from the SDR boat. In order to complete a more thorough inventory of species and communities the author will need to return to Mohotani.
A general ecological list of Mohotani’s vegetation zones are presented below and include the following plant communities. (Note: for a complete list of Mohotani’s plants see the Vascular Plant Checklist of Species)
a) Coastal Sea Cliff
b) Coastal Shrubland and Forest
c) Pisonia Forest
d) Secondary Non-Native Herb/Shrubland
Coastal Sea Cliff. The tall southern cliffs and lower coastal cliffs of southwestern Mohotani are characterized by native xerophytic plants that sparsely cover the crumbly basalt substrate including grasses and sedges such as Cyperus javanicus, Fimbristylis juncea, Leptochloa xerophila, Digitaria setigera, and Chrysopogon aciculatus; native herbs and shrubs include Waltheria indica, Boerhavia acutifolia, Boerhavia repens, Peperomia blanda var. floribunda, Portulaca lutea, Bidens cf henryi, Chamaesyce sachetiana, Colubrina asiatica, and Nicotiana fatuhivensis. In addition occasional native trees can be seen along small terraces where soil pockets can accumulate, including Cordia lutea, Maytenus crenata, Premna serratifolia, Waltheria tomentosa, Lebronnecia kokioides, and Thespesia populnea
Coastal Shrubland and Forest. Coastal shrub and forest is quite common along the southwestern slopes and just above the steeper sea cliff communities. This shrub/forest is densely inter-woven together and has a canopy of ca. 5-10 m in height. Many obvious sheep trails dissect the uniformity of this community and meander between the occasional small gulches that gently slope down from the upper eastern side of the island. The understory is severely degraded. The diversity of native plant species making up this community include Cordia lutea, Cordia subcordata, Celtis pacifica, Maytenus crenata, Premna serratifolia, Hibiscus tiliaceus, Waltheria tomentosa, Lebronnecia kokioides, Thespesia populnea, Eugenia reinwardtiana, Psydrax odorata, Sapindus saponaria, Ixora marquesensis, and Colubrina asiatica. There seems to be no consistently clear dominant combination of trees that form this community, yet there are some sections that are clearly dominated by fabulous giants of Cordia subcordata in addition to stands of Lebronnecia and Thespesia. Eugenia reinwardtiana is also distributed throughout this community with several thousand individuals, making Mohotani one of the best locations for this forest component.
The presence of Lebronnecia kokioides (Malvaceae) is significant in that it represents an endemic genus that is only known from one other location on Tahuata where just a few individuals remain. The author estimates that there are over 10,000 individuals of this species on Mohotani, with a healthy dynamic population of seedlings, juveniles, and mature individuals. Apparently the sheep seem to avoid eating this plant as it is reported to be poisonous with the chemical gossypol (Mueller-Dombois & Fosberg 1998). On the other hand, although the author observed around 70—100 mature trees of Ixora marquesensis (Rubiaceae) in a 4 sq km area, no regeneration was observed of this species with the result of clearly endangering the survival of Ixora on Mohotani. In addition, the author observed little evidence for the regeneration of Eugenia reinwardtiana (Myrtaceae).
Secondary Non-Native Herb/Shrubland. Dominating the northern half of Mohotani and scattered between the coastal shrub and forest communities on the southern end there are regions of weedy, non-native herb and shrubland. These degraded regions appear to be the result of over-grazing by sheep. Dominant herbs and shrubs in this community include Senna occidentalis, Ageratum conyzoides, Asclepias curassavica, Abutilon hirtum, Ocimum gratissimum, Cyanthillium cinereum, Chamaesyce hirta, and Passiflora foetida, in addition to the weedy tree Psidium guineense. Sedges and grasses include Sporobolus fertilis, Urochloa reptans, Rhynchelytrum repens, and Kyllinga nemoralis. The non-native fern, Pityrogramma calomelanos, is also common throughout this community.
Pisonia Forest. The central and upper regions of Mohotani are dominated by dense sections of Pisonia grandis forest. These forests reach heights of up to 20 m and can only be rivaled by some of the stands that the author observed along the northwestern coastal valleys of Ua Huka, including Ha`ahue, Vaipaia, Hanainamoa, and Ha`ahevea valleys (Wood 2004). Sections of this community are composed of pure stands of only Pisonia. Other sections of Pisonia forest include understory trees of Cordia subcordata, Celtis pacifica, Eugenia reinwardtiana, Ixora marquesensis, Sapindus saponaria, and Pandanus tectorius. Along the borders of these great Pisonia forests there occasional trees of Terminalia glabrata var. brownii and Ficus prolixa var. prolixa in addition to occasional pure stands of Pandanus tectorius.
Cultivated plantings of Aleurites moluccana, Inocarpus fagifer, Cocos nucifera, and Artocarpus altilis can be found both within and around the open borders of the Pisonia forest community.
Birds.
Acrocephalus mendanae (Sylviinae), Marquesas Reed-Warbler (komako): The reed-warbler was seen only occasionally hunting around small trees and shrubs in the lower, middle and upper forest zones.
Aerodramus ocistus (Apodidae), Marquesas Swiftlet (kopekapeka): Several kopekapeka were observed by the author hunting low over the interior canopy and around open areas of the plateau.
Anous stolidus (Sterninae), Brown noddy (no`i`o): Many brown noddy were observed in flight along coastal zones
Fregata minor (Fregatidae), Great Frigatebird (mokohe): Several dozen frigates were observed silently soaring above the steep eastern slopes. In the early morning the author was impressed with their fabulous speed and agility while diving through the air with their wings pulled back.
Gallus gallus (Phasianidae), Chicken (mo`a): Chickens were occasional throughout the southwestern forests of Mohotani.
Gygis alba (Sterninae), Common Fairy-Tern (kota`e): Fairy terns were the most abundant birds and commonly seen in the Pisonia trees. Groups of 5—7 would occasionally follow the author when he crossed open grassland in the upper zones that appear to have been overgrazed by sheep.
Lonchura castaneothorax (Estrildidae), Chestnut-Breasted Mannikin, (vini): a small brown-faced finch observed foraging in groups. Occasional throughout southwest Mohotani.
Pomarea mendozae motanensis (Monarchinae), Mohotane Monarch, (pati`oti`o): The author was continually followed by pairs and their juveniles as he explored in the upper southwestern forest zones. Thibault and Meyer (2001) estimate between 80-125 pair of the Pomarea mendozae motanensis. During this survey the author feels he covered a 4 sq kilometer area sufficiently enough to conclude that there were approximately 100 pair seen, and estimates one pair for every 40,000 sq m.
Procelsterna cerulea (Sterninae), Blue-gray Noddy: seen crossing the steep eastern slopes in the early morning.
Ptilinopus dupetithouarsii (Columbidae) White-capped Fruit-Dove (kuku): several juveniles with greenish gray caps were observed in the middle shrub and forest zone along with several white-capped adults.
Sterna fuscata (Sterninae), Sooty Tern, (keveka): seen crossing the steep eastern slopes in the early morning.
Sula leucogaster (Sulidae), Brown Booby (kena): several brown boobies were heard and observed in the Pisonia forests.
Sula sula (Sulidae), Red-Footed Booby (hauhe`e): several red-footed boobies were seen flying along the southwestern coastline.
Mollusca.
Two different genera of native tree-snails (1—3mm long) were observed upon the underside of leaves on four native tree species including Lebronnecia kokioides (Malvaceae), Eugenia reinwardtiana (Myrtaceae), Celtis pacifica (Cannabaceae), and Ixora marquesensis (Rubiaceae). The author is awaiting identification and suspect they represent previously undescribed species.
Diptera.
10 species of Inseliellum (black-flies [Diptera: Simuliidae]) have been described from the Marquesas to date (Craig 2004). This includes a single island endemic species on Mohotani which was recently collected by R. Englund in 31 Aug 2001 and published by D. Craig in 2004. The collection was made at 378 m elevation along the only section of running water that occurred while Englund was on island.
References
Adamson, A. M. 1936. Marquesan Insects: Environment, Bernice P. Bishop Museum, Bulletin 139, Honolulu, Hawaii.
Chubb, L. J. 1930. Geology of the Marquesas Islands, Bernice P. Bishop Museum, Bulletin 68, Honolulu, Hawaii.
Craig, D. A. 2004. Three new species of Inseliellum (Diptera: Simuliidae) from Polynesia, Zootasa 450: 1-18, Magnolia Press.
Craig, D. A., D. C. Currie and D. A. Joy 2001. Geographical history of the central-western Pacific black fly subgenus Inseliellum (Diptera: Simuliidae: Simulium) based on a reconstructed phylogeny of the species, hot-spot archipelagoes and hydrological considerations, Journal of Biogeography, 28, 1101-1127.
Linton, R. 1925. Archaeology of the Marquesas Islands, Bernice P. Bishop Museum, Bulletin 23, Honolulu, Hawaii.
Mueller-Dombois, D. and F. R. Fosberg. 1998. Vegetation of the Tropical Pacific Islands. Springer Press, New York.
Sachet, M. H., P. A. Shafer et J. C. Thibault 1975. Mohotani : Une Ile Protegee Aux Marquises, Bulletin ed la Societe des Etudes Oceaniennes (Polynesie Orientale) No. 193.
Thibault, J-C and J-Y Meyer 2001. Contemporary extinctions and population declines of the monarchs (Pomarea spp.) in French Polynesia, South Pacific, Oryx Vol 35 No 1.
Wood, K. R. 2004. The Vascular Plant Ecology of Ha`ahue Valley, Northwestern Ua Huka, Marquesas Islands (0-1500 ft elev.). Technical Report.
