L earning Objective 22

See video tape, Native Hawaiian Plants, What are They? on campus library reserve or local branch library.

For more detail, read in Hawaii: A Natural History by Carlquist, Chapter 3.

I. MODES OF LONG-DISTANCE DISPERSAL of flowering plants to Hawaii:

A. Categories of Dispersal of Original Colonist Plants.

Carlquist hypothesized that the means and percentages by which colonist plants were dispersed to Hawaiian islands was 1.4% by wind, 22.8% by water and 74.8% by wings, or bird transfer. (See Figure 22-A)

 

B. Distribution is by plant propagules.

A plant propagule is any part of the plant which can give rise to a whole new plant: seeds, spores, stems, leaves, roots.

C. How did Carlquist make such a determination?

1. He is an plant anatomist, a person who studies the cellular and finer details of a plant's structure. Also because he is a field botanist as well, (one who studies plants in their natural habitat, the places they grow) he was able to link these small features to likely interactions with the environment.

2. For example, the small barbs on the Bidens fruit provide a way for the small fruit to "hitch a ride" on the outer surface of an animal - its fur or wings. In another example, when he studied the fruit of a coastal plant, Naupaka, he could link the spongy tissue of the fruit to its ability to float and be dispersed by water.

Bidens

Naupaka

3. Thus, he was able to examine the present-day native Hawaiian plants, which are descendants of the first colonists and see if they still had distinct anatomical features which could explain their original dispersal.

II. ANALYSIS OF THE DIFFERENT DISPERSAL MODES.

A. Distribution by Wind: 1.4%.

1. Wind distribution is by air flotation. Propagules must be very small and light in order to stay suspended in the air for thousands of miles.

2. An example is 'Ohia lehua (Metrosideros polymorpha) which has very small seeds, which are easily dispersed, even today, to new lava flows. It has reached almost all the high islands of the Pacific. Ferns are a group of non-flowering plants whose tiny spores (one-celled) are airborne and have been widely dispersed worldwide including to Hawaii.

O'hia

Fern spores

 

3. A likely wind pattern responsible is the JET STREAM of the northern Hemisphere. The jet stream is an ultra-high speed continuous band of wind found at the 30,000 to 40,000 foot elevation. (See Figure 22- B)

   

4. The jet stream accelerates over south-east Asia and can decelerate (slow down) over the Hawaiian Islands. When it is over Asia, strong up-drafts from tropical storms could shoot air currents up into it, possibly carrying very small seeds of flowering plants and also spores of ferns. At top speeds of 122 mph, it can be over the Hawaiian Islands in two days.

5. At high speeds, the wind can carry larger particles than at lower speeds. These plant propagules could be carried at high speeds, and, when the wind velocity slowed, they would drop out of the jet stream. It is much like a turbulent, fast-moving stream, carrying large boulders downstream and dropping them as it becomes slower and weaker.

6. The jet stream goes over some of the Indo-Pacific region. This region is also the area of the largest source of colonist plants for Hawai'i. It provides primarily tropical plants which are more adapted to Hawai'i than those of the drier and colder American continent, a closer region.

7. Another source of plant dispersal by wind is the Kona storms. They come from the south in winter and could bring plant materials from the American continent and other Pacific Islands.

8. Testing of the Possibility of Long-Distance Dispersal by Wind.

a. This theory, when first proposed, had little supporting data. It seemed rather unlikely. However, an entomologist (a person who studies insects) was also trying to address this problem of the dispersal of small insects to the Islands.

b. To test the wind dispersal by jet stream thoery, J. Linsey Gressitt, an entomologist, took an airplane up, flew in the jet stream and collected a number of small insects in a fine meshed collecting net which was being towed behind the plane.

Clearly, fern spores and small seeds could also travel by this means. His research indicates that dispersal by jet stream is certainly possible.

c. A type of plant which has the tiniest seeds among the flowering plants is the orchid. Also, they are found in great numbers in the Indo-Pacific region. however there are only three native orchids species found in Hawai'i, and it is puzzling why there are not more native species, since they would be prime candidates for wind dispersal.

Anoectochilus sandvicensis

Liparis hawaiensis 'Awapuhiakanaloa

Photos of these endemic orchids by Gordon Daida are taken from Dr. Gerald Carr's Web site.

d. One possible explanation is that the tiny seeds are not resistant to the cold, dry air found in the jet stream. An even greater factor could be the lack of pollinators.

Most orchids are only pollinated by one specific insect pollinator. Because the flower and the pollen cluster (called the pollinium) is uniquely-shaped, only one insect can successfully pollinate the orchid flower. In order for the orchid to successfully establish itself on the island, its specific pollinator must also arrive at the same time. That is highly unlikely!

B. Distribution by Water: 22.8%.

1. Two ways by which plant propagules could be dispersed by water are oceanic drift (14.3% ) and by a rare rafting event ( 8.5% .)

2. By oceanic drift:

a. Seeds and fruits which disperse this way must have two characteristics: adaptation for flotation and resistance to lengthy salt water immersion.

b. Examples of native coastal plants with these characteristics are those listed here: the Beach Morning Glory, or Pohuehue, 'Akulikuli, Wiliwili and Naupaka. Stems may break off the 'Akulikuli plant ( Sesuvium) and these float readily because they are fleshy. The native dry lands tree, Wiliwili (Erythrina), comes from a genus of whose seeds float, and the beach shrub, Naupaka ( Scaevola), has white, spongy floatable fruits.

Pohuehue

Akulikuli/TD>

Wiliwili

Naupaka

 

3. Rafting: a rare event which has been reported by sailors.

a. It involves a large piece of river bank or soil held together by roots of plants which actually form a floating platform. They can be formed after heavy rainfall and flooding near coasts. Such rafts have been reported off the coast of South America several 100 miles from land!

b. Candidates for this type of dispersal are Koa ( Acacia koa), with its closest relatives in Australia and islands off the coast of Africa. Ma'o, Hawaiian cotton, and the native hibiscus species, Koki'o, are others that may have rafted to the Hawaiian islands.

Koa

Ma'o

Photos by Gerry Carr, Univeristy of Hawaii, Department of Botany

4. The percentage of plants arriving to Hawaii by water is low considering that many beach plants readily disperse by water.

a. One possible explanation for this could be the pattern of oceanic currents around Hawaii. (See Figure 22-C)

   

Any plants floating up from the Southern hemisphere would have to cross three major currents that would take them either east or westwards, and not north to the Hawaiian Islands. It is unlikely that many water-dispersed plants could cross all three currents and still head north!

b. Many plants common to the South Pacific are not native to Hawai'i such as Coconut (Niu) , Mangroves, the Tropical Almond (Terminalia) and Kamani (Calophyllum inophyllum). Their absence in the Hawaiian flora, but widespread presence in other Pacific Islands, may support this observation of water dispersal difficulty.

C. Dispersal By Wing (Birds): 74.8%.

1. Birds are clearly the most successful mode of dispersal of flowering plants to Hawaii, accounting for three-quarters of colonists.

2. Plant propagules could be carried in several ways:

Externally: 13%. Embedded in mud on feet or other parts.

10.3%. Attached by viscid or sticky substances.

12.8%. Attached by mechanical devices.

Internally: 39%. Carried in digestive tract of birds.

3. Birds that could be involved in dispersal would have to be migratory and eat fruits and seeds from plants:

a. Waterfowl like teals, ducks and geese occasionally include Hawaiian Islands in their migratory routes. They eat marsh plants as well as other plant materials.

b. Shore birds:

One shore bird which regularly visit Hawaiian islands is the Pacific Golden Plover (Kolea). It leaves its arctic nesting grounds in August, returning in May. This tiny bird has been documented to make 3,000+ mile trip from Alaska in 36 hours!

Also coming to Hawai'i are the following birds: the bristle-thighed curlew (Kioea: which breeds in Alaska and Siberia), the ruddy turnstone ('Akekeke: arctic breeding), the sanderling (Hunakai [which means sea foam]: arctic breeding) and the wandering tattler ('Ulili from Alaska).

4. Birds may travel from island to island which can further the dispersal of plants. Also their ranges are far-reaching: to other Pacific Island chains, Indo-Malaysia, most of Polynesia, New Zealand and Tasmania.

5. Though these birds primarily eat mollusks and other small animals of the shoreline, they will eat certain amounts of fruits and seeds.

6. Questions about retention of seeds and fruits in digestive systems of birds have to be addressed if bird dispersal is a viable mode of plant dispersal. Biologists (like Vernon Proctor) documented that shore birds and waterfowl can retain some seeds for hundreds of hours during long migratory flights. The larger and harder the seeds, the longer the retainment. Seeds are not often damaged in bird's stomachs and often have enhanced germination, as a result. This clearly indicates that bird dispersal can be effective.

7. External ways plant propagules traveled on birds.

a. embedded in mud.

This can happen if seeds are small and grow in wet, marshy places. Possible examples are Drosera (Sundew) and lobelias.

(Lobelias are small low-growing plants in the rest of the world which are represented in Hawai'i by many endemic genera. We generally refer to all the descendent genera from Lobelia as the Hawaiian lobelioids.)

Drosera is a small insectivorous plants which is found in bogs worldwide and is associated with muddy environments.

b. attached by viscid or sticky substance to bird body: 10.3%

Examples are Plantago, which has seeds that develop slime coating when soaked in water, then dry like glue and Chamaesyce ('Akoko) with a natural sticky coating.

Boerhavia (Alena), a common beach plant of Pacific, has sticky fruits held a few inches above ground which could easily come in contact with a walking bird.

Pisonia (Papala) has such a sticky substance on fruits that is was used like super glue by Hawaiians to trap birds for their feathers to make the famous Hawaiian cloaks.

Other plants have juices or pulp of the fruit which can dry and glue small seeds to feathers as birds feed on them. One examples is Clermontia (Lobellioid), in which a white latex oozes from split fruit, and 'Ohelo (Vaccinium reticulatum) which has a juicy red fruit, a relative of the cranberry.

c. Mechanical means are effective ways to disperse plants: 12.8%

Especially noteworthy is the genus Bidens (ko'oko'olau)which is called "Spanish needle", "dagger" or "beggar's tick" because its sharp-pointed fruit digs into clothing or fur or feathers.

 

Another is Dubautia scabra with bristles at tip of fruits.
 

An indigenous sedge, Uncinia, has a recurved barb at end of fruit, like fishhook.

   

8. Internal Ways Plants are carried are by bird's digestive tract: 39% of total dispersal.

a. This explains dispersal of many plants which are not easily dispersed by wind or water or have no obvious device to attach to bird.

b. If a fruit or seed has bright color or characteristics that appeal to birds, it is a likely sign that they are eaten by birds and dispersed.

c. Examples of colored fruit or seeds of native plants which are likely candidates for bird dispersal.

Blue: Dianella sandwicensis ('Uki'uki) Photo by M. Bauer

Red: Vaccinium reticulatum ('Ohelo or 'Ohelo 'ai) Photo by Dave Boynton

Black: Coprosma ernodeoides (Kukaenene which means goose droppings) Photo by Gerald Carr

Orange: Astelia and Coprosma species. (Coprosma rhynchocarpa is shown here.) Photo by Charles Lamoureaux

Purple : Sandalwood (Santalum) (`Iliahi) Photo by Gerald Carr

Black shiny seeds: Pittosporum (Ho'awa) Photo by Priscilla Millen

Green : Cassytha filiformis Indigenous, parasitic (Kauna'oa) Photo by Lani Stemmerman

White: Cyrtandra a relative to African violet. (Cyrtandra latifolium sp. Gesneriaceae is shown here) Photo by George Linney

Yellow: Nothocestrum spp. ('Aiea)

9. Other features of native plant flora are related to bird dispersal.

a. Families with mostly dry fruits often have primarily fleshy-fruited members in Hawaiian flora.

b. For example, the normally dry-fruited mint has native species with a fleshy fruit with the following modification: a succulent outer layer forms over the dry fruit structure which is at first green, then purple at maturity. An amazing example of a dry fruit turning into a fleshy one!

c. Exocarpus (Heau) is a broom-like shrub or small tree which has the fruit embedded in a bright, swollen red or yellow receptacle (end of stem on which flower sits) which makes it very bird-attractive.

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