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Flowers, Pollination & Ocean Dispersal In Coral Trees (Erythrina)

Depending on the botanical authority, there are 112 to possibly 116 different species of coral trees distributed throughout the world. Erythrina flowers are commonly pollinated by perching birds (passerines) and hummingbirds. Species that are pollinated by perching birds typically have gaping corollas in which the upper banner or standard is ovate or obovate and widely separated from the wing and keel petals which enclosed the stamens. The relatively short bills of these birds can easily reach the liquid nectar at the base of the banner. During feeding, pollen from the exerted stamens is transferred to the bird's head and breast. Compared with hummingbird flowers, the nectar in passerine flowers has a lower sugar content and higher amino acid concentration. Passerine pollination is commonly found in Old World species of Erythrina in Africa and in about a dozen New World species. Hummingbird flowers are typically elongate and tubular, with the standard or banner elongated and the keel and wing petals reduced. The long, probing bill of the hummingbird is well-adapted for reaching the liquid nectar reward at the base of the petals deep inside the tubular corolla. These flowers have relatively low concentrations of amino acids and a high concentration of sucrose-rich nectar to satisfy the energy requirements and high metabolic rates of hummingbirds. They are typically found in New World species of Erythrina in the Americas.

Erythrina is such an extraordinary genus of flowering plants that four international botanical symposia were devoted to it during the 1970s and 1980s. Some of the world's leading botanists discussed a wide range of topics, from the taxonomy and pollination of these trees, to their remarkable geography and dispersal. A number of articles from the symposia were published in several scholarly journals, including Lloydia Volumes 37 & 40 (1977), Annals of the Missouri Botanical Garden Vol. 66 (1979) and Allertonia Vol. 3 (1982). Erythrina flowers are pollinated by perching birds (passerines) or hummingbirds, depending on the species. Flowers pollinated by perching birds typically have gaping corollas in which the upper banner is widely separated from the wing and keel petals, while hummingbird flowers typically have slender, tubular corollas.

The exact mechanism for dispersal of Erythrina species throughout all the major continents of the world is an enigma. Over 100 species are distributed in the following regions: Mexico (27), Central America (25), South America (21), West Indies (9), Tropical Africa (26), South Africa (5), Continental Asia (6), Melanesia and South Pacific (6), and Australia (2). One additional species (E. fusca) occurs on all the continents except Africa, and three additional species are native to the continental United States and the Hawaiian Islands. Since a few species occur in more than one major continental group, the grand total is 116.

But why are populations of coral trees so widespread throughout the world? Although the distribution of some ancient plant groups, such as the cycads, correlates rather nicely with continental drift, the time frame when all the continents were joined together is much too early for Erythrina. More plausible theories for the intercontinental dispersal of coral trees include seed dispersal by ocean currents, rafting, and perhaps migratory birds. Although most species have seeds that sink in water, there are some with buoyant seeds that may have drifted or rafted to some of the Polynesian Islands. According to Bob Gun and John Dennis (World Guide to Tropical Drift Seeds and Fruits, 1976), the buoyancy of Erythrina seeds is due to lightweight cotyledonary tissue. E. Charles Nelson (Sea Beans and Nickar Nuts, 2000) states that coral beans are true long-distance drift seeds which can remain buoyant in salt water for at least one year. He documents several coral bean collections from beaches of the British Isles. It is not surprising that the most widely distributed of all coral trees (E. fusca) has buoyant seeds. Another species with buoyant seeds (E. variegata) occurs in Asia, Polynesia and Australia. One remarkable observation by the naturalist H.B. Guppy (Plant Dispersal, 1906) describes abundant seeds of an unknown Erythrina floating off the coast of Ecuador. Because of their small size they must have been a colorful, red-seeded species present in large numbers. It is interesting to note that E. velutina occurs on the Galapagos Islands 600 miles off the coast of Ecuador, where it is pollinated by finches. More than half of the known species have bright red seeds that may have also been dispersed by birds or mammals. In fact, the shiny red seeds are collected by people and used for necklaces, earrings, good-luck charms, and rosaries in Africa and Caribbean Islands. With thick, impervious seed coats, Erythrina seeds could pass through the digestive tract of large mammals and birds. The fact that wild populations of coral trees are represented on practically all major continents and many islands is truly remarkable.

An interesting coral tree endemic to dry, leeward slopes of all the main Hawaiian islands is called wiliwili (E. sandwicensis). Although the flowers are usually red, some of the populations have color variations of red to orange and white. This Hawaiian endemic belongs to a group of closely related passerine species in the section Erythraster, including E. tahitensis, endemic to Tahiti; E. variegata, a widespread species in tropical Asia and the South Pacific; and E. velutina, another widespread species in northern South America, the West Indies and the Galapagos Islands. Although the seeds of wiliwili sink in water, it may have originated from an ancestral species with buoyant seeds that reached these islands many thousands of years ago. In fact, a related tropical Asian species (E. variegata), which is commonly planted as a street tree in Hawaii, has seeds that float in water. The seeds of this species sometimes show up along Hawaiian beaches and tidal waters; however, they are usually of local origin. Trees of E. variegata are fairly easy to identify because the pods are longer than most cultivated coral trees, often up to 12 inches or more in length.

According to Sherwin Carlquist (Hawaii: A Natural History, 1980), the seeds of wiliwili (Erythrina sandwicensis) have gradually lost their ability to float in seawater. Although the reasons are not identical, Carlquist compares this with the loss of flight in certain insects and birds. "If a plant shifts its ecological preference, it will tend to lose contact with the agent responsible for its dispersal." Carlquist's hypothesis does not apply to the three species of sea beans on the Hawaiian Islands, including Mucuna gigantea, M. sloanei and Dioclea wilsonii (D. violacea), which appear to have arrived on these islands naturally and have retained their buoyancy and ecological preference. According to the second edition of Manual of the Flowering Plants of Hawaii by W. L. Wagner, D.R. Herbst and S. H. Sohmer (1999), all three lianas appear to be indigenous, and one variety of M. sloanei (var. persericea) is endemic to Maui. Unlike populations of Erythrina on these islands, the sea bean species appear to have maintained their ecological preference for wet, lowland forests where they sprawl over rocks, shrubs and trees along streams and near the ocean.

Although natural populations of wiliwili typically occur on the drier, leeward slopes of the islands, Carlquist postulates that they probably arrived by drifting (either buoyant seeds or floating limbs bearing seed pods) because of their proximity to the ocean. Perhaps E. velutina arrived on the Galapagos Islands by a similar mechanism. In fact, mainland iguanas probably rafted to the Galapagos Islands and gave rise to the present-day populations of marine iguanas. Based on the speed of the currents and local winds, it has been estimated that a raft from Ecuador could reach these islands in about two weeks. It is interesting to note that the Hawaiian wiliwili has a soft, light wood that was (and still is) highly prized for the outriggers of traditional canoes. It was also used for fishnet floats and surfboards.

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