Hawaiian Honeycreepers: Evolution in Hawaii

Untamed Science is a science video blog. In this episode the focus is on the evolution of Hawaiian honeycreepers and the geology of the archipelago. The documentary does an excellent job in describing the formation of the volcanic island chain with a brief animation showing yhe crossection of the Earth’s crust and the hot molten mantel. On the other hand the story is rather incomplete. In Hawai’i, honeycreepers and a group of plant species called lobeliads belonging to the bellflower family (Campanulaceae) evolved in an intricate interaction involving nectar feeding, pollination and seed dispersal. Pollination by birds (ornithophily) is a remarkable adaptation for plants.

The episode mentions two honeycreeper species the I’iwi (Vestiaria coccinea) and the ‘Apapane (Himatione sanguinea). The shape of the I’iwi beaks fit to Hawaiian lobeloid flowers precisely and the bird can draw nectar more efficiently than the other bird species. During nectar feeding most honeycreepers provide pollination service. The way I’iwi birds forage on lobelia flowers maximizes their defensive visual vigilance against natural predators such as hawks and owls.

This short observation was recorded at The Nature Conservancy’s Waikamoi Preserve on Maui. It shows one of the classic examples of co-evolution between a plant and a bird. The long bill of the scarlet ‘i’iwi (Vestiaria coccinea) and the curved, tubular flower of the blue ‘ōpelu (Lobelia grayana), a native lobelia have evolved together due to selective forces that increase the survival success of both species.

Although birds are excellent agents of seed dispersal, 40% of the Hawaiian lobeliad species have wind-dispersed seeds. The remaining 60% of the species are grouped under in one genus Cyanea and all are bird pollinated and bird dispersed. Thus Hawai’i became an impressive scene for adaptive radiation of 23 species of Hawaiian honeycreepers (more than 16 species became extinct since Human arrival around 1600) and lobelioids with 126 species described. This is the largest adaptive radiation on any island archipelago. The origin and rapid speciation of Hawaiian lobeloids have been resolved by an extensive molecular genetic study. Accordingly, the colonization of the islands began by a single wave of bird-pollinated and wind-dispersed ancestral migrants dating back to 13 million years pre-dating the formation of the Hawaiian island chain. The oldest island Kaua `i is 4.7 Myr old. Therefore the origin of the Hawaiian lobeliads began on a former island near French Frigate Shoals and Gardner Pinnacles which are so old that they are now almost completely submerged.

One characteristic Hawaiian endemic tree Metrosideros polymorpha benefits greatly from multiple bird species including honeycreepers for pollination of it’s red flowers that are adapted to provide secondary perching opportunity. Evolution of branching architecture is especially enticing for the birds since secondary perching helps them save energy during foraging in tree canopy. Here you can see another honeycreeper species the ‘Apapane providing pollination service to Metrosideros tree:

 

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