Ohaha (Brighamia rockii)
Hawai’i is home to so many interesting species of plants, many of which are found nowhere else in the world. One group however, stands out among the rest in that it represents the largest plant radiation not just in Hawai’i, but on any island archipelago!
I am of course talking about the Hawaiian lobelioids (Campanulaceae). Many of you will be familiar with members of the genus Lobelia, which include the lovely cardinal flower (Lobelia cardinalis) and the great blue lobelia (Lobelia siphilitica), but the 6 genera that comprise the Hawaiian radiation are something quite different altogether.
'Oha Wai (Clermontia samuelii). Photo by Forest and Kim Starr licensed under CC BY 2.0
Numbering roughly 125 species in total (in addition to many extinct species), it was long thought that the diversity of Hawaiian lobelioids were the result of at least 3 separate dispersal events. Thanks to recent DNA analysis, it is now believed that all 6 genera are the result of one single dispersal event by a lobelia-like ancestor. This may seem ridiculous but when you consider the fact that this invasion happened back when Gardner Pinnacles and French Frigate Shoals were actual islands and none of the extant islands even existed, then you can begin to grasp the time scales involved that produced such a drastic and varied radiation.
Delissea sp.
Sadly, like countless Hawaiian endemics, the invasion of the human species has spelled disaster. Hawaiian endemics are declining at an alarming rate due to threats like introduced pigs and rats that eat seeds, devour seedlings, and even go as far as to chew right through the stems of adult plants. To make matters worse, many species evolved to a specific suite of pollinators.
ʻŌlulu (Brighamia insignis)
Take, for instance, the case of the ʻŌlulu (Brighamia insignis). It is believed to have evolved a pollination syndrome with a species of sphinx moth known as the fabulous green sphinx moth (Tinostoma smaragditis), which is also believed to be extinct. Similarly, the ʻŌhā wai nui (Clermontia arborescens) evolved for pollination by the island's endemic honey creepers. Due to avian malaria and other human impacts, many honey creepers are endangered and some have already gone extinct. Without their pollinators, many of these lobelioids are doomed to slow extinction if they haven't disappeared already.
While it may be too late to bring back species that have likely gone extinct, that doesn’t mean conservation of these incredible plants is off the table. Indeed, many efforts are being put forth by institutions like the National Tropical Botanical Garden and the Chicago Botanic Gardens to help conserve and restore some of these species. Along the way, the Hawaiian lobelioids are teaching us important and timely lessons on the need for understanding and protecting all pieces of Earth’s ecosystems, rather than individual parts in isolation.