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Running Head: Hawaiian lobelioid adaptive radiation
The Adaptive Radiation of the Hawaiian Lobelioids
Megan Edgar
Miami University
November 17, 2014
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Hawaiian lobelioid adaptive radiation
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Introduction
The Hawaiian lobelioids are a group of angiosperms from the Campanulaceae family
(Givnish et. al., 2009). They arrived in the Hawaiian Islands in a single colonization event
(Givnish, Montgomery, & Goldstein, 2004) approximately 13 million years ago (Pender,
Morden, & Paul, 2014). Comprised of six different genera and 126 species, this single group
makes up over 1/8 of Hawaiian native flora (Givinish et. al., 2009). Inhabiting a wide range of
habitats from alpine bogs, to sea cliffs, to dry forests, to the dark understory of tropical
rainforests (Pender et. al., 2004), this family of endemic flowering plants represents the best
example of adaptive radiation found anywhere in the world (Givinish et. al., 2009).
Geology and Biogeography of Hawaii
To understand how a single colonization event could lead to such a wide variety of
genera and species, one must first understand the geology and biogeography of the Hawaiian
Islands. As the most isolated archipelago in the world, the islands are 3200 kilometers from the
nearest continent (Sakai, Wagner, & Mehroff, 2002). Formed as the Pacific plate passes over a
hot spot, each island in the chain follows a specific geological life cycle (Price & Clauge, 2004).
Rising up from the sea floor, these shield volcanoes reach their maximum height approximately
.5 million years after they reach the surface of the ocean. Once an island stops growing, it settles
rapidly over the next million years. The island then experiences a several million year long
period of erosion in which ocean waves, wind, and rain slowly wear the island down to sea level.
At this point, the island becomes an atoll, and eventually returns to the sea.
While the hot spot has been producing islands for approximately 85 million years (Sakai
et. al., 2002), only those formed in the last 30 million years remain above sea level (Price &
Clauge, 2004). From Hawai’i, the youngest active island reaching to 4170 meters (13,377 feet)
above sea level (National Park Service, 2014), to Kure, the oldest atoll barely rising above sea
level, the islands stretch 2575 kilometers (1600 miles) in a northwesterly diagonal across the
Pacific (Price & Clauge, 2004). At various times in history, depending both on rising sea levels
and the volcanoes’ growth stages, several of the islands have connected. Kaua’i and Ni’ihau
were connected by a land bridge, and Maui, Moloka’i, Kaho’olawe, and Lana’i once formed one
large island called Maui Nui. A land bridge also existed between Maui Nui and O’ahu for a short
time. These changing connections between the islands helped shaped the evolution of organisms
Hawaiian lobelioid adaptive radiation
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as they experienced various periods of gene flow and isolation within the island chain depending
on the geology of that time (Price & Clauge, 2004).
As the most isolated archipelago in the world, colonization events on any Hawaiian
Island are few and far between. Price and Clauge (2004) estimate 1 colonization occurs in 98,000
years for plants, 1 in 68,000 years for insects, and less than 1 in 1 million years for avian species.
The only natural means of transport to the Hawaiian Islands occurs via water (sea), wind (air), or
wing (flight) (Stone, 1967). Organisms must be able to survive a substantial amount of time in
salt water, be small and light enough to be transported via the wind, or perhaps find transport in
the feathers of a bird. Even if an organism makes it to Hawai’i, arrival on an island does not
guarantee colonization. An individual member of a species may arrive and thrive on its own, but
then not be able to reproduce. A species is only considered a native member of the flora and
fauna if it has a continuously successful means of reproducing in its new habitat (Stone, 1967).
Both geology and the location of the Hawaiian Islands on Earth contribute to their
possessing an exceptional variety of ecological niches (Carlquist, 1980). The islands rest at
roughly 20° North latitude. For most parts of the world, this latitude is characterized by
consistent subtropical conditions. There is little variation in day length, and temperatures are
warm and stable throughout the year. Hawai’i, however, is unique. Subtropical conditions exist,
but so does nearly every other climactic condition found on Earth (‘Ohu Gon III, 2014). On
islands with volcanoes that reach over 3000 meters (10,000 feet), one can find every biome that
exists in the world except tundra. Reaching up out of warm subtropical Pacific waters, these tall
peaks interact with prevailing trade winds to create their own weather (Carlquist, 1980).
Precipitation varies widely across short distances, and temperatures fluctuate greatly with
elevation changes. This allows for very different habitats to exist in close proximity to one
another, and provides organisms an amazing array of microclimates and niches in which to
evolve. Once a colonization event happens, and a species has established itself as native flora or
fauna, the diversity of the Hawaiian landscape combined with it’s isolation from any continent,
provides unprecedented opportunity for adaptive radiation (Sakai et. al., 2002).
Natural History of the Hawaiian lobelioids
Although it is clear that Hawaiian Island geology and biogeography provide ideal
conditions for species that colonize to evolve, the degree to which the Hawaiian lobelioids have
adapted to the wide range of niches in the islands is unique (Givnish et. al., 2009). No other
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group in the world shows such extensive adaptive radiation. What is it that allowed one member
of the Campanulaceae family to arrive in Hawai’i and evolve so many diverse forms?
First, it is estimated the colonization event for the Hawaiian lobelioid family took place
approximately 13 million years ago (Givnish et. al., 2009; Pender et. al., 2014). Researchers
speculate the first Campanulaceae colonist arrived on Gardner Island, which was unusually tall
for Hawaiian Islands at the time, and was the only island with the full range of wet habitats we
see in the islands today (Givnish et. al., 2009; Price & Clague, 2002). Price and Clague (2002)
state “the degree to which a lineage diversifies is a function of the area and range of habitats
available” (p. 2432). At the time of the colonist’s arrival, Gardner Island would have been
young, probably still growing, and likely housed many available and diverse niches to which an
organism could adapt and begin the process of speciation (Givnish et. al., 2009). Thirteen million
years makes the Campanulaceae colonization event once of the oldest for species that still exist
in Hawai’i (Givnish et. al. 2009). This, combined with the habitat diversity of Gardner Island,
gave Campanulaceae a good adaptive radiation head start.
Second, many members of the Campanulaceae family alive today are well adapted for
wind dispersal (Antonelli, 2009). This observation explains not only how a Campanulaceae
colonist could have arrived in Hawai’i in the first place, but also how the species may have been
transported around the islands once there. It is highly likely that seeds from the original colonist
moved around Gardner Island via wind, allowing the plant the opportunity to adapt to a wide
variety of niches in various locations. Once diversification and speciation began taking place on
Gardner Island, new varieties of the original colonist continued to spread their seeds throughout
Hawai’i. Today, for example, one gram of seeds from an endemic Hawaiian Campanulaceae
member of the Clermontia genera, Clermontia kakeana, contains 36,000 seeds. Such tiny seeds
allow for easy transport to new niches and subsequently provide more opportunities for
speciation.
Third, any species of Campanulaceae that evolved from the original colonist 13 million
years ago experienced a genetic bottleneck (Price and Clague, 2002). Gardner Island, which
formed 15.8 million years ago, was uniquely tall for the time period it existed. It wasn’t until the
formation of Kaua’i, 4.7 million years ago, that the Hawaiian Islands began to consistently grow
to great heights. Any living species that required a cool upland habitat went extinct during the 11
million year gap between Gardner Island and the growth of Kaua’i Island. Although it is likely
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some members of the Campanulaceae family were lost during this time period, the bottleneck
actually provided an opportunity for Campanulaceae to thrive. Most species alive at the time of
the bottleneck went extinct. Members of the Campanulaceae family survived. When Kaua’i
appeared, bringing with it the diversity of habitats associated with a tall Hawaiian island,
Campanulaceae experienced rapid adaptive radiation because there were no other species around
with which it had to compete. By the time new colonists made it to Kauai’i from the various
continents of the world, Campanulaceae had had ample time and opportunity to evolve into a
wide variety of different species without competition.
Fourth, approximately 5 million years ago, the Drepanidinae family of passerine birds
arrived in the Hawaiian Islands (Pender et. al., 2014). These birds, which are all extinct or highly
highly endangered due to the introduction of mongoose, the domestic cat, and an avian diseasecarrying mosquito, feed on nectar. At some point in Hawaii’s evolutionary history, the
Drepanidinae, or Hawaiian honeycreepers, began to co-evolve with members of the
Campanulaceae family, particularly the Cyanea (Givnish, Sytsma, Smith, & Hahn, 1994) and
Clermontia genera (Pender et. al., 2014). Over time the plants became ornithophilous, relying on
birds for pollination. This mutualistic relationship shaped the evolution of both Hawaiian
honeycreepers and Hawaiian lobeliods from 5 million years ago through today. Today, there are
a wide range of Cyanea and Clermontia plants, each with unique flowers adapted to the beaks of
particular Hawaiian honeycreeper species.
Fifth, members of the Cyanea genera in the Campanulaceae family experienced the
introduction of a predator approximately 3.7 million years ago (Givnish et. al., 1994). This
predator, likely the extinct flightless goose moa-nalo, caused the evolution of two adaptations.
One, some species of Cyanea developed prickles to deter the herbivorous geese from feeding on
them. Two, some Cyanea species of plants developed juvenile-adult leaf dimorphism in which
the juvenile form has prickles for protection from predation, but the adult form does not. The
introduction of a new predator spurred adaptations that led to speciation in Campanulaceae.
Hawaiian Lobelioids Today
Today, the Hawaiian members of the Campanulaceae family, the Hawaiian lobelioids,
comprise of six genera and 126 different species (Givnish et. al., 2004). Brighamia genera
species are halophytic (Gemmill, Ranker, Ragone, Perlman, and Wood, 1998), reside on steep
sea cliffs on Kau’i and Moloka’i, and are pollinated by an endemic moth (Carlquist, 1980).
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Lobelia genera species prefer wet habitats and boast several gigantic species that live in alpine
bogs found on Kaua’i, Maui, Moloka’i, and O’ahu. Trematolobelia genera species are found in
the wettest forests on all major islands. This genera is unique in its seed dispersal method. When
the climate is dry and windy, seed sacs allow Trematoblobelia seeds to shake out of little holes
onto the ground. Clermontia genera species are shrubs with few branches. They prefer clear
sunny openings in wet forests and also include a few species of successful epiphytes. The
Cyanea genera is the largest group of Hawaiian lobelioids and is also the most morphologically
diverse (Carlquist, 1980). Some species have prickles (Givnish et. al., 1994) and they grow in
densely vegetated low-light forests (Pender et. al., 2014). Many members of the genera are
pollinated by Hawaiian honeycreepers, which has contributed greatly diversity in flower
morphology. Plants in the Delissea genera are low lying and prefer shady forest habitats. It is
quite remarkable, given the vast diversity of Hawaiian lobelioids, that genetic testing consistently
shows this monophyletic group of flowering plants shares one common ancestor (Givnish et. al.,
2009; Haberle et. al., 2009).
The Future of Hawaiian lobelioids
Although the Hawaiian lobelioids have enjoyed 13 million years of adaptive radiation,
the future of this family of plants is uncertain. Like the vast majority of Hawaii’s endemic
species, habitat loss, new pressures from introduced species, and the extinction of species with
which plants have obligate relationships threaten their survival (Gemmill et. al., 1998). One of
the main characteristics of the Hawaiian Islands that allowed for such rapid and extensive
adaptive radiation in Hawaiian lobelioids – the incredible diversity of habitat and microclimates
across small distances – is now contributing to their demise. Once an area is altered by human
activity, or by invading species introduced by humans, there is no going back. And because
habitat areas are so small in the Islands, there may not be another area like the one that was lost
for a species to continue to thrive in. Conservation efforts are underway, but continued
development and the rampant growth of introduced invasive species make conserving Hawaii’s
native plants and animals a difficult challenge. Hopefully, the isolated cliffs, alpine bogs, and
forest areas that are difficult for humans to get to will keep at least some of the diverse Hawaiian
lobelioids safe from future human influences.
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Hawaiian lobelioid adaptive radiation
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