Evolution in Hawai‘i: How plants and animals reached the Hawaiian Archipelago
Hawai’i is an extremely remote place and therefore one of the best examples to study the evolution of biodiversity at geographically isolated locations under extreme geological conditions.
In this article you’re going to find out
- how plants and animals came to Hawai’i
- how they established their habitats and niches and adapted to those
- how Hawai’i’s exceptional biodiversity developed as a result and
- why there are many endemic species in Hawai’i
and it is structured as follows:
- Short introduction into the topic
- Hawaiian Archipelago
- Geographic isolation
- Geological formation
- Evolution of biodiversity
- Founder population
- Ecological succession (here: vegetation)
- Speciation and endemism
- Summary
This article turns a focus on the terrestrial biome.
1. Introduction
If you planned a Hawai’i vacation, you would quickly realize that getting there takes a long time from almost anywhere in the world. Since I am from Germany, my journey to Hawai'i would take especially long, about 20 hours, including one layover at the US west coast – one way. The time difference to Germany is 11 hours.
Image: Aerial photograph of the Ka'ena State Park in O'ahu, Hawai'i, by author
Once in Hawai’i, you would be impressed by the deep blue, wide ocean teeming with life; the lush vegetation whose intense green colors pose a stark contrast to the dry, basalt-blackish-grayish or rusty lava deserts; you would be amazed by the beautiful, varied flora and fauna.
Image: Koko Crater and Koko Head photographed from Makapu'u Lighthouse trail, O'ahu, Hawai'i, by author
This raises the question: How did all this come to be? How could this lush, biodiverse paradise develop on islands of volcanic origin, in the middle of the largest ocean on Earth, the Pacific, thousands of kilometers away from continents; on barren or sparsely populated islands that, at first glance, provide conditions hostile for life, as they form out of 1,200 °C hot lava?
Image: Papakōlea Green Sand Beach, Big Island, Hawai'i, photographed by author
2. The Hawaiian Archipelago
Geographic Isolation
To answer this question properly, it is necessary to take a closer look at the geographic location and extent of Hawai’i.
The Eight Main Hawaiian Islands
Eight islands form the main islands of the Hawaiian Archipelago. They are situated in the Pacific Ocean. These islands are, from northwest to southeast: Ni’ihau, Kaua’i, O’ahu, Moloka’i, Lāna'i, Maui, Kaho’olawe and the Island of Hawai’i (Big Island).
Image: Satellite image of the Hawaiian Islands. Source: Google Earth.
These eight main islands are part of the larger Hawaiian Archipelago and are located in its southeast (see next image). To the west of the main islands are the Northwestern Hawaiian Islands which extend to the westernmost island, the Kure Atoll. The Kure Atoll was formed approximately 30 million years ago and is the last emerged island.
Thus, the Hawaiian Archipelago consists of the Northwestern Hawaiian Islands plus the eight main islands and reaches from the Big Island in the southeast to the Kure Atoll. All of these islands are still emerged.
Westward of the Hawaiian Archipelago is the Hawaiian Emperor Bend. North of it, the chain of the submerged Emperor Seamounts (drowned volcanoes) extends. Together with the Hawai’i-Archipelago they form the Hawaiian-Emperor Seamount Volcanic Chain.
This chain has a total length of 5,700 km / 3,500 miles. The closest mainland is California at the Westcoast of the United States. It is 3,800 km / 2,300 miles far away, that is about 5.5 flight hours.
Image: Satellite image of the Hawaiian Emperor Volcanic Chain, modified by author. Source: Google Earth.
If the complete Hawaiian-Emperor Volcanic Mountain Chain was projected onto the continental USA, the Big Island would be in Miami, Florida, and the northwesternmost seamount, Meiji Seamount, would be in the southeast of Alaska.
How did the Hawaiian Islands form?
The Hawaiian Islands are of volcanic origin.
The map below shows the Pacific Plate (oceanic tectonic plate) as well as the Pacific Ring of Fire (= the rim / plate margin of the Pacific Plate is marked by zones of high volcanic activity). Hawai’i is located in the center of the Pacific Plate, far away from plate margins where volcanism typically occurs. However, the red dot indicates that Hawai’i is located above a volcanic hotspot. Hotspots are locations on Earth where magma from the Earth’s mantle erupts independent of plate margins.
Image: Map of Pacific Plate with Hawaiian Hotspot and Ring of Fire (Simkin et al. 2006).
The Hawaiian Hotspot has been active for about 80 million years and has gradually built the Hawaiian Islands, all the way from Meiji Seamount to the Big Island. Since the Pacific Plate slowly drifts to the northwest, the Hawaiian Islands form a chain, like a string of beads. Thus, the oldest islands and seamounts are located in the northwest, the youngest in the southeast.
Over time, emerged islands subside and become seamounts.
How did animals and plants come to Hawai'i?
Evolution in Hawai'i
Due to the geographic isolation and geological origin of the island, the question arises:
How has Hawaiian biodiversity and endemism developed?
In this chapter we will look at the
- founder population and the pioneer organism’s arrival on the islands
- development of a green forest by the concept of ecological succession
- process of speciation and adaptive radiation as well as endemism
Founder Population
Founder species are those species that reach a new habitat first and successfully establish a population. The founder population has been isolated from its original community (e.g. located on the continents) and continues to evolve independently in its new habitat (e.g. the Hawaiian Islands).
If only one or very few individuals of a species reach a new geographic area, a founder effect will inevitably occur. The founder effect describes the decline of genetic variation of the species arriving on the island compared to the one in its original population. Due to the spatial separation, gene exchange can no longer take place. Consequently, gene diversity will decrease and new species form by allopatric speciation (explained further below). In Hawai’i this is exactly what happened.
For a founder to survive and establish a population, a number of crucial conditions must be met, e.g.
- the founder must find a suitable habitat that provides everything they need to survive
- its food must be currently available
- in the case of sexual reproduction: the opposite sex must be present
- for species originally from temperate climates, adaptation to tropical and subtropical climates must occur
- and niche competition must be low (explained further below)
An average number of about eight species evolved from one terrestrial founder. In other words: One founder organism brings about eight new terrestrial species.This is a high ratio. By comparison, marine organisms have an average of two species per founder.
How did the first animals and plants (founders) get to Hawai'i?
The transport mechanisms are also known as the three W’s. They represent wind, waves and wings.
- wind stands for dispersal by air.
- waves stands for dispersal over the ocean.
- wings stands for the dispersal via birds or the arrival of birds in general.
And later, of course, humans imported plants and animals.
Rate of colonization
How often, on average, was a founder of a certain species able to reach a Hawaiian Island and settle there successfully, i.e. establish a population?
This table lists the statistic rate of successful colonization by a founder species in years (read it like this: every xx years founder species y succesfully established a colony). Notes: Only for terrestrial biota and time of colonisation is approx. 30 million years (= age of Kure Atoll, last emerged island).
| Insects | 70,000 years |
| Flowering plants | 105,000 years |
| Ferns | 265,000 years |
| Birds | 1,155,000 years |
| Spiders | 2,310,000 years |
| Mammals | 15,000,000 years |
Insects and flowering plants have the fastest colonization rates.
It is necessary to point out that a far greater number of plant and animal species arrived in the Hawaiian Islands than were able to establish populations. The exact percentage of unsuccessful arrivals is unknown, but it would not be surprising if it exceeded 95 or even 99 %.
It is likely that other species also established thriving populations, but are either extinct and/or absent in the fossil record. Theoretically, then, the rate of colonization could be higher.
Ecological succession (here: vegetation)
Ecological succession (here focused on vegetation) describes the changes in plant cover over time. When a new area is created that could potentially be covered with vegetation, such as a new fresh lava flow, one or more plant species will inevitably colonize that area. This group is the pioneer vegetation. This initial group prepares the habitat for subsequent communities with successional species by contributing food, shade, moisture retention, ideal temperature conditions, as well as soil and wetland. The succeeding communities, hence, are adapted to the ecological conditions provided by the preceding pioneer group.
The following exemplary plant succession illustrates the vegetation succession as it could occur in Hawai’i at about 1,200 m / 3,900 ft elevation in a relatively wet region with fresh lava flows. The pictures shown are randomly chosen and do not correspond to this specific habitat, but are accurate for the given stage.
The vegetation succession begins with (1) the Pioneer stage, followed by (2) the Middle stage, and (3) the Later stage, and culminates the (4) Climax stage.
(1) Pioneer stage
As soon as the lava has cooled sufficiently, the activity of cyanobacteria and the germination of mosses, lichens and ferns begins in crevices and pockets of soil where moisture from previous rain or other atmospheric wet deposition has been contained. The spores of ferns probably arrived in Hawai’i mainly by wind. After 4-5 years, the pioneer vegetation has covered the lava floor.
Image: Stereocaulon vulcani Lichen, Kipahoehoe Natural Area Reserve, by brewbooks, on flickr, CC BY-SA 2.0.
(2) Middle stage
In the middle stage, besides lichens and ferns, flowering plants now colonize the lava flow, e.g. the endemic tree Metrosideros polymorpha, which is called 'Ōhia lehua in Hawaiian, is particularly important here. It is assumed that the seeds of 'Ōhia lehua were carried to Hawai’i primarily via birds. The plants grow in small soil pockets. The soil forms from organic material created by the pioneer plants and from the weathering basalt lava (including important minerals).
This photo depicts ferns and lichens that are still present.
This photo beautifully displays the growth of vegetation in crevices and also a larger tree is already present, leading to the next stage.
Image: Forest und Kim Starr, auf flickr, CC BY 2.0
(3) Later stage
After about 100-150 years, 'Ōhi'a lehua reaches a considerable height. A new species of ferns established, the Hawaiian tree ferns, which can grow up to 5 m / 16 ft tall. The former shrubs have also been replaced by successional plants. The forest floor, tree trunks and branches are covered with a thick layer of moss. Fungi grow on dead trees and decompose organic material.
Image: Forest und Kim Starr, auf flickr, CC BY 2.0
(4) Climax stage
After 300-400 years, more shrubs and trees have established. A rainforest has now developed in which 'Ōhi'a lehua and the Hawaiian tree ferns are the most important species.
Image: Hanakāpīʻai Stream along the Hanakāpīʻai Trail, Kaua'i, Hawai'i, photographed by author
How did evolution in Hawai’i produce such a high number of species, particularly endemic species?
Unfortunately, I only found a 2002 figure for the number of species because the Hawai’i Biological Survey website, which does the inventory, is unfortunately not working properly. In 2002, a total of 23,680 species, both aquatic and terrestrial, were recorded. Of those, 40% are endemic. Considering only terrestrial species, 90% are endemic to Hawai’i.
Unfortunately, many (endemic) species are either threatened with extinction, or populations are critically endangered. Many flowering plants, insects and birds are affected gravely. A great number of flowering plants, mollusks, insects and birds already are extinct. The reasons are: the invasion of foreign plants and animals, climate change, as well as habitat loss due to human interference.
Speciation in Hawai’i: How did so many species evolve?
Three processes will be discussed in detail:- Allopatric speciation and vicariance
- Island hopping
- Adaptive Radiation
Allopatric speciation by geographic vicariance
Speciation is triggered by the initial irreversible, permanent spatial separation (possible barriers: oceans, mountains, canyons) of an originally continuously distributed species or population. The separation results in the interruption of the gene flow and this in turn leads to a gene differentiation of the new populations, which corresponds to the founder effect introduced above. When gene differentiation occurs, successful mating with the original population is inhibted and new species emerge as a result of reproductive isolation.
For Hawai'i, vicariance exists not only because the founder population came from the continents surrounding Hawai’i with the ocean as a barrier. Specifically, vicariance also arises from subsidence of volcanic islands as they evolve from a high volcanic island to an atoll, thus, either splitting previously connected islands into multiple islands (example: Maui Nui) or habitats are separated otherwise.
Allopatric speciation as a speciation process can occur much more easily and intensely on an archipelago like Hawai'i than on continents, which brings us to the second important speciation mechanism:
Island hopping
The individual Hawaiian Islands are somewhat distant from each other and, thus, isolated, but still close enough to allow at least occasional biotic exchange (migration). Because Hawai’i is an island chain whose islands are constantly being formed, island hopping could occur over several million years. The ancestors of today's Hawaiian species arrived on Hawaiian Islands that are now seamounts (submerged islands, mountains).
The following example is ought to briefly illustrate island hopping. Imagine two islands, Island 1 and Island 2, in close proximity to each other and an initial species A that occupies Island 1.
Species A migrates from Island 1 to Island 2, where it becomes the founder (A) and is isolated on Island 2 for an extended period of time, becoming the new species B.
Species B migrates back to Island 1 (or migrates to a new island) and becomes founder (B) there. The founder species (B) eventually becomes the new species C.
Species A migrates again to Island 2, becomes founder species (A) there and evolves into new species D.
Of course, this is only an example, and the speciation path could also take different courses.
Adaptive Radiation
As soon as the founder species has arrived on an island, has successfully colonized it, and has established a functioning population, the occupation of all possible niches by adaptive radiation takes place. The Hawaiian Islands have numerous possible niches to occupy due to the specific microclimates and habitats.
If an organism is able to colonize a new area, and given the fact that most or all of the ecological niches are unoccupied, only partially occupied, or lacking competing dominant forms, that founder species will almost inevitably bring forth a number of new forms of this species that use many of these available habitats, by adapting to them. Thus, new niches are used and new species are created.
These soft conditions are almost always given on oceanic islands. Then, establishing a successful, thriving population and filling niches is a piece of cake for founders. Due to the absence of serious competition by superior species, they have time to adapt to the diverse island habitats and ecological circumstances that would be denied to them in a continental setting. In general, unadapted species in early stages would not survive such an adaptation (a transition) in a highly competitive continental ecosystem where rapid, efficient adaptation is required.
Endemism in Hawai’i
As stated preciously, the rate of speciation is much higher than the rate of immigration. Consequently, a plethora of new species can develop from one single founder. This, a consequence of geographic isolcation, is the reason for such a high degree of endemism of terrestrial species.
Excellent examples of adaptive radiation are the Silversword Alliance and the Hawaiian Honeycreepers, which will here be introduced briefly.
Silversword Alliance
The Silversword Alliance is a group of three endemic plant genera in the sunflower family (Asteraceae) and includes 30 endemic species that all evolved from a single species of targrass from California about 5-6 Ma ago (= age of the Ni'ihau and Kaua'i). Pictured below is the Haleakalā Silversword (scientific name: Argyoxiphium sandwicense, Hawaiian name: 'ahinhina). This plant grows only in the cinder cones on the peaks of Haleakalā volcano, the highest volcano on the Island of Maui, at an elevation of 2,100-2,400 m / 6,800 - 7,800 ft.
Image: Haleakalā Silversword, by Forest and Kim Starr, on flickr, CC BY 2.0
Hawaiian Honeycreepers
Hawaiian Honeycreepers are birds endemic to Hawai’i. They belong to the family of finches or Fringillidae and there to the tribus of Drepanidini. The group of Hawaiian Honeycreepers comprises more than 60 species, which includes fossil, extinct and recent ones. Today, there are only 17 living species. Two of these species are shown in these photos:
Image: Drepanis coccoinea, Hawaiian: 'I’iwi, by Michael Klotz, on flickr, CC BY-NC 2.0
Image: Chlorodrepanis virens, Hawaiian: 'amakihi, by Michael Klotz, on flickr, CC BY-NC 2.0
The Hawaiian Honeycreepers are particularly remarkable because they represent an impressive and extreme example of adaptive radiation in vertebrates. Some even claim that the Hawaiian Honeycreepers are even more exciting than the Darwin's Finches.
The clade of Hawaiian Honeycreepers is characterized by a wide variation in phenotype, with the beak shape, tongue, and diet playing particularly prominent roles.
They are likely a sister taxon of the Eurasian-native Common Rosefinche and arrived in Hawai'i from Asia. The arrival of the Hawaiian Honeycreeper ancestor was about 7.2 million years ago. Radiation began about 5.8 million years ago, which coincides with the formation of the Islands of Ni'ihau and Kaua'i.
Unfortunately, many species of Hawaiian Honeycreepers as well as many other endemic species of Hawai'i are threatened with extinction.
Summary
- The geographic isolation and the archipelago setting have been critical to the evolution of Hawaiian biodiversity.
- There is an estimated number of species of at least 24,000. Out of these, 90 % are endemic terrestrial organisms.
- The arrival of founder organisms occurred randomly, and is is assumed that the successful establishment of a population was rare.
- Fresh lava flows are colonized according to vegetation succession.
- Vicariance, island hopping, and adaptive radiation produced the species-rich flora and fauna with high endemism. Autochthonous speciation oupaces immigration, resulting in high endemism.
- The Silversword Alliance and Hawaiian Honeycreepers are excellent examples of adaptive radiation.
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Sources
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Further Reading
Burney, D. A; James, H. F.; Burney, L. P.; Olson, S. L.; Kikuchi, W.; Wagner, W. L.; Burney, M.; McCloskey, D.; Kikuchi, D.; Grady, F. V.; Gage II, R.; Nishek, R. (2001): Fossil evidence for a diverse biota from Kaua‘i and its transformation since human arrival. In Ecological Monographs 71 (4), S. 615-641. DOI: 10.1890/0012-9615(2001)071[0615:FEFADB]2.0.CO;2.
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About the author
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Daniela is convinced that gaining deep insights into planet Earth and travel destinations creates meaningful, grounding and memorable life + travel experiences. She explains fundamental geological processes that form and shape landscapes and combines these insights with philosophical and philanthropical views in her online courses, articles, and newsletter. She currently studies for a master's degree in geosciences at the University of Cologne. She is the owner and founder of EarthyMe, EarthyUniversity and the Science of Travel blog and the STORIES OF EARTH newsletter. |
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