By Emily Underwood | Illustrations by Neal Uno
Sometime between three and five million years ago, a bird took wing from the California coast, heading west across the Pacific Ocean. Scientists can only guess what kind of bird it was—some have proposed it was a migratory shorebird called the Pacific golden plover—or why it flew into the sunset. But with as much certainty as can be expected for an event that happened so long ago, they do know that the bird flew more than 2,000 miles until it landed on what was then Hawai’i’s youngest island: Kauaʻi.
Scientists know this epic flight occurred because the bird was carrying a tiny stowaway—the seed of a California native plant called a tarplant, a member of the sunflower (Asteracae) family. What happened next is considered one of the most remarkable events in the history of plant evolution: After the bird landed on Kaua‘i, the tarplant seed took root. Over the next few million years, the plant’s descendants diverged into more than 30 new species, known collectively as the Hawaiian silversword alliance.
Each member of the alliance evolved to fill a distinct ecological niche. On the cindery slopes of Haleakalā Crater on Maui, for example, Hawaiian silverswords, or ‘ahinahina (Argyroxiphium sandwicense) developed into large, rosette-shaped plants with silvery, sword-shaped leaves that reflect sunlight and protect the plant from extreme temperatures. Other descendants adapted to the wet summit bog of Kaua‘i’s Mount Wai’ale’ale, rainforests, mesic forests, and bare lava flows.
Biologists call this rapid proliferation of species from a single ancestor an adaptive radiation. A classic example comes from the finches of the Galápagos Islands: During the voyage of the HMS Beagle, naturalist Charles Darwin collected finches on different islands that had a range of beak shapes, each suited to a different type of food. He later hypothesized that the birds had all descended from one species, contributing to his theory of evolution by natural selection. In Hawai‘i, adaptive radiations contributed to a wild proliferation of fauna and flora, including more than 1,000 flowering plant species, 50 species of the finch-like birds called honeycreepers, and roughly 800 different species of fruit fly.
Such radiations provide biologists with precious glimpses into how new species evolve, and can also provide valuable clues about how to preserve biodiversity in the future—not just on islands, but in other hotspots for rare and threatened species, including California, says Bruce Baldwin, a professor of integrative biology at UC Berkeley and curator of the Jepson Herbarium. For Baldwin, who has spent decades studying the evolutionary histories of both Hawaiian and Californian plants, understanding a plant’s ancient past is key to charting a course for its future survival. “It’s important to understand how evolution has occurred, so we have an understanding of the importance of different landscapes and habitats in fostering future evolution and resilience to change,” he says.
It’s not just the speed of the silversword alliance’s radiation that has long impressed biologists, says Jonathan Price, a professor of geography and environmental science at the University of Hawai‘i. It’s the improbability of the original tarplant seed reaching Kaua‘i in the first place. At least from the perspective of a brief human lifespan, the odds are so low that “it seems almost impossible,” he says.
Hawai‘i is one of the most isolated places on Earth. The closest continent, western North America, is more than 2,000 miles away. If a seed were very light, it might have been able to ride a high atmospheric current. If ocean currents were in its favor, it could have floated, most likely attached to a buoyant tree branch or fruit. Regardless of what route the seed took, the odds of a mainland plant getting a seed to Hawai‘i were akin to winning the lottery, says University of Hawai‘i botanist Sterling Keeley. “How many tickets do you have, and how many opportunities?”
Of course, over millions of years such improbable events do happen. “One of nature’s greatest currencies is events that happen rarely,” says Nick Jensen, conservation program director for CNPS. As early as the 1850s, botanists such as Asa Gray suspected that the North American tarplants and Hawaiian silversword alliance were connected. Then one summer day in 1953, botanist Sherwin Carlquist was hiking on Maui when he smelled the sharp, resinous scent of silverswords in bloom. The smell reminded him of tarplants he’d smelled in Southern California, as did the shape of the silverswords’ maroon blossoms.
Building on Gray’s hypothesis, Carlquist amassed a wealth of anatomical observations supporting the notion that the silverswords in Hawai‘i had descended from a California tarplant. Many scientists were skeptical of the connection, however. Not until the 1980s, when Baldwin was a graduate student at UC Davis, did it become possible to confirm that the groups were related through genetic analysis. Using newly developed tools for analyzing plant DNA, Baldwin was able to show that the entire Hawaiian silversword alliance had descended from a single, ancestral North American tarplant closely related to Muir’s tarplant (Carlquistia muirii), which grows in the Sierra Nevada and Santa Lucia Range.
Today, most scientists agree that the tarplant seed likely traveled to Hawai‘i by bird, says Baldwin. The seeds are only a couple of millimeters long and sticky, so it’s easy to imagine one getting wedged in a bird’s feathers, he says. When the bird arrived in Kaua‘i, it likely landed on high volcanic slopes climatically similar to northwestern habitats in California, minus the harsh seasons, he adds. With no need to die back in winter, the tarplant’s descendants could grow all year long and spread out. And spread out they did: As new islands emerged, the silversword alliance colonized them, too.
If the tarplant’s unrestrained proliferation sounds a bit, well, weedy, that’s probably a fair assessment, Baldwin says. In the early stages of their colonization, members of the silversword alliance “may have been pretty disruptive arrivals, in terms of having an explosion of success.”
But, he and other botanists say, there are important differences between early plant colonists like the tarplant, and the invasive species that Hawaiian land managers are now scrambling to eradicate.
The first difference has to do with timing. For much of Hawai‘i’s existence, new plant arrivals were spaced out by thousands, if not millions of years. In a recent study co-authored with botanist Warren Wagner, Price tallied the number of plant species that made the journey, and found that just 259 flowering plants managed to reach the archipelago over a 30-million-year period, among them species of mint, violet, and raspberry from North America, and other plant groups from places as far-flung as Southeast Asia, Central and South America, Australia, and New Zealand.
It’s important to understand how evolution has occurred, so we have an understanding of the importance of different landscapes and habitats in fostering future evolution and resilience to change.
– Bruce Baldwin
The descendants of these early arrivals evolved into what botanists consider Hawai‘i’s “native” flora, including roughly 1,400 species, subspecies, and varieties of plants. More than 90 percent of these plants are found nowhere else on Earth, and have evolved unique adaptations to life in the archipelago over millions of years. An ocean away from the stressors of the mainland, for example, many plants lost their defenses. Nettles lost their stings, evolving into soft understory plants, and raspberries lost their prickles. Some plants also lost their dispersal mechanisms, jettisoning sticky seed parts designed for hitchhiking on animals, or gaining curlicue-shaped seeds that twist into the ground like a stake.
Today, hundreds of new plant and animal species arrive in Hawai‘i on a weekly basis: In one recent study, scientists found more than 200 species of non-native insects on cargo ships in a Maui port over a two-week period, says Keeley. While most of these species don’t survive or cause problems, some have the power to radically reshape the islands’ ecosystems—not over millennia, but in decades or less.
These are the species biologists consider “invasive.” Notorious examples include the black rats (Rattus rattus) that European colonists brought to the islands 150 years ago, which have driven many plant and bird species to extinction, and the mosquitos that arrived by ship in the 1830s, carrying a form of avian malaria, says Kawika Winter, a professor of ecology and biocultural restoration at the University of Hawai‘i, who studies the coevolution between plants and cultures. Other examples include non-native pigs that eat the soft parts of tree-fern trunks, and introduced ornamental plants that grow so big they shade out native vegetation, Keeley says.
Today, biologists estimate that at least 100 native Hawaiian plant taxa have gone extinct since humans arrived on the islands; nearly 400 species are listed as federally threatened or endangered. Although a common narrative about Hawai‘i is that the archipelago was a pristine ecological paradise before humans arrived, bringing with them wave after wave of extinction, the story isn’t quite that simple, Winter says.
From Winter’s perspective, the question of what makes a plant “native” shares some philosophical and technical similarities with the question “When did Hawaiians become Hawaiian?” ”Ultimately, no matter if you’re talking about cultures and peoples, plants, or animals, it’s when something separates out . . . and can be clearly identified as its own thing.”
When the first Polynesians landed in Hawai‘i over a millennium ago, Winter says, they brought a carefully curated “biocultural kit” of plants, each chosen for its multiple cultural uses and meanings. The kit included plants like taro, a staple food plant and medicine that also conveys stories about Polynesian identity and values, coconut, sugarcane, bamboo, and breadfruit. Although botanists do not consider these plants “native” to Hawai‘i—and some could certainly be considered “invasive,” for their tendency to spread— most are crop plants that require human cultivation, Winter notes. In some cases, Polynesians developed these crops into varieties not found anywhere else, that “were unique from our ancestral homeland in the South Pacific,” he says.
Some of the species the Polynesians brought permanently changed the islands: A little brown rat called Rattus exulans, for example, rapidly wiped out many large-seeded tree species, causing a shift to forests that reproduce mostly with small seeds. This had a cascade effect on other Hawaiian species, causing a string of extinctions up the food chain. Over time, however, Polynesians developed a system for managing and cultivating the islands’ flora that maintained a high level of biodiversity, Winter says. “That is a really important part of the story that often gets left out when conservationists talk about Hawaiian extinctions,” he says.“It’s often ‘humans showed up and everything went extinct,’” he says. “But that’s not really what happened.”
Over centuries, as the Polynesians engaged with Hawai‘i’s biodiversity, they created a new language for it, as well as a plethora of uses, traditions, and stories for both native and introduced plants. Silverswords illustrate some of these rich connections: In addition to medicine, the group’s flowers are important for the garlands, or leis, that Native Hawaiians and their ancestors use to express love, providing a rich palette of meanings and symbols, Winter says. “The intentionality that goes into every step of making a lei imbues the lei with all of your aloha, or the love that you have for somebody,” he says. Little by little, the plants of Hawai‘i influenced Polynesians’ identity, becoming a vehicle for conveying their morals and values. At some point in that process, Polynesians became Native Hawaiians, Winter says.
There was a time when both humans and biodiversity could thrive together, so what are the lessons of that ancestral path?
– Kawika Winter
When Europeans brought the black rat and malaria-bearing mosquitoes to the archipelago in the 1800s, extinctions skyrocketed. As Hawai‘i was swept into the global market economy, colonists exported the islands’ valuable sandalwood and koa trees and razed the land for sugar plantations. With every extinction, Native Hawaiians lost not only species, but ancestral practices for resource management and ecosystem-based management, Winter says. Now, he is among those attempting to reinvigorate and learn from practices such as maintaining a mosaic of different habitat types across the landscape, and reserving an untouched part of the forest as sacred. “There was a time when both humans and biodiversity could thrive together, so what are the lessons of that ancestral path?” he asks. “For some of us, it’s in the more recent past, but all of us have an ancestral past where we work in closer connection to the plants and animals and places around us.”
The modern silverswords
Today, many members of the silversword alliance are struggling for survival. Nibbled by feral goats and uprooted by tourists, the iconic Maui silversword was declared a federally endangered species in 1992, appeared to be recovering for a brief period, then started to plummet again. One culprit for the plants’ population decline is climate change: As temperatures rise, moist, cool air from the east-west trade winds is getting trapped at lower elevations by a layer of warmer air, making higher elevations even hotter and drier. Counterintuitively, the shift is affecting lower-elevation plants more severely, because plants that develop in wetter conditions are less tolerant of drought in adulthood, a recent study found.
Other members of the silversword alliance are failing to reproduce because they’ve lost their pollinators, or the population is too sparse for sexual reproduction between plants. Many plants can reproduce sexually in two ways, either by receiving pollen from another plant via a pollinator, or by self-fertilizing. But most members of the silversword alliance are limited to the first option, which can pose a problem when populations are low. Groups that can’t self-fertilize, like most of the silversword alliance, are extremely rare in remote archipelagos, so it may be that more than one seed arrived on a bird or that more than one bird brought tarplant seeds over from California, Baldwin says. “If you had birds coming in every year, you could eventually bring in more individuals, but it is a real mystery how they actually got established—just an extremely unlikely event.”
Studying the evolution of Hawaiian flora has shifted Baldwin’s view of California’s native plants. “We always talk about California as a cultural and biological island, with its isolated Mediterranean-like climate. But when you study an actual island system, you can draw some interesting comparisons and contrasts,” he says.
California contains North America’s most diverse—and threatened—flora, with more than 6,500 types of native plants, 288 of which have been listed as rare, threatened, or endangered under state and federal laws. In California, most botanists think of “native” plants as those that evolved here before European contact, since that’s when the speed of plant arrivals from distant origins vastly accelerated, notes Andrea Williams, director of plant science at CNPS.
Everything’s a lineage—that’s just the nature of life.
– Kawika Winter
As in Hawai‘i, this diversity has been shaped by a uniquely varied topography that packs an extremely wide array of habitat types closely together. “One thing we’ve learned is that landscapes that are highly heterogeneous—where you have a lot of habitat diversity over a small area—are areas that seem to not only lead to higher diversification rates but allow diversity to persist,” Baldwin says. “So with climate change, for example, to be able to move easily upslope or downslope or around the slope without having to disperse over large distances is key to the long-term survival of lineages.”
California began shifting to a Mediterranean-like climate only around 15 million years ago, a rapid change on an evolutionary timescale, Baldwin says. That shift resulted in the extinction of many plant species that could not tolerate drought, but also created ecological opportunities for new species to diversify, particularly in the state’s drier regions, says Baldwin. Plants that evolved before the shift to a drier climate include species such as California’s giant sequoias (Sequoiadendron giganteum), which date back to the age of the dinosaurs. These ancient plants have survived either by shifting their ranges, or remaining in relatively stable areas known as “refugia.” Much of California’s plant diversity and high number of rare species derives from this combination of “new” species— the product of rapid radiations—and “old” species that survived in stable refugia, says Williams.
An important part of preserving California’s biodiversity is to identify and protect refugia, Baldwin says. Also key is the preservation of plants within the same species, which have local adaptations that affect their ability to survive in different geographic areas. Many plants can adjust to change through such local adaptations, shifting the timing of their reproduction and other parts of their life cycles, their means of dispersal, or other aspects of their biology. For many species, however, conditions are changing so rapidly, and communities of plants are so fragmented, that adjusting “is harder and harder for them,” says Loralee Larios, a professor of plant ecology at the University of California, Riverside.
In the end, the decision to attempt to save a species comes down to human values, says Williams. In a world where humans have influenced every ecological system, there’s no way to avoid making value-based decisions about how we affect the environment. “We have to make these decisions, and clearly state why we’re making them,” she says. When it comes to conserving plant lineages such as the silversword alliance, the decisions are about valuing biodiversity, resilience, and local adaptation: “It represents the unique and irreplaceable interaction of life with its environment over thousands or millions of years.”
With our short lives and attention spans, it can be easy to lose sight of the fact that so far as plants are concerned, we’re still in the middle of a vast evolutionary story. “Everything’s a lineage—that’s just the nature of life,” Winter says.
On Guadalupe Island, off the coast of Mexico’s Baja California, Baldwin and colleagues recently confirmed that tarplants have undergone yet another adaptive radiation that follows a similar pattern to the silversword alliance on Hawai‘i. The find confirms observations that Sherwin Carlquist made in the 1960s, Baldwin says: “Here you put California tarplants on another isolated oceanic island, independently of the Hawaiian occupation of tarplants, and you see similar types of change.”
Rather than merely a destination for plants, Hawai‘i is turning out to be the source of a number of species that have traveled to other places. In their recent analysis of plant migrations to the island, for example, Price and Wagner found more than 30 species that have island-hopped to other places. Among them is the iconic flowering Hawaiian koa tree (Acacia koa) that traveled more than 9,000 miles to La Réunion Island in the Indian Ocean, a journey suited to the meaning of the tree’s name: “bold,” or “fearless.”
Meanwhile, off the southeastern shore of the Big Island, a new Hawaiian island is growing. Sometime within the next 100,000 years, scientists estimate, the volcano will reach the surface of the Pacific and start a new chapter in the story of Hawaiian plants—and possibly, in the tarplant’s journey.
Emily Underwood is the Publications Editor for CNPS.