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Hawaii's forests reveal a hidden ecological transformation
Deep in O'ahu's mountains, a lush understory of pepper, cinnamon, and guava trees thrives-yet not a single native plant remains. Ecologists say these forests, teeming with non-native species, offer a preview of Earth's ecological future.
An accidental experiment in biodiversity
Corey Tarwater, an ecologist at the University of Wyoming, began studying O'ahu's ecosystems in 2014. What she found defied expectations: vibrant forests humming with the calls of pale green warbling white-eyes and red-billed leiothrixes-birds native to Asia, not Hawaii. "It's beautiful," she says, "but if you walk around any forest near Honolulu, there's not one native plant species left."
Human activity wiped out O'ahu's original flora and fauna. Settlers cleared forests for farms and introduced predators like rats and mosquitoes, driving mass extinctions. Today, the island's lowland forests are a patchwork of species from Brazil, Indonesia, the Himalayas, and Australia. Even the birds-once iconic honeycreepers found nowhere else-are gone, replaced by global castaways like the red-whiskered bulbul.
"I followed it as it flitted between trees until it stopped to feed on a fruit I immediately recognised-guava, a plant native to Brazil. That moment captured the essence of what's happening in Hawaii: a South American plant being eaten by an Asian bird on a remote Pacific island."
Jef Vizentin-Bugoni, ecologist at Universidade Federal de Pelotas
The rise of 'novel ecosystems'
O'ahu's forests are a prime example of what scientists call "novel ecosystems"-self-sustaining habitats shaped by human activity. Unlike botanical gardens or zoos, these systems don't rely on constant human intervention. "They're wild but changed," says Jens-Christian Svenning, an ecologist at Aarhus University. "If you removed all people from the planet, Hawaii would be on a different evolutionary trajectory."
Novel ecosystems emerge when human actions-like deforestation, species introductions, or climate change-alter landscapes beyond recognition. In the Netherlands, coastal defenses created salty lakes where exotic species flourish. In Brazil, abandoned farms became freshwater floodplains. Even retired quarries and mines can transform into thriving habitats, like a Scottish shale mine now home to rare orchids.
A planet reshaped
How widespread are these ecosystems? A 2013 study estimated that 30-40% of Earth's terrestrial ecosystems have already become "novel." Svenning's projections suggest half the planet's land could follow by 2100. Yet the concept remains contentious. Critics argue it's unclear when an ecosystem crosses the threshold into "novel," and some systems, like Florida's warm-water vents, still depend on human infrastructure.
Hawaii's story is especially stark. Its isolation bred unique species-flightless ibises, bird-hunting owls, and "flightless megaducks" that once dispersed seeds. Of 142 bird species found nowhere else, 95 are now extinct. Mosquitoes, introduced in the 1800s, spread avian malaria, pushing native birds to the brink. "O'ahu is the cautionary tale for all Pacific Islands," Tarwater warns.
Unexpected lifelines in a broken system
Non-native species aren't just invaders-they're filling gaps left by extinctions. Tarwater's research shows that non-native birds now disperse seeds for Hawaii's last native fruiting plants, like the violet-berried uki'uki. "It was shocking how embedded they are," she says. Yet the relationship is double-edged: these birds also spread invasive plants, crowding out endangered natives.
In O'ahu's forests, ecologists observe behaviors once seen only in ancient ecosystems. Mixed flocks of Himalayan and South American birds take flight together in search of fruit. Some non-native birds have even evolved smaller bills to eat O'ahu's smaller fruits. "It's like the Amazon," Vizentin-Bugoni says, "but with species from all over the planet."
Conservation in a 'freakosystem'
Traditional conservation-eradicating invasives to restore pre-human states-often fails in novel ecosystems. In O'ahu, land managers now use unconventional tactics, like playing bird calls to lure seed-dispersing species to restoration sites. "We trick them into eating native fruit," Tarwater explains. Yet gaps remain: no non-native species can replace the moa-nalo's role in dispersing large seeds.
The debate over novel ecosystems is fraught. Some see them as ecological betrayals; others, like San Francisco's beloved eucalyptus forests, have become cherished local landmarks. "It's not as simple as them being good or bad," Tarwater says. A 2025 global map revealed that over half of Earth's land-including protected areas-is now highly altered, forcing conservationists to rethink their approach.
A glimpse into Earth's future
O'ahu's forests are a living laboratory for a planet in flux. As climate change and biodiversity loss accelerate, novel ecosystems may become the norm. The challenge, ecologists say, is learning to protect what remains while adapting to what's already changed. "The challenges will remain, and likely increase," Tarwater says. "We know it's not black-and-white."
