When humans bring new plants to an ecosystem, it can slowly push out the original inhabitants. Research shows us how to identify this threat before plant species become “the living dead.”

Tiny purple flowers burst their way out of a rocky landscape

extinction countdownAuthor’s note: My “Extinction Countdown” column will mark its 20th anniversary this summer. As that milestone approaches, let’s look back at some previous entries, which I’ll update for the world we find ourselves in today. A version of this article was published in 2016 in Scientific American.

Japanese knotweed. Purple loosestrife. Kudzu. Mesquite. Giant hogweed. Bitou bush. What do these plants have in common? Easy: They’re among the most “invasive” plant species on the planet. When humans bring these highly adaptable, fast-growing plants to new ecosystems, whether it’s on purpose or by accident, native species often get squeezed out and pushed toward extinction.

But, unlike predators such as rats and cats — which have threatened animal species and caused extinctions around the globe — have displaced plants like kudzu ever actually driven another plant species extinct? The authors of a 2016 paper published in the journal AoB Plants couldn’t document any confirmed cases.

Not yet, anyway. But that’s only because globalization is a relatively recent phenomenon.

“The main reason why there is no clear evidence of extinction that can be exclusively attributed to plant invasions is that invasions have not been around long enough,” co-author Dave Richardson of the Centre for Invasion Biology at Stellenbosch University, South Africa, said in a prepared release. “Our research shows that plant extinction is an agonizingly slow process. However, red flags are evident in numerous locations around the world — species that now exist in fragmented populations, with radically reduced opportunities to reproduce.”

Richardson and co-author Paul Downey from the University of Canberra looked at these “red flags” and came up with a six-point “extinction trajectory” for native plant species facing threats from displaced vegetation:

    1. Plants die more quickly than they can be replaced by their offspring in some locations.
    2. Plants disappear from some locations entirely, but potential offspring remain as “propagules,” seeds or spores that could regenerate a new cohort of individuals.
    3. Some locations lose both individual plants and their propagules. With no plants or seeds, this is a local extinction.
    4. The last locations hosting a species lose their individual plants, but in some places seeds or spores remain in the soil.
    5. The species is entirely lost in the wild, with no individuals or propagules. The only survivors are held in botanic collections.
    6. The remaining plants are lost, and the remaining seeds or spores are no longer capable of becoming new plants.

Downey said that this research suggests we need to start managing threatened plants much earlier than we currently do.

“If we wait until we have sufficient evidence to show that extinctions are occurring, it will be too late to save a great number of species,” he said. Hundreds of plants species, the authors warn, may already be functionally extinct and exist now only as “the living dead.”

The biggest risk point for many plant species appears to exist somewhere between points 2 and 4 on Downey and Richardson’s scale. As we’ve seen with many endangered plants, figuring out how to keep a species alive in a botanical setting is not as easy as simply sticking a seed in the ground. Many plants require very specific conditions in which to germinate — some rely on fire, for example, while others need to be consumed by an animal, after which stomach acids soften a seed’s outer layer before it is pooped back out. Other plants require specific pollinators, which may also disappear as humans destroy an ecosystem’s delicate balance.

Will we discover the details on how these endangered plants propagate in time to save them? That seems unlikely for many species. Another 2016 paper in Conservation Biology warned that plants in general remain understudied while scientists concentrate on mammals and other more charismatic species, much in the same way that scientists also ignore “ugly” creatures. The authors called this “plant blindness” and suggest that it could have severe implications for conservation of many species now and in the future.

As Downey and Richardson wrote in their paper, the lack of evidence for extinctions “does not mean we should disregard the broader threat.” In fact, that may just make it more urgent.

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Previously in The Revelator:

The Monumental Effort to Replant the Klamath River Dam Reservoirs

John R. Platt

is the editor of The Revelator. An award-winning environmental journalist, his work has appeared in Scientific American, Audubon, Motherboard, and numerous other magazines and publications. His “Extinction Countdown” column has run continuously since 2004 and has covered news and science related to more than 1,000 endangered species. He is a member of the Society of Environmental Journalists and the National Association of Science Writers. John lives on the outskirts of Portland, Ore., where he finds himself surrounded by animals and cartoonists.