There’s an invader at your door. If you’re reading this story, no matter where you are—a train station in Madrid, a cabin in Alaska, a beach in Sydney, or even a remote field station in Antarctica—you probably see it every day. You walk right past it, never realizing that this little patch of green, inconspicuous and unremarkable, hides a fascinating tale of evolution, scientific history, survival, and domination.It’s a tale that started on the northern coast of the Mediterranean Sea, within the last glacial period, perhaps some 50,000 years ago. It was probably springtime, and plant life was thriving in the bare land newly exposed by retreating glaciers. As the theory goes, increasing aridification during that era pushed the growth of grasslands, and glacial movement likely brought the pollen of one type of grass, a mountain species called supina, in proximity to infirma, a lowland grass. The two species were related, both members of the large botanical genus Poa, but as different as two cousins could be. P. supina was perennial, surviving multiple years, while P. infirma was an annual plant, living, dropping seeds, and dying all in a single season.Poa annua makes more seeds than many plants its size. A single plant can produce thousands of seeds, and a square meter patch can send out up to 100,000.We’re taught in our earliest biology classes that species are defined by reproductive isolation. Only those of the same species can breed. But in nature, nothing is that simple. Hybrids happen all the time. Many are born stunted, or sterile, like mules, evolutionary dead ends with no way to pass on their genes. But every once in a while, a genetic loophole allows for the rise of something entirely new, endowed with the most useful traits of both of its parents.The seeds produced from that chance meeting of P. supina and P. infirma were such a new species, eventually given the name Poa annua. Both parent species were genetically diploid, meaning they had two sets of each chromosome, just like humans do. But rather than take one from each parent, P. annua took both. It was born in that imagined Mediterranean springtime as what geneticists call an allotetraploid, a species with four sets of each chromosome. In one generation, it had doubled its number of genes, giving it a much larger variety of traits and greater flexibility than either parent. From there, it was off to the races.Weekly Newsletter[contact-form-7]Poa annua makes more seeds than many plants its size. A single plant can produce thousands of seeds, and a square meter patch can send out up to 100,000. This reproductive explosion provides a smorgasbord to birds, small mammals, and other predators who spread the seed even as they feed on it. P. annua probably spread its range for a while on its own, whereever it found available space to settle. Then, it met a new ally, a seed disperser that put sparrows and mice to shame.We cannot know when humans first encountered Poa annua—or recognized it as a unique species. What we do know is that when Carl Linnaeus published his Species Plantarum, the seminal accounting of plant species, in 1753, P. annua was included therein. By 1800, colonists had brought it in seed mixes from Europe to North America, intent on feeding their livestock familiar European species rather than the abundant native grasses of the new continent. As colonizers moved west, P. annua followed with them both by design and by accident. The tiny seeds caught a ride in their seed bags as well as on their boots and clothes, and the invading grass took hold everywhere their horses’ hooves and wagon wheels tread.In the meantime, back in England, a groundbreaking experiment was about to change horticulture and world history forever, and P. annua played a central role.As rhetoric and Victorian studies scholar Margaret Flanders Darby writes, Nathaniel Bagshaw Ward was a London doctor in the early 1800s. Like many physicians of his day, he spent his free time as a naturalist, applying his scientific knowledge to understanding the natural world. One day in 1829, he embedded a moth chrysalis into a sealed glass jar with moistened soil, hoping to see it hatch. Day after day he waited, but for naught; no moth emerged. Instead, writes Darby, two little green shoots appeared from the soil. One grew into a small fern. The other was Poa annua.Ward monitored his jar for at least three years and was amazed to find that the two plants survived and grew without him adding any water or soil. He had created a self-contained environment for them, like a modern-day terrarium. He immediately started constructing larger cases of wood and glass. Within roughly a decade, these Wardian cases became a common sight in wealthy households around London and elsewhere in Europe, Darby writes.More to ExploreThe Accidental Invention of TerrariumsMatthew WillsDecember 9, 2018Victorian London became obsessed with Ward’s cases, which protected plants from the city’s toxic pollution—and piqued peoples’ imaginations.But the real impact of Ward’s creation wasn’t a passing fad for rich urban gardeners. Before his cases, the survival rate of transplants from far-off shores was abysmal, packed as they were in the hulls of ships on long voyages. The East India Company’s John Livingstone estimated in 1819 that only one in a thousand plants survived these journeys, according to historian and curator Luke Keogh. Wardian cases allowed for reliable shipping of seedlings and even mature trees. By the middle of the nineteenth century, the cases provided the fuel for some of the biggest economic engines of the time. Commodities like tea, rubber, and cinchona—the source of the antimalarial drug quinine—began to move freely around the world. Massive colonial profit centers, like the Assam tea plantations in British-controlled India and Southeast Asian rubber farms, owed their establishment to Ward, writes Keogh.This little grass is controversial in golf; some courses work intensely to establish it as a nearly indestructible green and fairway grass while others seek to annihilate it as a hated weed.The cases also ushered in a new age of harmful species introduction. Japanese honeysuckle and many other invasive Asian and European plants arrived healthy in North America, thanks to Wardian cases. Though introduced intentionally, they spread more quickly and invasively through their new environs than anyone could have predicted. In thousands of Wardian cases carrying tons of soil back and forth between every colonized continent, the tiny seeds of P. annua undoubtedly found even more new places to settle.In addition to accidental spread, humans have found reason to spread P. annua intentionally: Golf. This little grass is controversial in the sport, writes W. Mary Lush. some courses work intensely to establish it as a nearly indestructible green and fairway grass while others seek to annihilate it as a hated weed. True to its flexible nature, P. annua adapts quickly to different mowing schedules and heights of the various parts of a particular golf course. In botany, an ecotype occurs when a plant adapts to different environmental conditions with different size, appearance, or other traits. A single hole on a golf course in Minnesota was found to host four separate ecotypes of P. annua.A photograph of Poa annua from The genera of grasses of the United States, with special reference to the economic species, 1922 via Wikimedia CommonsToday, Poa annua thrives on every continent, including Antarctica, where it has made its most recent, and most impressive invasion of both the islands and mainland. As a research team led by Marco A. Molina-Montenegro explains, P. annua is the only known flowering plant to successfully colonize the southern continent. Botanists once thought arid deserts would stymie its advance. Then it started popping up in Mexico, Utah, Australia, and other dry regions.P. annua is even adaptable enough to take on either of the life strategies of its parents. When conditions allow, it grows perennially, living multiple years like its parent species, P. supina. But when it needs to reproduce quickly and colonize shifting patches of habitat, it can also live as an annual, like P. infirma. The same individual plant can take on either lifestyle, leaning on its ancestral mother’s or father’s genes as need be. (Indeed, the US Golf Association discusses this unique characteristic at length on its website.) Rather than compromise either side of its heritage, it has become a more successful annual than its annual parent, and likewise for the perennial side of the family.As we have realized the extent of P. annua’s spread, humans have started to fight back, applying herbicide in golf courses, backyards, and parks—all with limited success. P. annua’s flexibility tends to win out. In the end, there are two strategies for limiting the spread of this invasive survivor. The first, and most obvious, is to see it’s not introduced in the first place. Remote and vulnerable terrain, like that of the Antarctic islands with their lack of lush, tall natural vegetation, should be approached with care. We should carefully remove any seeds from boots and clothes to reduce the chance of introduction and then monitor closely for sneaky pioneers that survive our best efforts.The other strategy is less obvious: Let your grass grow long. In England and some of its colonies, P. annua used to be known as “path grass” because it grew in disturbed areas like foot paths but not in established meadows. As incredibly adaptable as it is, it was called an “opportunistic colonizer” by the late weed scientist Larry Mitich: P. annua doesn’t compete particularly well with tall grass in natural conditions. It takes advantage of short grass and bare patches, which can come about by excessive mowing or livestock grazing. Wilder landscapes provide less opportunity for the invader to get a foothold while also providing more native biodiversity than a well-manicured lawn. In the end, the only real answer to this incredible, adaptable grass may be a combination of careful prevention and benign neglect, letting nature be the barrier against its own greatest survivor.Support JSTOR Daily! Join our membership program on Patreon today.The post A Most Opportunistic Colonizer appeared first on JSTOR Daily.