Planning the Perfect Alien Invasion

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There are alien invaders just outside your back door, but they probably aren’t of the extraterrestrial variety. Image by Carrol MacDonald

It sounds like a conspiracy theory: there are invaders all over the world, lurking in plain sight. The thought of it immediately calls to mind fantastical Dr. Who-worthy plots where little green martian men wear the skins of humans and walk among us while they plot the ultimate destruction of all mankind. In truth, the alien invaders outside your doorstep are (most likely) not of the extraterrestrial variety. For hundreds, even thousands of years, plants, animals, and microbes have been hitchhiking with humans as they move around the world. These alien species find themselves in new habitats and even on new continents with a world of opportunity in front of them. Each launches an invasion into these new habitats. Some find limited success, barely able to eke out a living in their new home. Others become invasive species, exploding in numbers and spreading widely, dominating, out-competing, and even exterminating the existing native plants and animals.

In a study published this month in Ecology Letters, a group of French researchers has asked the question “What makes an alien invasion successful?”.  To do this, the researchers quite literally looked at the invaders outside their backdoors, and focused on the 10 million hectares of grasslands that are found in France. They analysed a massive amount of information from 50 000 grassland vegetation plots that have been surveyed by botanists over the last 20 years. More than 1 million individual plants have been identified, representing over four thousand different species! Among these thousands of grassland species were 160 alien invaders – plant species that had been introduced from other places intentionally through agriculture and horticulture, or by accident through the movement of humans and goods. The scientists scored the success of these invaders: how big their local populations were and how pervasive they had become in different habitats. They then used mathematical modeling to identify those characteristics associated with the most successful invasions.

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Can you spot the invaders? The most successful alien plant species have similar functional roles and are closely related to their native counterparts. Photo by: Xiao Lin

You might think that the best invasion involves brute force domination, with bigger, stronger alien species out-competing the puny locals, but the researchers found this was rarely the case. The majority of successful invading plants are shorter than the surrounding native plants and do not have larger leaves, meaning their success can’t be attributed to cutting off surrounding native plants’ access to light. Instead successful plant invasions are more of a stealth attack. The best invaders actually resemble native species: they are closely related and share similar functional roles to members of the existing community. The scientists hypothesize that this high degree of similarity between invaders and locals is a result of environmental filtering over very large habitat scales. The invaders must have many of the same ecological adaptations as the locals in order to survive in that habitat’s combination of climatic and geologic conditions.

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Goldenrod, Solidago canadensis, is a particularly successful invader in French grasslands. Photo by: Kim Starr.

However, on smaller scales, the similarities between invaders and locals end. Invasions are won with strength in numbers, and successful alien species are typically make many, small, light seeds that are easily transported to new places and can quickly take advantage of any available patch of dirt. The invader’s short stature means they are less likely to be eaten by herbivores, and are better competitors for real-estate, crowding out native species. Persistence pays off in invasions too, and the best invaders get an early start. The more time that has lapsed since the introduction of an alien plant, the more likely it is to have found a foothold and become pervasive in its new community.

The researchers analyses have not only pinpointed what makes alien species successful when they invade a grassland, but the model they have developed also allows them to assess the invasive potential of newly introduced alien species. Armed with this kind of information, authorities will be able to identify high-risk invaders early and manage the developing invasions, hopefully preventing invaders from exterminating the locals. Although these researchers may not have discovered the key to protecting earth from hostile extraterrestrials, their work contributes to preserving native biodiversity.

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