Ask Sam: Do Species From Here Become Invasive Elsewhere?

Feb 8, 2019

Yup. It always comes back to squirrels.
Credit Flickr Creative Commons | Denis Fournier

Every other Friday on Morning Edition NHPR’s Sam Evans-Brown tracks down answers to questions about the environment and outdoors for our listeners in a segment we call “Ask Sam.”

Kenneth from Concord Asks: “On Eat the Invaders we hear all about invasive species that have come to New England or New Hampshire specifically. What invasive species from our region are terrifying other parts of the world. Are there any muscles that are screwing up Asian rivers, or is it just that everything’s coming here?”

The short answer to this: of course!

The “greatest hits” are the North American bullfrog (which has spread all over the world, including to some places that were interested in having them around for their delicious legs), the largemouth bass and the North American gray squirrel. (Hilariously, it seems that the now-hated grey squirrel was brought to the United Kingdom in order to make Victorian estates look more picturesque.)

But the list is long. More than 52 million red-eared sliders, a turtle from the south, have been shipped all around the world as pets and released when owners learned that turtles are boring pets. Scandinavia has been waging a campaign to have the American lobster declared invasive. The Alaskan lupine was introduced in 1945 as part of a campaign to revegetate what was believed to be the most environmentally degraded place in the world, Iceland. Raccoons, American mink, and (most notably in Tierra del Fuego) beavers have all run amok after being released from fur-farms that went belly-up following the collapse of the market for fur clothing. And of course there are some mid-century biocontrol horror stories, such as the rosy wolf snail, which was introduced around the world to eat other pest snails and wiped out 56 endemic species on the Society Islands in French Polynesia.

But Is There Something More to This?

Here’s a tantalizing possibility: what if it’s true that there are certain places that tend to produce more invasive species than others? That’s the hypothesis that Jason Fridley of Syracuse University and Dov Sax of Brown set out to test.  The model they worked up proposes to predict what parts of the world are more likely to produce invasive species. (I should note, they are plant biologists, though I’m not sure why this same idea wouldn’t apply to other kingdoms as well.)

What are the characteristics of a place that is more likely to produce invaders? First, bigger.

“If you just think about evolution as a series of happy experiments... if you have a really small population, you probably don’t have the same number of possibilities of increasing your fitness, really,” explained Fridley. More organisms means more evolution. “So one thing you could look to is just kind of the size of the habitat” said Fridley, as a proxy for population sizes.

But what else drives evolution? Competition. To capture this, Fridley and Sax turned to a measure of diversity, “How many lineages do those plants have to compete with in their home range.”

Lastly, “the third one is time,” said Fridley, “Adaptation takes time.” You’ll get more invasive species from places that have had more stable habitats for longer.

When Fridley and Sax did the math, put in all the data into the equation, where do you think it predicted invasive species will tend to come from? “It turns out that temperate Asia is the most diverse temperate flora in the world and that’s where we get a huge number of our invaders,” said Fridley.

Eurasian milfoil. Asian Long-horned beetle. Asian shore-crab. Asian carp. Chestnut blight, emerald ash borer, purple loosestrife, and kudzu are all from Asia. The list goes on and on.

All of this is not to say that invasions can’t go in both directions, or that things like “introduction effort” and the history of biological trade don’t matter. It is merely to say that from an evolutionary fitness perspective, evolutionary theory predicts that some parts of the world should simply produce species that are tougher than others. This would mean they would export more invasions than they would import.

So while yes it's true that we mostly hear about invaders from abroad because we generally do a bad job of paying attention to news from abroad, and yes it's true that some places don't have the same number of scientists on the ground watching for invasions so maybe that's another reason we don't hear about them. But wouldn't it be interesting if part of the explanation was because of the fundamentals of evolution?

Sam Evans-Brown, is host of NHPR’s Outside/In which you can subscribe to where-ever you get your podcasts. If you’d like to submit a question you can record it as a voice memo on your smartphone and send it to, OR call the hotline, 1-844-GO-OTTER.