The term "invaders" doesn't quite conjure up feelings of welcome or belonging. But some biologists are trying to change the perception of biological invaders — and instead want us to think of them as migrants searching for the right place to live.

CBC Radio science columnist Torah Kachur looks at the debate about how we view so-called invasive species.

What's the concern about invasive species?

It's mostly because there are several high-profile horror stories out there. Take, for example, rabbits in Australia. In the 1800s, rabbits were introduced to the island continent and quickly managed to cause millions of dollars of damage to crops. They caused extinction of native plant life and ate so many plants that the soil started to erode.

In the 1950s, officials introduced a virus to kill the rabbits. It did successfully reduce the population, but rabbits are still a major concern for nature in Australia.

Zika is another invader — it's an invasive virus, and we all know the terror it has induced across the Americas.


A Parks Canada employee pulls up a patch of garlic mustard, a plant species considered invasive. ((CBC))

There are more examples closer to home. Asian carp are swimming their way into the Great Lakes, where they are voracious eaters and are squeezing out the native fish from their natural habitat. 

Garlic mustard is an invasive plant found coast-to-coast in Canada that's spreading fast — expanding across roughly 6,400 square kilometres per year, according to the Nature Conservancy of Canada.

This plant quickly takes over ecosystems, disrupting butterfly migrations and reducing growth of native plants like sugar maples.

What's the current debate around invasive species?

There's a movement happening in some circles of ecologists who think maybe we should re-brand invasive species — which sound very unwelcome — into just non-native species.

That idea was covered in a 2011 essay in Nature, for example, and in a 2014 book by ecologist Ken Thompson called Where Do Camels Belong? Why Invasive Species Aren't All Bad

The idea is to think of them more like immigrants to a new ecosystem — which may cause disruption, or may fit right in. In fact, they may provide a benefit.

Every native species was an invader at one point. The honey bee is a crucial pollinator that's absolutely essential to the health of our crops. But it was, in fact, an invasive species not that long ago. Honey bees were brought to North America by European settlers around 400 years ago for honey and wax.  It turns out they were a perfect fit for our crops and climate, and now they are an essential part of pollination.

Some non-native species are particularly good at growing in areas that have been disrupted by the worst invasive species on the planet — humans. The eucalyptus tree, for example, may grow where no native species is suited anymore.

So for all the stories of the dangers of invasive species, there are stories of non-native species that fit right into the ecosystem, providing important diversity in different habitats.

How do researchers know if an invasive species will fit in, or not?

That's the hard part, says University of Washington conservation biologist Julian Olden.

"The paradigm is usually 'guilty until proven innocent' for non-natives," he said. 

"So we try to use kind of short-cut, risk assessment approaches to determining whether something is likely to become invasive. So there are certain characteristics or traits of species that might pre-dispose them to be highly invasive. And we try to screen and use those traits to determine when something will ultimately turn bad."

Those kinds of traits would be lack of predators, a rapid lifecycle, or non-seasonal reproductive times.

How controversial is this approach to non-native species?

Julian Olden

University of Washington conservation biologist Julian Olden says the distinction between an 'invasive' or 'non-native' species may rely on what we value in an ecosystem. (

It's quite controversial, but this is a very tricky field. Ecosystems are always in flux, and there's always change — whether it's environmental shifts or migration or the introduction of new species that change the natural balance of an ecosystem.

More importantly, it comes down to whether something is labelled as "non-native" or "invasive." The label depends on your perspective, and what you value, says Julian Olden.

 "For example, zebra and quagga mussels filter out vast amounts of water and make murky water clear," he said. "And clear water is valued by a lot of people who live on shorelines of lakes."

But the mussels also reproduce quickly, and filter out food that is necessary for other organisms to survive — such as fish that people who live along lakes catch and eat.

"So there's competing values in our ecosystems, and non-native species often have different effects on those different values," said Olden.

That's the grey area where scientists really don't like to be — what value system do we place on zebra and quagga mussels? Do we consider them invasive, or just non-native and potentially even beneficial?

What difference does the 'non-native' vs. 'invasive' label make?

It actually makes quite a big difference when it comes to management. Just the terms "invasive" or "invader" imply the species should be stopped. "Non-native," though, is just a migrant into a new habitat. In other words, a simple label could drastically change the outcome to the species in question.

Species like the zebra and quagga mussels are considered to be invasive — and a lot of money is spent trying to eradicate them from the lakes and streams across Canada.


Zebra mussels can be beneficial in that they filter water, but are considered invasive because they reproduce quickly, and filter out food that is necessary for other organisms to survive. (John L. Russell/Associated Press)

But here's where things can get even more complicated. What if an invasive species is wreaking havoc on an ecosystem in one area, but is actually critically endangered in its native ecosystem?

This is what is happening to the wattle-necked softshell turtles – they're native to China, and they are endangered. But on the Hawaiian island of Kauai, they are considered invasive. So do we try and eradicate them from Kauai — and potentially doom the entire species?

That was the topic of a recent paper in Conservation Biology. The authors didn't really come to a conclusion, but these kind of questions are hardly black-and-white.

In the end, the debate will continue about what to do about invasive species — if anything.

In the meantime, the prudent management decision from the public's point of view is to keep nature in the balance it has found for itself and don't mess with it. That means making sure you aren't transporting zebra mussels on watercraft between lakes, and pulling garlic mustard when you see it.

The wait-and-see approach on the impact of a new species should be left to the experts.