The Burmese python, which has spread throughout the Everglades in Florida as a result of accidental or intentional releases by pet owners (Image Credit: US NInvaders, Aliens, and tational Park Service, Public Domain Mark 1.0, Image Cropped)

Language is important. It’s a lesson many biological scientists would have learned a long time ago if we hadn’t kept social sciences at such a wary arm’s length. Ecologists have a tendency to label and relabel ecological concepts (anyone up for a debate about the word ‘niche’?), species and even global phenomena (think global warming vs. climate change) based on anything from shifts in public perception to new findings that challenge our earlier labels.

Being an ‘invasive biologist’ (in only one sense, though I’m not saying there weren’t torches and pitchforks when I first arrived in Norway), the language around invasive species has always fascinated me. As much as some of us want to think so, there doesn’t seem to be much real consistency about the use of the terms ‘invasive’ or ‘alien’, despite decades of debate on the subject. Just last week, a paper warning on the dangers of invasive species was published, authored by several eminent invasive species ecologists, including Petr Pyšek, Helen Roy (my interview with whom is linked below) and Dan Simberloff. It included a section entitled “What is an Invasive Species” which immediately gave two different definitions. So I want to briefly go over a) why the terminology here is important, b) what the components of an invasive species generally are and c) why they’re each so difficult in their own ways.

Why Does it Matter

First of all, the importance I’ve ascribed to this argument isn’t just the delusion of some self-important PhD candidate. Professors have been at constant odds about this for decades. Mark Davis, Dan Simberloff and James Russell have been going back and forth across several publications for most of the 2010s, with Davis calling for less stigma against invasive species and Russell labelling this ‘invasive species denialism’. I interviewed Dr. Helen Roy late last year and she mentioned that modern studies often conflate invasive species with anything non-native.

Related: Breaking Down the Social Stigma of Invasive Species with Professor Helen Roy

So why is it important? I don’t necessarily share all of Mark Davis’ views on invasive species (clearly – I’m still calling them invasive) but his words in our 2018 interview (linked below) have stayed with me.

“The problem with declaring harm is that you then obligate society to spend some money and do something about it. So labeling something as harmful is a very important act.”

Not all, but a lot of definitions these days mandate that an invasive species must have a negative effect (we’ll get into that later on). So when we label something as invasive, we’re saying that it’s harmful. Think of someone who moved in with you versus someone who invaded your home. One evokes thoughts of companionship, the other thoughts of Jodie Foster and Kristen Stewart in a criminally underrated 2000s film. Whether you believe in the nativism paradigm or invasive species denialism, you should really accept that ‘invasive’ (and to a certain extent, even ‘alien’) is a loaded term.

Related: Mark Davis: Rethinking Invasive Biology

As Mark puts it, when something is labelled as harmful, we’re expected to do something about it. And as always, the resources with which to ‘do something’ are limited, for ecologists and environmental managers. As much as climate sceptics might believe that ecologists are all sitting on ‘big pots of money’, we aren’t.

The Concept of Harm

The problem with labelling something as harmful is that it’s inherently subjective. Many organisations, including the IUCN, define that the harm in question must occur in relation to the ecosystem, human health or the local economy.

First problem – those issues often clash. Red King Crabs, a species which is objectively bad for Norwegian coastal ecosystems, are great for the fishing industry. While they contribute to the homogenisation of ecosystems and reduce coastal biodiversity, over 50 million US dollars was brought into the Norwegian economy in 2016 alone from Red King Crab exports. The Sitka Spruce is another species that takes over Norwegian ecosystems, yet its placement on the Norwegian Alien Species list drew stern reactions from the timber industry, as this could potentially lead to public backlash against its use.

In other areas, efforts to get rid of invasive species have been severely hampered by the charisma of the animals in questions. The grey squirrel is a huge problem in parts of Europe, outcompeting the local (much cuter) red variety. Yet attempts to eradicate it have been hampered by the public, which perceive it as cute, which gives culling an air of cruelty. There are plenty of regions around the world where a harmful species has gained iconic status. Brown trout in Australia are an example of a fish that do almost as much damage as carp do, but are loved by the fishing community. The same goes for the ring-necked pheasant, which has become a state symbol in the USA state of South Dakota.

It’s a tricky topic. On one hand, as Helen Roy put it, “if [a local community] have a cultural attachment to a particular species, that is a very valid reason to conserve that species”. Yet that species’ ongoing persistence in that area could make its spread to other nearby regions, which do not have the same cultural attachment to the species and are rightly worried about the presence of an invader. There’s often also a knowledge gap about the true harm of such a species. And while it may be tempting to just assume the public don’t know what’s best for their surrounding and start eradication programs regardless of whether or not support is there, that can foster resentment of the scientific community, something we can hardly afford these days.

The second big problem is the timescale that harm can occur on. Let’s do away with the economic and human health sides, and focus on purely ecological harm. An issue here is that often the true impact of a species’ presence is not obvious until that species is already well-established. Not all species have an obviously negative impact immediately like the cane toad. Declaring any alien species invasive and getting rid of them early on might be more costly, but as the saying goes, an ounce of prevention is worth a pound of cure, and we could potentially save ourselves a lot of grief down the line by indiscriminately removing new species.

Figuring out which species are worth acting against early on and which can be left alone is a difficult task. It’s a field which ecologists and geneticists alike are putting some serious effort into currently.

Related: The Genetics of Invasion

The (Point of ) Origin of the Species

Let’s now move away from the concept of harm, and look at the definition that the Pyšek paper first uses – that in invasive species is simply one that is capable of establishing itself substantial distances from the point of introduction. I personally don’t like this definition, as I don’t see that there’s really a reason to get rid of a species if it’s just existing, BUT I can’t deny that it makes the definition simpler.

But there’s still the matter of the point of origin of the species. Where does a species ‘belong’? You’ll often hear people refer to species as not native to their country, but this concept is patently ridiculous – other species rarely have an understanding of geopolitics. I work with the northern pike and the European perch, two species which are invasive to certain regions of Norway, despite being native to other parts of the country. They are often regarded with much more disdain than a species like the rainbow trout, which is completely alien to the country.

This isn’t a problem for many species – most of the invaders we hear about are from completely different continents. Think the Burmese python in the US, the camel in Australia or the Ring-Necked Parrot in Europe. But regionally alien species present an increasing problem, and it comes in the form of climate change. More and more, we’re seeing species move slowly towards their respective poles. This might not be a problem for some species, as whilst they might get outcompeted by these new arrivals, they themselves might be expanding their habitats at the other end of their range. But what happens when some sort of barrier prevents them from moving any further north or south, and their geographic range is whittled away by competitors encroaching on their space as the climate warms? Many species at the northernmost range of Africa won’t have the ability to cross over to Europe to track more appropriate conditions. And of course people in these regions might ascribe certain cultural significance to animals which they start to see disappearing.

But are these species moving further north ‘invasive’? Generally we think of invaders as a result of direct human action. Climate change is certainly a product of human action. But a few studies have shown that species simply tracking changing climate doesn’t strike people as a cause of direct human activity the same way that regular translocation does. Managers often shrug, and begrudgingly accept these new species. After all, maintaining a status quo in an ecosystem that has fundamentally change an important environmental variable (temperature) is a costly process, and potentially even futile.

The Timescale

This is the last big complicating factor in the definition of an invasive species. Rewilding – the concept of bringing back a species to an area it once flourished in – and species invasions are really two sides of the same coin. And public opinion can again play a huge role here. Species like the musk ox, whose ancestors went extinct in Europe around 9000 years ago, have been welcomed back, and are now even part of the coat of arms of a Norwegian municipality. Wolves, which were nearly wiped out in Norway and Sweden as recently as the 60s and 70s, have faced a much tougher welcoming committee. Try mentioning the reintroduction of bears into the UK and see what happens.

Artsdatabanken, a Norwegian institution which publishes lists of both Endangered and Alien species, defines an alien species as one that was not present 200 years ago, making the musk ox alien. The IUCN classification of reintroduction only applies to species that were wiped out no more than 300 years ago. Know what was still present in Europe less than 300 years ago? The lion. Yet you’d be hard-pressed to find anyone who would consider a lion as a non-invasive species if it did reestablish on continental Europe.

On a scale closer to home for me, there is a lot of sediment core work which now shows that species we think of as regionally alien in many Scandinavian lakes may have actually been the original inhabitants. But it’s not an easy sell, telling a local angler that they just have to deal with the reintroduction of a fish that will kill off what they perceive to be their local species, especially if that reintroduced fish has been gone for a millenia.

Is There A Better Way?

I’d like to think so. As I’ve said, the conflation of the terms ‘non-native’ and ‘invasive’ can be confusing, so I’m happy that there’s has been a shift towards more neutral terminology for a while. In the 70s and 80s any non-native species was classified as invasive, and that’s generally no longer the case. The use of ‘exotic’ and ‘non-native’ have begun to replace the more pejorative ‘alien’ in many cases. There were even suggestions to move towards stage-based definitions of alien species’ establishment back in the early 2000s (Colautti and MacIsaac), but these never seemed to catch on.

Personally, I’m familiar with the problems that creating new definitions and terminology every decade or two create. I think it would honestly add more confusion to an already confused subfield of ecology. ‘Non-native’ is a perfectly good replacement for ‘alien’, but a decent modifier which signals a non-native species’ negative impact hasn’t arrived yet.

Honestly, I don’t think a complete overhaul of language is going to solve any problems here. But I do think that a better understanding of how the public relates to a species and how the language surrounding invasive species affects both scientific and public perception of a species would benefit invasive biology.

Sam Perrin is a freshwater ecologist currently completing his PhD at the Norwegian University of Science and Technology who now has to go and fit the above article into two paragraphs in his thesis. You can read more about Sam’s research on his Ecology for the Masses profile here, and follow him on Twitter @samperrinNTNU.